The invention relates to the field of sorption purification of surface and groundwater with a high content of titanium and its compounds and can be used for water purification to produce drinking water that is safe for health. A method for purifying surface and groundwater from titanium and its compounds involves bringing contaminated water into contact with an adsorbent, where carbon nanotubes are used as an adsorbent, which are placed in an ultrasonic bath and act on the carbon nanotubes and the water being purified in a mode of 1-15 min, with a frequency ultrasound 42 kHz and power 50 W. The technical result consists in 100% purification of water from titanium and its compounds due to the very high adsorption characteristics of carbon nanotubes. 4 ill., 2 tables, 4 ex.
Drawings for RF patent 2575029
The invention relates to the field of sorption purification of surface and groundwater with a high content of titanium and its compounds and can be used to purify water from titanium and its compounds to obtain drinking water that is safe for health.
There is a known method for purifying water from heavy metal ions, according to which a calcined activated natural adsorbent is used as an adsorbent, which is a siliceous rock of mixed mineral composition from deposits in Tatarstan, containing wt.%: opalcristobolite 51-70, zeolite 9-25, clay component - mont morillonite, hydromica 7-15, calcite 10-25, etc. [RF Patent 2150997, IPC B01G 20/16, B01G 20/26, publ. 06/20/2000]. The disadvantage of this known method is the use of hydrochloric acid to activate the material, which requires equipment that is resistant to aggressive environments. In addition, the method uses a rather rare rock of complex mineral composition and there is no data on the content of titanium and its compounds.
There is a known method for producing granular adsorbent based on shungite [Auth.St. USSR No. 822881, IPC B01G 20/16, publ. 04/23/1981].
The disadvantage of this method is the use of the less common mineral shungite, which is pre-modified with ammonium nitrate, calcination at high temperatures, which requires appropriate equipment and energy consumption, as well as processing in aggressive environments. There is no data on the effectiveness of water purification from titanium.
There is a known method, taken as an analogue, of obtaining organomineral sorbents based on natural aluminosilicates, namely zeolite, by modifying pre-heat-treated aluminosilicate with polysaccharides, in particular chitosan [RF Patent No. 2184607, IPC C02F 1/56, B01J 20/32, B01J 20/26 , B01J 20/12, publ. 07/10/2002]. The method makes it possible to obtain sorbents suitable for effective purification of aqueous solutions from metal ions and organic dyes of various natures.
The disadvantages of sorbents obtained by the described method are their high degree of dispersion, which does not allow water purification by current through the sorbent layer (the filter quickly becomes clogged), as well as the possibility of washing off the chitosan layer from the sorbent over time due to the lack of fixation on a mineral basis and no data on effective purification from heavy metal compounds, such as titanium and its compounds.
A method for clarification and disposal of industrial water from filter structures of water treatment stations is described [Patent for invention RU No. 2372297, IPC C02F 1/5, C02F 103/04, publ. 11/10/2009].
The essence of the invention lies in the use of a complex coagulant, which is a mixture of aqueous solutions of sulfate and aluminum oxychloride in a dose ratio of 2:1 for aluminum oxide.
This patent provides examples of groundwater purification for drinking water supply.
The disadvantage of the described method is the poor efficiency of purification from impurities; 46% of the sediment floated, and the rest was in suspension.
There is a known method of water purification by treatment in a supply pipeline with a cationic flocculant [RF Patent No. 2125540, IPC C02F 1/00, publ. 01/27/1999].
The invention relates to methods for purifying water from surface drains and can be used in the field of domestic and drinking or technical water supply.
The essence of the invention: in addition to the flocculant, a mineral coagulant is introduced into the pipeline in a mass ratio to the flocculant from 40:1 to 1:1.
The method ensures an increase in the efficiency of aggregation of suspended substances, which makes it possible to reduce the turbidity of settled water by 2-3 times. After using this method, further complete sedimentation in settling tanks is necessary. Thus, according to the described method, 100% removal of metals was not achieved, water hardness decreased from 5.7 mg-eq/l to 3 mg-eq/l, turbidity decreased to 8.0 mg/l.
The disadvantage of the analogue is the poor efficiency of removal of metals and organic impurities; there is no data on titanium content.
The sorption efficiency of carbon nanotubes (CNTs) is described as the basis of an innovative technology for the purification of water-ethanol mixtures [Zaporotskova N.P. and others. Bulletin of VolSU, series 10, issue. 5, 2011, 106 pp.].
The work carried out quantum mechanical studies of the processes of adsorption of heavy alcohol molecules on the outer surface of single-walled carbon nanotubes.
The disadvantage of the described sorption activity of CNTs is that only theoretical quantum mechanical calculations are carried out, while experimental studies were carried out for alcohols. There are no examples for cleaning metals.
The positive effect of carbon nanotubes on the purification process of water-ethanol mixtures has been proven.
Currently, special hopes in the development of many areas of science and technology are associated with carbon nanotubes CNTs [Harris P. Carbon nanotubes and related structures. New materials of the XXI century. - M.: Tekhnosphere, 2003. - 336 p.].
A remarkable feature of CNTs is associated with their unique sorption characteristics [Eletsky A.V. Sorption properties of carbon nanostructures. - Advances in physical sciences. - 2004. -T. 174, No. 11. - P. 1191-1231].
A filter based on carbon nanotubes for purifying alcohol-containing liquids is described [Polikarpova N.P. and others. Bulletin of VolSU, series 10, issue. 6, 2012, 75 p.]. Experiments were carried out on the purification of alcohol-containing liquids using filtration and transmission methods, and the mass fraction of CNTs was determined that leads to the best result.
The experimental studies carried out proved that the treatment of a water-ethanol mixture with CNTs helps to reduce the content of fusel oils and other substances. The disadvantage of this analogue is the lack of data on the purification of water from metals.
The work studied the sorption/desorption of Zn(II) in successive cycles by activated carbon and CNTs. The adsorption of Zn(II) by activated carbon decreased sharply after several cycles, which was attributed to the low removal of metal ions from the inner surface of the activated carbon pores.
The hydrophobic nature of CNTs causes their weak interaction with water molecules, creating conditions for its free flow.
Noy A., Park N.G., Fornasiero F., Holt J.K., Grigoropoulos S.P. and Bakajin O. Nanofluidics in carbon nanotubes // Nano Today. 2007, vol. 2, no. 6, pp. 22-29.
The adsorption capacity of CNTs depends on the presence of functional groups on the surface of the adsorbent and the properties of the adsorbate.
For example, the presence of carboxyl, lactone and phenolic groups increases the adsorption capacity for polar substances.
CNTs, which have no functional groups on their surface, are characterized by a high adsorption capacity for non-polar pollutants.
One way to create a membrane is to grow CNTs on a silicon surface using carbon-containing vapor using nickel as a catalyst.
CNTs are molecular structures resembling straws made from sheets of carbon a fraction of a nanometer thick, 10 -9 m thick, essentially an atomic layer of ordinary graphite rolled into a tube - one of the most promising materials in the field of nanotechnology. CNTs can also have an expanded structure [WCG website http://www.worldcommunitygrid.org/].
Membrane technology, which is widely used to obtain drinking water for the inhabitants of our planet.
There are two significant disadvantages - energy consumption and membrane fouling, which can only be removed by chemical methods.
Productive and antifouling membranes can be created based on carbon nanotubes or graphene [M. Majumder et al. Nature 438, 44 (2005)].
The closest to the claimed invention in terms of technical essence and achieved result is a method for producing sorbents for water purification [RF Patent 2277013 C1, IPC B01J 20/16, B01J 20/26, B01J 20/32, publ. 01.12.2004]. This patent is taken as a prototype. This method relates to the field of sorption water purification, specifically to the production of sorbents and purification methods, and can be used to purify drinking or industrial water with a high content of heavy metal ions and polar organic substances. The method involves treating natural aluminosilicate with a solution of chitosan in dilute acetic acid in a ratio of aluminosilicate to chitosan solution equal to 1:1, at pH 8-9.
In table 1 shows a comparative description of sorbents obtained according to the invention, taken as a prototype [Patent 2277013]. Examples are given on sorption in relation to dyes and on the sorption of copper, iron and other metal ions from solutions.
The disadvantage of the prototype is the low adsorption capacity for heavy metals (SOE) mg/l for copper Cu +2 (from 3.4 to 5.85); there is no data on the adsorption of titanium and its compounds. COE, mg/l for Fe +3 varies from 3.4 to 6.9.
The objective of the invention is to develop a method for purifying surface and groundwater from titanium and its compounds using carbon nanotubes and exposure to ultrasound, which will produce high-quality, clean drinking water and increase the efficiency of purification of surface and groundwater due to the high adsorption characteristics of CNTs.
The problem is solved by the proposed method of purifying surface and groundwater from titanium and its compounds using CNTs, using ultrasound with a power of 50 W with an ultrasound frequency of 42 kHz for 1-15 minutes.
The method is carried out as follows. The adsorbent is a single-walled carbon nanotube that has the ability to actively interact with titanium atoms and its cations (Ti, Ti +2, Ti +4).
One gram of CNTs of 98% purity is added to 99 g of water to remove Ti, Ti +2, Ti +4, and then the entire contents are placed in an ultrasonic bath UKH-3560 and exposed to ultrasound for 1-15 minutes at a power of 50 Watts and at a frequency ultrasound 42 kHz.
After filtration, water samples taken for analysis are examined. Atomic emission analysis is used to determine the content of titanium and its compounds in water samples before treatment of CNTs and after treatment of water samples with CNTs in an ultrasonic bath.
The proposed “Method for purifying surface and groundwater from titanium and its compounds using carbon nanotubes and ultrasound” is confirmed by examples that will be described below.
Implementation of the method in accordance with the specified conditions makes it possible to obtain absolutely pure water with zero content of titanium and its compounds (Ti, Ti +2, Ti +4).
The technical result is achieved by the fact that the CNT acts as a capillary, absorbing Ti atoms and titanium cations Ti +2 and Ti +4, the dimensions of which are comparable to the inner diameter of the CNT. The diameter of CNTs varies from 4.8 Å to 19.6 Å depending on the conditions for obtaining CNTs.
It has been experimentally proven that the cavities of CNTs are actively filled with various chemical elements.
An important feature that distinguishes CNTs from other known materials is the presence of an internal cavity in the nanotube. The Ti atom and its cations Ti +2, Ti +4 penetrate into the CNT under the influence of external pressure or as a result of the capillary effect and are retained there due to sorption forces [Dyachkov P.N. Carbon nanotubes: structure, properties, application. - M.: Binom. Knowledge Laboratory, 2006. - 293 pp.].
This enables selective adsorption by nanotubes. In addition, the highly curved surface of CNTs allows quite complex atoms and molecules to be adsorbed on its surface, in particular Ti, Ti +2, Ti +4.
Moreover, the efficiency of nanotubes is tens of times greater than the activity of graphite adsorbents, which are today the most common cleaning agents. CNTs can adsorb impurities both on the outer surface and on the inner surface, which allows for selective adsorption.
Therefore, CNTs can be used for final purification of various liquids from ultra-low concentration impurities.
CNTs have an attractive high specific surface area of the CNT material, reaching values of 600 m 2 /g or more.
Such a high specific surface area, several times higher than the specific surface area of the best modern sorbents, opens up the possibility of their use for purifying surface and groundwater from heavy metals, in particular Ti, Ti +2, Ti +4.
Synthesis of CNTs. Using the CVDomna carbon nanotube synthesis facility, carbon nanomaterial CNT was obtained, which was used to purify surface and groundwater from titanium and its compounds.
Experimental studies have been carried out to purify water from titanium and its compounds.
To determine the optimal amount of CNTs, it is necessary to bring the content of titanium and its compounds to ultra-low quantities. This concentration of CNTs was found and in subsequent experiments the optimal concentration was used in the amount of 0.01 g per 1 liter of analyzed water.
Atomic emission analysis showed the presence of atomic Ti and its cations (Ti +2, Ti +4) in the water samples under study, from which we can conclude that it is titanium and the Ti +2, Ti +4 cations that interact with carbon nanotubes. The radius of a Ti atom is 147 pm, i.e. Titanium cations can either intercalate into the cavity of a carbon nanotube and be adsorbed inside (Fig. 1) or adsorbed on its outer surface, also forming a bridging structure with the carbon atoms of the hexagons (Fig. 2), forming connected molecular structures.
The introduction of Ti and its cations into the CNT cavity is possible by step-by-step approach of Ti to the nanotube along its main longitudinal axis and the penetration of titanium atoms and its cations into the nanotube cavity with their further adsorption on the inner surface of the CNT. Another variant of Ti adsorption is also known, according to which one titanium atom can create stable Ti-C bonds with carbon atoms on the outside of a carbon nanotube in two simple cases, when Ti is in 1/4 and 1/2 of all hexagons (Fig. 3) .
That is, the adsorption of titanium and its cations on the surface of CNTs is not only a theoretically proven fact, but also experimentally proven in research.
The inventive sorbent is a conglomerate of single-walled carbon nanotubes that have the ability to actively interact with titanium and its cations, forming stable bonds, and the possibility of adsorption of titanium atoms and its compounds on the internal and external surfaces of CNTs with the formation of bridging structures with two Ti-C bonds, if Ti +2 or four for Ti +4. When purifying water contaminated with titanium and its compounds, CNTs are used; titanium is adsorbed on the CNT surfaces due to van der Waals forces, that is, titanium and its compounds from the free atom and cations Ti +2 and Ti +4 become bound into a molecular connection (Fig. 4).
The possibility of implementing the invention is illustrated by the following examples.
Example 1. Groundwater from well 1) with a depth of 40 m was taken for testing for the content of qualitative elemental composition, as well as quantitative analysis for the content of titanium and its compounds before purification with CNTs and after CNT adsorption and ultrasonic treatment. Ultrasound exposure time 15 min. The content of Ti and its compounds after purification is 0% (Table 2).
Example 2. Groundwater from well 2) with a depth of 41 m, in contrast to well 1), this water was located at a distance of 200 m from well 1) of the Bereslavsky reservoir (Volgograd). Ultrasound exposure time 15 min. The content of Ti and its compounds after purification is 0% according to the invention (Table 2).
Example 3. Water taken from a water tap (Sovetsky district, Volgograd) was purified using CNTs and exposure to ultrasound for 15 minutes, a power of 50 W and an operating ultrasound frequency of 42 kHz (Table 2).
Example 4. Everything is the same as in example 1, but the ultrasound exposure time is 1 minute.
Example 5. Groundwater from well 1) 40 m deep was taken for analysis for the content of titanium and its compounds, and then purified according to the prototype [Patent RU 2277013].
Ultrasound exposure time 15 min (experiment 1, 2, 3, 5). Ultrasound exposure time 1 min (experiment 4).
The advantages of the claimed method based on CNTs include a very high degree of adsorption of titanium and its compounds. According to the results of the experiment, 100% purification of the test waters from titanium and its compounds is ensured under optimal conditions.
CLAIM
A method for purifying surface and groundwater from titanium and its compounds using carbon nanotubes (CNTs) and ultrasound, including bringing contaminated water into contact with adsorbents to capture heavy metals, characterized in that carbon nanotubes are used as an adsorbent, which are placed in an ultrasonic bath , influencing CNTs and purified water in the mode of 1-15 min, with an ultrasound frequency of 42 kHz and a power of 50 W.
Owners of patent RU 2430879:
The invention relates to nanotechnology and can be used as a component of composite materials. Multiwalled carbon nanotubes are produced by pyrolysis of hydrocarbons using catalysts containing Fe, Co, Ni, Mo, Mn and their combinations as active components, as well as Al 2 O 3 , MgO, CaCO 3 as carriers. The resulting nanotubes are cleaned by boiling in a solution of hydrochloric acid followed by washing with water. After acid treatment, heating is carried out in a stream of high-purity argon in a furnace with a temperature gradient. In the working area of the furnace the temperature is 2200-2800°C. At the edges of the oven the temperature is 900-1000°C. The invention makes it possible to obtain multiwalled nanotubes with a metal impurity content of less than 1 ppm. 3 salary f-ly, 9 ill., 3 tables.
The invention relates to the field of producing high-purity multiwalled carbon nanotubes (MWCNTs) with a metal impurity content of less than 1 ppm, which can be used as components of composite materials for various purposes.
For mass production of MWCNTs, methods are used based on the pyrolysis of hydrocarbons or carbon monoxide in the presence of metal catalysts based on metals of the iron subgroup [T.W.Ebbesen // Carbon nanotubes: Preparation and properties, CRC Press, 1997, p.139-161; V.Shanov, Yeo-Heung Yun, M.J.Schuiz // Synthesis and characterization of carbon nanotube materials (review) // Journal of the University of Chemical Technology and Metallurgy, 2006, No. 4, v.41, p.377-390; J. W. Seo; A. Magrez; M.Milas; K.Lee, V Lukovac, L.Forro // Catalytically grown carbon nanotubes: from synthesis to toxicity // Journal of Physics D (Applied Physics), 2007, v.40, n.6]. Because of this, the MWCNTs obtained with their help contain impurities of the metals of the catalysts used. At the same time, a number of applications, for example, for the creation of electrochemical devices and the production of composite materials for various purposes, require high-purity MWCNTs that do not contain metal impurities. High-purity MWCNTs are primarily necessary for the production of composite materials subject to high-temperature processing. This is due to the fact that inorganic inclusions can be catalysts for local graphitization and, as a result, initiate the formation of new defects in the carbon structure [A.S. Fialkov // Carbon, interlayer compounds and composites based on it, Aspect Press, Moscow, 1997, p. 588 -602]. The mechanism of the catalytic action of metal particles is based on the interaction of metal atoms with a carbon matrix with the formation of metal-carbon particles with the subsequent release of new graphite-like formations that can destroy the structure of the composite. Therefore, even small metal impurities can lead to disruption of the homogeneity and morphology of the composite material.
The most common methods for purifying catalytic carbon nanotubes from impurities are based on their treatment with a mixture of acids with different concentrations when heated, and also in combination with exposure to microwave radiation. However, the main disadvantage of these methods is the destruction of the walls of carbon nanotubes as a result of exposure to strong acids, as well as the appearance of a large number of undesirable oxygen-containing functional groups on their surface, which makes it difficult to select conditions for acid treatment. In this case, the purity of the resulting MWCNTs is 96-98 wt.%, since the metal particles of the catalyst are encapsulated in the internal cavity of the carbon nanotube and are inaccessible to the reagents.
Increasing the purity of MWCNTs can be achieved by heating them at temperatures above 1500°C while maintaining the structure and morphology of carbon nanotubes. These methods allow not only to clean MWCNTs from metal impurities, but also contribute to the ordering of the structure of carbon nanotubes due to the annealing of small defects, increasing the Young's modulus, reducing the distance between graphite layers, and also removing surface oxygen, which subsequently ensures a more uniform dispersion of carbon nanotubes in polymer matrix, necessary to obtain higher quality composite materials. Calcination at a temperature of about 3000°C leads to the formation of additional defects in the structure of carbon nanotubes and the development of existing defects. It should be noted that the purity of carbon nanotubes obtained using the described methods is no more than 99.9%.
The invention solves the problem of developing a method for purifying multiwalled carbon nanotubes obtained by catalytic pyrolysis of hydrocarbons, with almost complete removal of catalyst impurities (up to 1 ppm), as well as impurities of other compounds that can appear during acid treatment of MWCNTs, while maintaining the morphology of carbon nanotubes.
The problem is solved by a method for purifying multiwalled carbon nanotubes obtained by pyrolysis of hydrocarbons using catalysts containing Fe, Co, Ni, Mo, Mn and their combinations as active components, as well as Al 2 O 3 , MgO, CaCO 3 as carriers, which is carried out boiling in a solution of hydrochloric acid with further washing with water, after acid treatment, heating is carried out in a stream of high-purity argon in a furnace with a temperature gradient, in the working area the temperature is 2200-2800 ° C, at the edges of the furnace the temperature is 900-1000 ° C, as a result of which multi-walled nanotubes with a metal impurity content of less than 1 ppm are obtained.
Heating is carried out in ampoules made of high-purity graphite.
The heating time in an argon flow is, for example, 15-60 minutes.
Argon is used with a purity of 99.999%.
A significant difference of the method is the use of a furnace with a temperature gradient for cleaning MWCNTs, where metal impurities evaporate in the hot zone, and condensation of metal particles in the form of small balls occurs in the cold zone. To carry out the transfer of metal vapors, a flow of high-purity argon (with a purity of 99.999%) is used with a gas flow rate of about 20 l/h. The oven is equipped with special seals that prevent exposure to atmospheric gases.
Preliminary desorption of water and air oxygen from the surface of the MWCNT and the graphite ampoule, in which the sample is placed in a graphite furnace, as well as blowing them with high-purity argon, makes it possible to avoid the impact on the purified MWCNT of gas transport reactions involving hydrogen and oxygen-containing gases, leading to the redistribution of carbon between its highly dispersed forms and well-crystallized graphite-like forms with low surface energy (V.L.Kuznetsov, Yu.V.Butenko, V.I.Zaikovskii and A.L.Chuvilin // Carbon redistribution processes in nanocarbons // Carbon 42 (2004) pp.1057-1061; A.S. Fialkov // Processes and devices for the production of powdered carbon-graphite materials, Aspect Press, Moscow, 2008, pp. 510-514).
Catalytic multiwalled carbon nanotubes are produced by pyrolysis of hydrocarbons using catalysts containing Fe, Co, Ni, Mo and their combinations as active components, as well as Al 2 O 3, MgO, CaCO 3 as carriers (T.W. Ebbesen // Carbon nanotubes: Preparation and properties, CRC Press, 1997, p.139-161; V.Shanov, Yeo-Heung Yun, M.J.Schuiz // Synthesis and characterization of carbon nanotube materials (review) // Journal of the University of Chemical Technology and Metallurgy, 2006, no. 4, p.377-390; J.W.Seo; M.Milas; K.Lee, V. Lukovac, L.Forro // Catalytically grown carbon nanotubes: from synthesis to toxicity / / Journal of Physics D (Applied Physics), 2007, v.40, n.6).
In the proposed method, to demonstrate the possibility of removing impurities of the most typical metals, purification is carried out for two types of MWCNTs synthesized on Fe-Co/Al 2 O 3 and Fe-Co/CaCO 3 catalysts containing Fe and Co in a 2:1 ratio. One of the most important features of the use of these catalysts is the absence of other carbon phases other than MWCNTs in the synthesized samples. In the presence of the Fe-Co/Al 2 O 3 catalyst, MWCNTs with average outer diameters of 7-10 nm are obtained, and in the presence of the Fe-Co/CaCO 3 catalyst, MWCNTs are obtained with large average outer diameters of 22-25 nm.
The obtained samples are examined by transmission electron microscopy, X-ray spectral fluorescence method on an ARL - Advant "x analyzer with a Rh anode of an X-ray tube (measurement accuracy ± 10%), and the specific surface of the samples is measured by the BET method.
According to TEM data, the initial samples consist of highly defective MWCNTs (Fig. 1, 6). Fragments of tubes in the area of bends have smooth, rounded contours; A large number of fullerene-like formations are observed on the surface of the tubes. Graphene-like layers of nanotubes are characterized by the presence of a large number of defects (breaks, Y-like connections, etc.). In some sections of the tubes, there is a discrepancy in the number of layers on different sides of the MWCNTs. The latter indicates the presence of open extended graphene layers, mainly localized inside the tubes. Electron microscopic images of heated MWCNTs in a flow of high-purity argon at temperatures of 2200°C - Fig. 2, 7; 2600°C - Fig.3, 8; 2800°C - Figures 4, 5, 9. In the samples after calcination, smoother MWCNTs with fewer internal and near-surface defects are observed. The tubes consist of straight fragments on the order of hundreds of nanometers with clearly defined kinks. As the calcination temperature increases, the dimensions of the straight sections increase. The number of graphene layers in the walls of the tubes on different sides becomes the same, which makes the MWCNT structure more ordered. The inner surface of the tubes also undergoes significant changes - metal particles are removed, the internal partitions become more ordered. Moreover, the ends of the tubes close - the graphene layers forming the tubes are closed.
Calcination of samples at 2800°C leads to the formation of a small number of enlarged cylindrical carbon formations, consisting of graphene layers embedded in each other, which may be associated with the transfer of carbon over short distances due to an increase in graphite vapor pressure.
Studies of samples of initial and heated MWCNTs using the X-ray fluorescence method showed that after heating samples of multiwalled carbon nanotubes at temperatures in the range of 2200-2800°C, the amount of impurities decreases, which is also confirmed by transmission electron microscopy. Heating MWCNT samples at 2800°C ensures almost complete removal of impurities from the samples. In this case, not only impurities of catalyst metals are removed, but also impurities of other elements that enter the MWCNTs at the stages of acid treatment and washing. In the initial samples, the ratio of iron to cobalt is approximately 2:1, which corresponds to the initial composition of the catalysts. The aluminum content in the initial tubes obtained using samples of the Fe-Co/Al 2 O 3 catalyst is small, which is associated with its removal when treating the nanotubes with acid when washing the catalyst. The results of studying the content of impurities using the X-ray spectral fluorescence method are given in tables 1 and 2.
Measurement of the specific surface area by the BET method showed that with increasing temperature, the specific surface area of MWCNT samples changes insignificantly while maintaining the structure and morphology of carbon nanotubes. According to TEM data, the decrease in specific surface area can be associated both with the closing of the ends of the MWCNTs and with a decrease in the number of surface defects. With increasing temperature, it is possible to form a small proportion of enlarged cylindrical formations with an increased number of layers and a length to width ratio of approximately 2-3, which also contributes to a decrease in the specific surface area. The results of the study of the specific surface area are given in Table 3.
The essence of the invention is illustrated by the following examples, tables (tables 1-3) and illustrations (Figures 1-9).
A sample of MWCNTs (10 g), obtained by pyrolysis of ethylene in the presence of a Fe-Co/Al 2 O 3 catalyst in a flow quartz reactor at a temperature of 650-750°C, is placed in a graphite ampoule with a height of 200 mm and an outer diameter of 45 mm and closed with a lid ( 10 mm in diameter) with a hole (1-2 mm in diameter). The graphite ampoule is placed in a quartz ampoule and the air is pumped out using a vacuum pump to a pressure of at least 10 -3 Torr, followed by purging with high-purity argon (99.999% purity), first at room temperature and then at a temperature of 200-230°C to remove oxygen-containing surface groups and traces of water. The sample is heated at a temperature of 2200°C for 1 hour in a flow of high-purity argon (~20 l/h) in a furnace with a temperature gradient, where in the working area the temperature is maintained at 2200°C, and at the edges of the furnace the temperature is 900-1000°C WITH. Metal atoms evaporating from MWCNTs during heating are removed from the hot part of the furnace to the cold part by a flow of argon, where the metal is deposited in the form of small balls.
After calcination, the resulting material is examined by transmission electron microscopy and X-ray fluorescence method. Figure 1 shows electron microscopic images of the original MWCNTs, and Figure 2 shows MWCNTs heated at 2200°C. Using the BET method, the specific surface area of MWCNT samples is determined before and after calcination. The data obtained indicate a slight decrease in the specific surface area of the samples after calcination when compared with the specific surface area of the original MWCNT sample.
Similar to example 1, characterized in that a sample of the original MWCNTs is heated at 2600°C for 1 hour in a flow of high-purity argon (~20 l/h) in a furnace with a temperature gradient, where the temperature in the working area is maintained at 2600°C, at The temperature at the edges of the oven is 900-1000°C. Images of heated MWCNTs obtained by transmission electron microscopy are shown in Figure 3. High-resolution TEM images show the closed ends of the nanotubes.
Similar to example 1, characterized in that a sample of the original MWCNTs is heated at 2800°C for 15 minutes in a flow of high-purity argon (~20 l/h) in a furnace with a temperature gradient, where the temperature in the working area is maintained at 2800°C, at The temperature at the edges of the oven is 900-1000°C. Images of heated MWCNTs obtained by transmission electron microscopy are shown in Figure 4.
Calcination at 2800°C leads to the formation of a small number of enlarged cylindrical formations with an increased number of layers and a length to width ratio of approximately 2-3. These enlargements are visible in TEM images (Figure 5).
Similar to example 1, characterized in that the original MWCNTs were obtained in the presence of a Fe-Co/CaCO 3 catalyst. Images of the original MWCNTs and MWCNTs heated at 2200°C, obtained by transmission electron microscopy, are shown in Figures 6, 7, respectively. TEM images of the original MWCNTs show metal particles encapsulated in the tube channels (marked with arrows).
Similar to example 4, characterized in that a sample of the original MWCNT was heated at 2600°C. Transmission electron microscopy images of heated MWCNTs are shown in Figure 8. High-resolution TEM images show the closed ends of the nanotubes.
Similar to example 4, characterized in that a sample of the original MWCNT was heated at 2800°C for 15 minutes. Transmission electron microscopy images of heated MWCNTs are shown in Figure 9. The images show the formation of a small fraction of enlargements.
| Table 1 | ||||
| X-ray spectral fluorescence data on the content of impurities in MWCNTs after heating, obtained using the Fe-Co/Al 2 O 3 catalyst | ||||
| Element | ||||
| Initial MWCNTs | MWCNT_2200°C example 1 | MWCNT_2600°C example 2 | MWCNT_2800°C example 3 | |
| Fe | 0.136 | 0.008 | footprints | footprints |
| Co | 0.0627 | footprints | footprints | footprints |
| Al | 0.0050 | footprints | footprints | footprints |
| Sa | footprints | 0.0028 | 0.0014 | footprints |
| Ni | 0.0004 | footprints | footprints | footprints |
| Si | 0.0083 | 0.0076 | footprints | No |
| Ti | No | 0.0033 | footprints | footprints |
| S | footprints | No | No | No |
| Cl | 0.111 | No | No | No |
| Sn | 0.001 | 0.001 | footprints | footprints |
| Ba | No | No | No | No |
| Cu | 0.001 | 0.001 | footprints | footprints |
| traces - element content below 1 ppm |
| table 2 | ||||
| X-ray spectral fluorescence data on the content of impurities in MWCNTs after heating, obtained using the Fe-Co/CaCO 3 catalyst | ||||
| Element | Estimation of impurity content, wt.% | |||
| Initial MWCNTs | MWCNT_2200°C example 4 | MWCNT_2600°C example 5 | MWCNT_2800°C example 6 | |
| Fe | 0.212 | 0.0011 | 0.0014 | 0.001 |
| Co | 0.0936 | footprints | footprints | footprints |
| Al | 0.0048 | footprints | footprints | footprints |
| Sa | 0.0035 | 0.005 | 0.0036 | footprints |
| Ni | 0.0003 | footprints | footprints | footprints |
| Si | 0.0080 | 0.0169 | 0.0098 | footprints |
| Ti | No | footprints | 0.0021 | 0.0005 |
| S | 0.002 | No | No | No |
| Cl | 0.078 | No | No | No |
| Sn | 0.0005 | footprints | footprints | footprints |
| Ba | 0.008 | No | No | No |
| Cu | footprints | footprints | footprints | footprints |
| Table 3 | |
| Specific surface area BET of initial and heated MWCNT samples | |
| MWCNT sample (catalyst) | Ssp., m 2 /g (±2.5%) |
| MWCNT_ref (Fe-Co/Al 2 O 3) | 390 |
| MWCNT_2200 (Fe-Co/Al 2 O 3) example 1 | 328 |
| MWCNT_2600 (Fe-Co/Al 2 O 3) example 2 | 302 |
| MWCNT_2800 (Fe-Co/Al 2 O 3) example 3 | 304 |
| MWCNT_ref (Fe-Co/CaCO 3) | 140 |
| MWCNT_2200 (Fe-Co/CaCO 3) example 4 | 134 |
| MWCNT_2600 (Fe-Co/CaCO 3) example5 | 140 |
| MWCNT_2800 (Fe-Co/CaCO 3) example 6 | 134 |
Captions for the figures:
Fig.1. Electron microscopic images of the initial MWCNT sample synthesized on the Fe-Co/Al 2 O 3 catalyst. On the left is a low-resolution TEM image. On the right, below is a high-resolution TEM image, which shows the defective walls of MWCNTs.
Fig.2. Electron microscopic images of a MWCNT sample heated at a temperature of 2200°C, synthesized on a Fe-Co/Al 2 O 3 catalyst. On the left is a low-resolution TEM image. Right, bottom - high resolution TEM image. The MWCNT structure becomes less defective, and the ends of the nanotubes close.
Fig.3. Electron microscopic images of a MWCNT sample heated at a temperature of 2600°C, synthesized on a Fe-Co/Al 2 O 3 catalyst. On the left is a low-resolution TEM image. On the right, below is a high-resolution TEM image showing the closed ends of the MWCNTs. The walls of MWCNTs become smoother and less defective.
Fig.4. Electron microscopic images of a MWCNT sample heated at a temperature of 2800°C, synthesized on a Fe-Co/Al 2 O 3 catalyst. On the left is a low-resolution TEM image. On the right, below is a high-resolution TEM image, which shows less defective MWCNT walls.
Fig.5. Electron microscopic images of a MWCNT sample heated at a temperature of 2800°C, synthesized on the Fe-Co/Al 2 O 3 catalyst, displaying the appearance of defects in the MWCNT structure, which are cylindrical formations consisting of graphene layers nested inside each other, which are shown on the right top high resolution TEM image.
Fig.6. Electron microscopic images of the initial MWCNT sample synthesized on the Fe-Co/CaCO 3 catalyst. On the left is a low-resolution TEM image. On the right, below is a high-resolution TEM image, which shows the uneven surface of the MWCNT. On the right, at the top, catalyst particles are visible encapsulated inside the carbon nanotube channels (marked with arrows).
Fig.7. Electron microscopic images of a MWCNT sample heated at a temperature of 2200°C, synthesized on a Fe-Co/CaCO 3 catalyst. On the left is a low-resolution TEM image. On the right, below is a high-resolution TEM image, which shows smoother walls of the MWCNTs.
Fig.8. Electron microscopic images of a MWCNT sample heated at a temperature of 2600°C, synthesized on a Fe-Co/CaCO 3 catalyst. On the left is a low-resolution TEM image. Bottom right is a high-resolution TEM image showing the closed ends of the MWCNTs. The walls of MWCNTs become smoother and less defective.
Fig.9. Electron microscopic images of a MWCNT sample heated at a temperature of 2800°C, synthesized on a Fe-Co/CaCO 3 catalyst. On the left is a low-resolution TEM image. Right, bottom - high resolution TEM image.
1. A method for purifying multiwalled carbon nanotubes obtained by pyrolysis of hydrocarbons using catalysts containing Fe, Co, Ni, Mo, Mn and their combinations as active components, as well as Al 2 O 3 , MgO, CaCO 3 as carriers, by boiling in a solution of hydrochloric acid with further washing with water, characterized in that after the acid treatment, heating is carried out in a stream of high-purity argon in a furnace with a temperature gradient, where in the working area the temperature is 2200-2800°C, at the edges of the furnace the temperature is 900-1000°C , resulting in multi-walled nanotubes with a metal impurity content of less than 1 ppm.
2. The method according to claim 1, characterized in that heating is carried out in ampoules made of high-purity graphite.
None of the common methods for obtaining CNTs makes it possible to isolate them in their pure form. Impurities in NT can be fullerenes, amorphous carbon, graphitized particles, and catalyst particles.
Three groups of CNT purification methods are used:
1) destructive,
2) non-destructive,
3) combined.
Destructive methods use chemical reactions that can be oxidative or reductive and are based on differences in the reactivity of different carbon forms. For oxidation, either solutions of oxidizing agents or gaseous reagents are used, and for reduction, hydrogen is used. The methods allow the isolation of high purity CNTs, but are associated with tube losses.
Non-destructive methods include extraction, flocculation and selective precipitation, cross-flow microfiltration, size exclusion chromatography, electrophoresis, and selective reaction with organic polymers. As a rule, these methods are low-productivity and ineffective.
At the same time, it has been shown that purification of SWCNTs obtained by the laser-thermal method by filtration with sonication makes it possible to obtain a material with a purity of more than 90% with a yield of 30–70% (depending on the purity of the initial soot).
Extraction is used exclusively to remove fullerenes, in large quantities they are extracted with carbon disulfide or other organic solvents.
The bulk of the catalyst and catalyst carrier are removed by washing in sulfuric and nitric acids, as well as their mixtures. If the catalyst carrier is silica gel, quartz or zeolites, hydrofluoric acid or alkali solutions are used. To remove aluminum oxide, concentrated solutions of alkalis are used. Catalyst metals occluded in the CNT cavity or surrounded by a graphite shell are not removed.
Amorphous carbon is removed either by oxidation or reduction. For reduction, hydrogen is used at a temperature of at least 700 o C; for oxidation, air, oxygen, ozone, carbon dioxide or aqueous solutions of oxidizing agents are used. Oxidation in air begins to occur at 450 o C. In this case, part of the CNT (mainly the smallest diameter) is completely oxidized, which contributes to the opening of the remaining tubes and the removal of catalyst particles that were not removed during the primary acid treatment. The latter are removed by secondary washing in acid. To obtain the purest product, acid and gas purification operations can be repeated several times, combined with each other and with physical methods.
In some cases, primary acid purification is carried out in two stages, using first dilute acid (to remove the bulk of the catalyst and support) and then concentrated acid (to remove amorphous carbon and clean the CNT surface) with intermediate filtration and washing operations.
Since metal oxide particles catalyze the oxidation of CNTs and cause a decrease in the yield of the purified product, an additional passivation operation is used by converting them into fluorides using SF 6 or other reagents. In this case, the yield of purified CNTs increases.
Several methods have been developed at Rice University (USA) for the purification of materials produced by arc and laser-thermal methods. The “old” method included oxidation operations with 5 M HNO 3 (24 h, 96 o C), neutralization with NaOH, dispersion in a 1% aqueous solution of Triton X-100, and cross-flow filtration. Its disadvantages include coprecipitation of Ni and Co hydroxides together with CNTs, difficulties in removing graphitized particles and organic Na salts, foaming during vacuum drying, low filtration efficiency, long process times, and low yield of cleaned tubes.
The “new” method involved oxidation with 5 M HNO 3 for 6 hours, centrifugation, washing and neutralization of the precipitate with NaOH, re-oxidation of HNO 3 with repeated centrifugation and neutralization, washing with methanol, dispersion in toluene and filtration. This method also does not allow achieving complete purification, although the yield of CNTs (50–90%) is superior to the “old” method.
The use of organic solvents directly after boiling in acid makes it possible to remove 18–20% of impurities, half of which are fullerenes, and the other half are soluble hydrocarbons.
SWCNTs obtained by the arc method (5% catalyst consisting of Ni, Co and FeS with a ratio of 1:1:1) were first oxidized in air at 470 o C for 50 minutes in a rotating laboratory oven, then metal impurities were removed by repeated washing with 6 M HCl , achieving complete discoloration of the solution. The yield of SWCNTs containing less than 1 wt.% nonvolatile residue was 25–30%.
A process has been developed for cleaning arc SWCNTs, which includes, in addition to oxidation in air and boiling in HNO 3 , treatment with an HCl solution and neutralization, ultrasonic dispersion in dimethylformamide or N-methyl-2-pyrrolidone, followed by centrifugation, evaporation of the solvent and vacuum annealing at 1100 o C.
The purification of pyrolytic SWCNTs and MWCNTs is described in two stages: by long-term (12 hours) sonication at 60 o C in an H 2 O 2 solution to remove carbon impurities in the first stage and sonication for 6 hours in HCl to remove Ni impurities in the second. After each stage, centrifugation and filtration were performed.
To purify SWCNTs obtained by the HiPco method and containing up to 30 wt.% Fe, a two-stage method is also described, including oxidation in air (in particular, in a microwave oven) and subsequent washing with concentrated HCl.
An even greater number of stages (dispersion in hot water during sonication, interaction with bromine water at 90 o C for 3 hours, oxidation in air at 520 o C for 45 minutes, treatment with 5 M HCl at room temperature) were used to purify MWCNTs, obtained by pyrolysis of a solution of ferrocene in benzene and containing up to 32 wt.% Fe. After washing and drying at 150°C for 12 hours, the Fe content decreased to several percent, and the yield was up to 50%.
Oxidation by gases can lead to the development of porosity of NT and NV, and prolonged boiling in nitric acid can lead to the complete degradation of these substances.
With a relatively large amount of silicon (laser-thermal method), the primary product is heated in concentrated hydrofluoric acid, then HNO 3 is added and treated at 35–40 o C for another 45 minutes. Operations involve the use of highly corrosive media and the release of toxic gases.
To remove the zeolite used in the production of SWCNTs by catalytic pyrolysis of ethanol vapor, the product oxidized in air is treated with an aqueous solution of NaOH (6 N) with short-term (5 min) sonication, and the residue collected on the filter is washed with HCl (6 N).
Separation of SWCNTs from impurities of other forms of carbon and metal particles can be carried out by ultrasonic dispersion of the tubes in a solution of polymethyl methacrylate in monochlorobenzene, followed by filtration.
To purify SWCNTs, it is often recommended to use their functionalization. In particular, a method is described that includes three sequential operations: functionalization using azomethine ylide in dimethylformamide (see Section 4.5), slow deposition of functionalized SWCNTs by adding diethyl ether to a solution of tubes in chloroform, removal of functional groups and regeneration of SWCNTs by heating at 350 o C and annealing at 900 o C. At the first stage, metal particles are removed, at the second - amorphous carbon. The Fe content of HiPco tubes cleaned by this method is reduced to 0.4 wt.%.
Interaction with DNA can be used to separate metallic SWCNTs from semiconducting ones. Laboratories have a wide range of different single-stranded DNA, by selecting which it is possible to achieve selective envelopment and subsequent separation of the initial mixture into fractions by chromatographic method.
Physical methods include transferring the initial mixture into an aqueous solution using long-term ultrasonic treatment in the presence of surfactants or enveloping soluble polymers, microfiltration, centrifugation, high-performance liquid chromatography, gel permeation chromatography. Zwitterion grafting was used to obtain dispersions suitable for chromatography (see Section 4.5).
It is expected that the development of chromatographic methods will make it possible to separate CNTs not only by length and diameter, but also by chirality, and to separate tubes with metallic properties from tubes with a semiconductor type of conductivity. To separate SWCNTs with different electronic properties, selective deposition of metal tubes in a solution of octadecylamine in tetrahydrofuran was tested (the amine is more strongly adsorbed on semiconductor tubes and leaves them in solution).
An example of the use of non-destructive methods for purifying and separating CNTs by size is also a method developed by scientists from Switzerland and the USA. The starting material obtained by the arc method was transferred into an aqueous colloidal solution using sodium dodecyl sulfate (the surfactant concentration was slightly higher than the critical micelle concentration). As the surfactant concentration increased, CNT aggregates were obtained, which were filtered with intense sonication through track membranes with pores of 0.4 μm. After redispersing in water, the operation was repeated several times to achieve the desired degree of purification of the CNTs.
The capillary electrophoresis method is low-productive, although it allows not only to purify CNTs, but also to separate them by length or diameter. When separating, dispersions stabilized by surfactants or soluble polymers are used. For the purification and separation of CNTs by dielectrophoresis, see Section. 4.13.
A non-destructive method has been developed for separating purified and shortened CNTs into fractions with tubes of different sizes in cross (asymmetric) liquid flows.
To enlarge the catalyst metal particles, annealing is carried out in hydrogen at 1200 o C, after which the metals are dissolved in acid. Complete removal of catalyst metals and catalyst carriers, regardless of the form in which they are present in the mixture, can be carried out by high-temperature (1500–1800 o C) vacuum annealing. In this case, fullerenes are also removed, the CNTs increase in diameter and become less defective. To completely anneal defects, temperatures above 2500 o C are required. Vacuum annealing at 2000 o C is used to increase the resistance of MWCNTs to acid treatment.
To remove impurities from carbon fibers formed during the pyrolysis of hydrocarbons, freezing with liquid nitrogen is recommended.
The choice of one or another purification option depends on the composition of the mixture being purified, the structure and morphology of the NT, the amount of impurities and the requirements for the final product. Pyrolytic CNTs and especially CNFs contain less or no amorphous carbon.
When assessing the purity of CNTs, the greatest difficulty is determining the content of amorphous carbon impurities. Raman spectroscopy (see Chapter 8) gives only a qualitative picture. A more reliable, but at the same time labor-intensive method is spectroscopy in the near-IR region (Itkis, 2003).
In the USA, a standard for the purity of SWCNTs has been created.
reaction in sulfuric acid containing chromic anhydride. However, preliminary removal of the large fraction of nanodiamond granules is necessary. References 1. Spitsyn B.V., Davidson J.L., Gradoboev M.N., Galushko T.B., Serebryakova N.V., Karpukhina T.A., Kulakova I.I., Melnik N.N. Inroad to modification of detonation nanodiamond // Diamond and Related Materials, 2006, Vol. 15, p. 296-299 2. Pat. 5-10695, Japan (A), Chromium plating solution, Tokyo Daiyamondo Kogu Seisakusho K.K., 04/27/1993 3. Dolmatov, V.Yu. Ultrafine diamonds of detonation synthesis as the basis of a new class of composite metal-diamond galvanic coatings / V.Yu. Dolmatov, G.K. Burkat // Superhard materials, 2000, T. 1.- P. 84-94 4. Gregory R. Flocculation and sedimentation - the basic principles // Spec. Chem., 1991, Vol. 11, no. 6, p. 426-430 UDC 661.66 N.Yu. Biryukova1, A.N. Kovalenko1, S.Yu. Tsareva1, L.D. Iskhakova2, E.V. Zharikov1 Russian Chemical-Technological University named after. DI. Mendeleev, Moscow, Russia Scientific Center for Fiber Optics RAS, Moscow, Russia 1 2 PURIFICATION OF CARBON NANOTUBES OBTAINED BY THE METHOD OF CATALYTIC PYROLYSIS OF BENZENE In this work the results of experimental studies of purification and separation of multi-walled nanotubes by physical and chemical methods are presented. The efficiency of each stage has been controlled by studying of morphological characteristics of pyrolysis products. The paper presents the results of experimental studies of the purification and separation of multiwalled carbon nanotubes using physical and chemical methods. The effectiveness of each purification stage was monitored by changes in the morphological characteristics of pyrolysis products. The method of catalytic pyrolysis of hydrocarbons is one of the promising methods for the synthesis of carbon nanotubes. The method makes it possible to obtain single-walled, multi-walled nanotubes, oriented arrays of carbon nanostructures with appropriate organization of synthesis parameters. At the same time, the product obtained by pyrolysis of carbon-containing compounds, along with nanotubes, contains a significant amount of impurities, such as catalyst particles, amorphous carbon, fullerenes, etc. To remove these impurities, physical methods are usually used (centrifugation, ultrasonication, filtration) in combination with chemical (oxidation in gas or liquid media at elevated temperatures). The work tested a combined technique for purifying and separating multiwalled nanotubes from by-products, and determined the effectiveness of various reagents. The initial deposit was obtained by catalytic pyrolysis of benzene using iron pentacarbonyl as a precatalyst. The deposit was treated with hydrochloric, sulfuric and nitric acids. Aggregates of nanotubes were broken up with ultrasound at a frequency of 22 kHz. To separate the deposit into fractions, centrifugation was used (3000 rpm, processing time - up to 1 hour). In addition to acid, heat treatment of nanotubes using U S P E X I was also used in chemistry and chemical technology. Volume XXI. 2007. No. 8 (76) 56 air. To achieve the best purification, the optimal sequence of different methods was established. The morphological characteristics of the pyrolysis products and the degree of purification were monitored by scanning electron microscopy, Raman spectroscopy, and X-ray phase analysis. UDC 541.1 E.N. Golubina, N.F. Kizim, V.V. Moskalenko Novomoskovsk Institute of the Russian Chemical-Technological University named after. DI. Mendeleev, Novomoskovsk, Russia INFLUENCE OF NANOSTRUCTURES ON EXTRACTION FEATURES IN THE SYSTEM WATER – ErCl3 – D2EHPA – HEPTANE KINETICS The kinetic feature of extracted Er(III) the solution of D2EHPA in heptane (the concentrated area on kinetic curve, the high rate of its accumulation at dynamic interfacial layers in the beginning of the process, the extremal disposition in reviewed depending on the thickness of dynamic interfacial layers from concentration ratio element and solvent) are indicated at significant part of nanostructures in the process of extraction. The kinetic features of the extraction of erbium (III) by solutions of D2EHPA in heptane (concentration plateaus on the kinetic curves, the high rate of its accumulation in DMS at the beginning of the process, the extreme nature of the dependence of the observed thickness of DMS on the ratio of the concentrations of the element and the extractant) indicate the significant role of nanostructures in the extraction process. It is known that various nanoobjects can appear in extraction systems: adsorption layers, micelles, micellar gels, vesicles, polymer gels, crystalline gels, microemulsion, nanodispersion, emulsion. In particular, in the La(OH)3-D2EHPA-decane-water system an organogel is formed, the spatial structure of which is built from rod-shaped particles with a diameter of ≈0.2 and a length of 2-3 μm. The sodium salt of D2EHPA in the absence of water forms reverse cylindrical micelles with a radius of 53 nm. In the cross section of the micelle there are three molecules of NaD2EHP, oriented with polar groups towards the center and hydrocarbon chains towards the organic solvent. The state of such a lattice depends on the nature of the element. In the case of Co(D2EHP)2, macromolecular structures are formed with an aggregation number greater than 225. In the case of Ni(D2EHP)2 (possibly Ni(D2EHP)2⋅2H2O), aggregates with an aggregation number ≈5.2 appear. Under certain conditions, the formation of polymer molecular structures with a hydrodynamic radius of ≈15 nm is possible. When lanthanum is extracted with D2EHPA solutions, bulky and structurally rigid lanthanum alkyl phosphate is formed, which causes a decrease in the elasticity of the lanthanum alkyl phosphate monolayer at the phase interface. The formation of nanostructures affects both the equilibrium properties of the system and the kinetics of the process. Extraction of rare earth elements is complicated by the occurrence of numerous interfacial processes, such as the emergence and development of spontaneous surface convection (SSC), the formation of a structural-mechanical barrier, phase dispersion, etc. As a result of the chemical reaction between D2EHPA and the element, a sparingly soluble salt is formed, which causes the formation of nanostructures according to the “from smaller to larger” mechanism. The purpose of this work was to establish the influence of nanostructures on the kinetic features of erbium(III) extraction with solutions of D2EHPA in heptane. U S P E X I in chemistry and chemical technology. Volume XXI. 2007. No. 8 (76) 57
Purification of carbon nanotubes
None of the common methods for obtaining CNTs makes it possible to isolate them in their pure form. Impurities in NT can be fullerenes, amorphous carbon, graphitized particles, and catalyst particles.
Three groups of CNT purification methods are used:
destructive,
non-destructive,
combined.
Destructive methods use chemical reactions that can be oxidative or reductive and are based on differences in the reactivity of different carbon forms. For oxidation, either solutions of oxidizing agents or gaseous reagents are used, and hydrogen is used for reduction. The methods allow the isolation of high purity CNTs, but are associated with tube losses.
Non-destructive methods include extraction, flocculation and selective precipitation, cross-flow microfiltration, size exclusion chromatography, electrophoresis, and selective interaction with organic polymers. As a rule, these methods are low-productivity and ineffective.
Properties of carbon nanotubes
Mechanical. Nanotubes, as has been said, are an extremely strong material, both in tension and bending. Moreover, under the influence of mechanical stresses exceeding critical ones, nanotubes do not “break”, but are rearranged. Based on the high strength properties of nanotubes, it can be argued that they are the best material for a space elevator cable at the moment. As the results of experiments and numerical simulations show, the Young's modulus of a single-walled nanotube reaches values of the order of 1-5 TPa, which is an order of magnitude greater than that of steel. The graph below shows a comparison between a single-walled nanotube and high-strength steel.
1 - According to calculations, the space elevator cable must withstand a mechanical stress of 62.5 GPa
2 - Tensile diagram (mechanical stress y versus relative elongation e)
To demonstrate the significant difference between the current strongest materials and carbon nanotubes, let's conduct the following thought experiment. Let’s imagine that, as previously assumed, the cable for the space elevator will be a certain wedge-shaped homogeneous structure consisting of the strongest materials available today, then the diameter of the cable at GEO (geostationary Earth orbit) will be about 2 km and will narrow to 1 mm at the surface Earth. In this case, the total mass will be 60 * 1010 tons. If carbon nanotubes were used as the material, then the diameter of the GEO cable would be 0.26 mm and 0.15 mm at the surface of the Earth, and therefore the total mass would be 9.2 tons. As can be seen from the above facts, carbon nanofiber is exactly the material that is needed in the construction of a cable, the actual diameter of which will be about 0.75 m, in order to also withstand the electromagnetic system used to move the space elevator cabin.
Electrical. Due to the small size of carbon nanotubes, it was only in 1996 that it was possible to directly measure their electrical resistivity using the four-prong method.
Gold stripes were applied to the polished surface of silicon oxide in a vacuum. Nanotubes 2–3 μm long were deposited into the gap between them. Then, 4 tungsten conductors with a thickness of 80 nm were applied to one of the nanotubes selected for measurement. Each of the tungsten conductors had contact with one of the gold strips. The distance between the contacts on the nanotube ranged from 0.3 to 1 μm. The results of direct measurements showed that the resistivity of nanotubes can vary within significant limits - from 5.1 * 10 -6 to 0.8 Ohm/cm. The minimum resistivity is an order of magnitude lower than that of graphite. Most of the nanotubes have metallic conductivity, and a smaller part exhibits the properties of a semiconductor with a band gap from 0.1 to 0.3 eV.
French and Russian researchers (from IPTM RAS, Chernogolovka) discovered another property of nanotubes, such as superconductivity. They measured the current-voltage characteristics of an individual single-walled nanotube with a diameter of ~1 nm, a large number of single-walled nanotubes rolled into a bundle, as well as individual multiwalled nanotubes. Superconducting current at temperatures close to 4K has been observed between two superconducting metal contacts. The features of charge transfer in a nanotube differ significantly from those inherent in ordinary, three-dimensional conductors and, apparently, are explained by the one-dimensional nature of the transfer.
Also, de Geer from the University of Lausanne (Switzerland) discovered an interesting property: a sharp (about two orders of magnitude) change in conductivity with a small, 5-10o, bend of a single-walled nanotube. This property can expand the range of applications of nanotubes. On the one hand, the nanotube turns out to be a ready-made, highly sensitive converter of mechanical vibrations into an electrical signal and back (in fact, it is a telephone handset several microns long and about a nanometer in diameter), and, on the other hand, it is an almost ready-made sensor of the smallest deformations. Such a sensor could find application in devices that monitor the condition of mechanical components and parts on which the safety of people depends, for example, passengers of trains and airplanes, personnel of nuclear and thermal power plants, etc.
Capillary. Experiments have shown that an open nanotube has capillary properties. To open the nanotube, you need to remove the top part - the cap. One method of removal is to anneal the nanotubes at a temperature of 850 0 C for several hours in a flow of carbon dioxide. As a result of oxidation, about 10% of all nanotubes become open. Another way to destroy the closed ends of nanotubes is to soak them in concentrated nitric acid for 4.5 hours at a temperature of 2400 C. As a result of this treatment, 80% of the nanotubes become open.
The first studies of capillary phenomena showed that liquid penetrates into the nanotube channel if its surface tension is not higher than 200 mN/m. Therefore, to introduce any substances into nanotubes, solvents with low surface tension are used. For example, to introduce nanotubes of some metals into the channel, concentrated nitric acid is used, the surface tension of which is low (43 mN/m). Then annealing is carried out at 4000 C for 4 hours in a hydrogen atmosphere, which leads to the reduction of the metal. In this way, nanotubes containing nickel, cobalt and iron were obtained.
Along with metals, carbon nanotubes can be filled with gaseous substances, such as molecular hydrogen. This ability is of practical importance because it opens up the possibility of safe storage of hydrogen, which can be used as an environmentally friendly fuel in internal combustion engines. Scientists were also able to place inside a nanotube a whole chain of fullerenes with gadolinium atoms already embedded in them (see Fig. 5).
Rice. 5. Inside C60 inside a single-walled nanotube
