NEW REACTIVE SURFACE COATINGS FOR AL PARTICLES REACTING WITH WATER

Transcription

1 NEW REACTIVE SURFACE COATINGS FOR AL PARTICLES REACTING WITH WATER Mikhail Laritchev, Ilia Leipunsky, Olga Laricheva*, Pavel Pshechenkov, Alexey Jigatch, Mikhail Kuskov, Valentin Sedoi** Institute for Energy Problems of Chemical Physics RAS, , Moscow, Russia *Moscow Research Institute of Medical Ecology, , Moscow, Russia **Institute of High Current Electronics RAS, , Tomsk, Russia I. Introduction. The usage of ultra fine (UF) and nanosized (NS) aluminum powders as a source of thermal or chemical energy is of a special interest to some energetic systems [1, 2, 3]. The distinguishing characteristic of these powders is a high specific surface area, which determinates extensively the behavior of Al powders in considered systems. Several types of the surface of Al particles are usually using to provide different operations with Al powders: 1. Pure metal Al surface for a storage and operation in inert mediums or for a fusion reaction of specific surface coatings. 2. Metal Al surface covered with the inert aluminum oxides coatings for a storage, operation and utilization of Al powder in the air atmosphere and in other oxidative mediums. 3. Metal Al surface covered with the coatings ensuring a safety in an atmosphere of storage and operation and reacting with a thermal or chemical energy evolution during a utilization of Al powder in given medium. Particular case, it may be the coatings permitting Al powder storage and operation at an atmospheric air conditions and which are reactive at a condition of Al powder oxidation. The type 2 is now using the most extensively at least in scientific practice. Usually a "soft" oxidation of a surface of Al particles is applying to the formation of such surface oxide coatings passivating surface of particles. These oxide coatings intercept a process of further oxidation of a metal aluminum in air atmosphere at near room temperature and provide to Al particles a sufficient unreactiveness, which is necessary for organization of process of their application. In this case the passivating coatings may consist of a mixture of oxides and hydroxides of aluminum. Moreover surface of Al particles can be subjected by a special chemical modification changing the surface properties of particles surface (hydrophobization, hydrophilization and et cetera) [4]. The concrete structure of a coating depends on conditions of a "soft" oxidation and chemical modification of Al surface. Usually thickness of passivating oxide coating for UF Al particles is equal about several nanometers [5]. The advantages of manner 2 are the simplicity and cheapness of a synthesis of passivating coatings. The disadvantage is the high weight percentage of inert Al oxides surface film in composition of nano-particles after soft oxidation. It composes about 15 wt.% of Al powder with the average particles size 100 nm [6]. If the diameter of nano-particles decreases, this percentage will grow approximately with the law ~1/D pat, where D pat is the diameter of particle. It can make unprofitable the usage of aluminum UF and especially NS powders for energetic applications. Subject of our investigation is the type 3 (the coatings permitting Al powder storage and operation at an atmospheric air conditions and which are reactive at a condition of Al powder oxidation). The nano-powders covered with reactive surface coatings. Advantages of these coatings are: 1. Capability of storage and operation with Al powders at normal atmospheric conditions. 2. Production of additional amount of energy (chemical or thermal) due to reaction of coatings at conditions of Al powder application.

2 Unfortunately, it is difficult to design universal "reactive" passivating coatings for the all possible energetic applications of UF Al powder. It should be design directly for each energetic application UF aluminum powder. The "reactive" passivating coatings can be a films (organic and inorganic) covering a surface of Al particles and not containing Al atoms in the film structure or they can include the aluminum surface atoms in a film composition. For example, the coatings on the basis of nitrides, carbides, oxycarbides of aluminum, salts of aluminum and other compounds of aluminum can be used as reactive passivating coatings for different energetic applications of UF Al powders. The objective of this paper is the investigation of the properties of "reactive" passivating coatings of UF and NS Al powders which one were synthesized on the basis of aluminum carbide (Al 4 C 3 ) by means of the carbothermy methods (reaction of the carbon or carbon oxides with the metal aluminum or aluminum oxides at high temperatures). Aluminum carbide is inert with respect to dry air at normal atmospheric conditions. This substance reacts with the water, water vapor and oxygen at high temperature. It may be used to design the passivating coatings for Al particles stored and operated at normal atmospheric conditions and utilized with liquid water and water vapor at high temperature. We assume to use this substance to design the coating for aluminum powders utilizing in composition of Al/H 2 O slurries on the basis of polyacrylamide as gel agent. The object of investigation: Al powders manufactured by method Gen-Miller (Moscow, INEPCP RAS) [7] and by exploding wire method (Tomsk, ISCE RAS) [8] covered with aluminum carbide containing coatings. II. Electronic images of Al particles with the surface passivated by aluminum carbide <D>=0.1µ m % % ,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 D, µm a) Fig.1a. SEM images of Al particles of powder Al-СО2-1 manufactured by a condensation method of a Gen - Miller [6] (Moscow) with a surface of particles passivated by aluminum carbide. b) Fig. 1b. Particle size distribution for powder Al-CO2 (<d>=0.1 mkm). The results of our experiments demonstrate that the passivating coatings designed on the basis of aluminum carbide provide effective protection of a metal core of UF aluminum particles from oxidation at their contact to atmospheric air at room temperature and natural humidity. The several

3 months storage of the aluminum powders (manufactured by both methods) contacting at time to time to the atmospheric air in laboratory room was not resulted in the noticeable decreasing of amount of the metal aluminum in the powders. Fig.1 shows SEM images of Al particles of the powder Al-СО2-1 manufactured by the condensation method Gen-Miller (Moscow). Fig. 2 shows SEM images of Al particles of the powder Al-CO2-CO2 manufactured by the exploding wire method (Tomsk). In both cases the surface of aluminum particles was passivated by formation of surface coatings containing aluminum carbide. The average size of particles of Al powders and their shape were determined using the scanning electron microscopy (LEO EVO 4020) <D>=0.16µ m % % ,0 0,1 0,2 0,3 0,4 0,5 D, µm a) Fig. 2а. SEM images of Al particles of powder Al-СО2- CO2 manufactured by the exploding wire method [7] (Tomsk) with a surface of particles passivated by aluminum carbide. b) Fig. 2b. Particle size distribution for powder Al-CO2-CO2 (<d>=0.16 mkm). The carbide of aluminum was detected in composition of surface coating by means of gas chromatography analysis of gaseous products of reaction of Al powders with hot liquid water: Al 4 C H 2 O = 4Al (OH) 3 + 3CH 4 (gas) + Q (1) The presence of methane in the composition of gaseous reaction products evidences the presence aluminum carbide in composition of Al particles. Aluminum carbide was detected by means of X- ray structural analysis only in composition of Al-CO2-CO2 sample. It means that Al 4 C 3 has the crystal structure only for this sample. II. Study of Al powders passivated with coatings containing the aluminum carbide by derivatography. 1) Linear heating of Al powders passivated with a coating containing the aluminum carbide in inert flow of argon up to 900 o C gives no way of the identification of composition of passivated coatings. The only difference observed in properties of Al powders manufactured by a condensation Gen Miller method and by the exploding wire method is the amount of gases solved in volume of powders. This amount is larger for powders manufactured by the exploding wire method. 2) Linear heating of Al powder samples in the flow of atmospheric air up to 800 o C demonstrates (Figs.3 and 4) that collapse of coatings starts at temperatures up to 370 o C. The temperature of collapse can be different for different samples and depends from the coating structure and composition.

4 Fig.3. TG, DTA, T curves for Al powder passivated with aluminum carbide coatings (sample Al-CO2-CO2) linear heating in flow of atmospheric air up to 800 o C. The collapse of coatings starts at temperature up to 370 o C. Fig.4. TG, DTA, DTG, T curves for Al powder sample passivated with aluminum carbide coatings (Al-CO2-1) linear heating in flow of atmospheric air up to 800 o C. The collapse of coatings starts at temperature up to 520 o C. Maximum of heat evolution at temperature 600 o C. Fig.5. TG, DTA, DTG, T curves for Al powder sample passivated with aluminum carbide coatings (Al-CO2-1) linear heating in flow of atmosphere of water vapor up to 900 o C. The collapse of coatings reaches at temperature up to 500 o C and accompanied by hydrogen evolution. 3) Linear heating of Al powder samples in the water vapor flow up to 900 o C shows (Fig.5) that the reaction of metal Al with water vapor takes place at temperature of a collapse of passivative coating and is accompanied by gaseous hydrogen evolution. The partial pressure of hydrogen in

5 furnace of derivatograph was measured by Hydrogen Analyzer (AH-1, AlfaBassence, Russia [8]) analyzing the composition of flow passed the furnace. III. Study of evolution of gaseous reaction products for Al/H 2 O slurry samples heated linear in atmosphere of saturated water vapor. The Al/H 2 O slurry samples were studied at a linear heating in the atmosphere of a saturated water vapor. The last allowed to prevent the change of water content in samples as the result of water evaporation. Fig. 6 shows the scheme of apparatus for investigation of the reactivity of Al powders + gel composition at a linear heating in saturated water vapor up to 100 o C The possibility of simultaneous measurement of both: temperature of a slurry sample and temperature of water heating a sample allows to realize the measurement of temperature of a start of reaction between the metal aluminum and the water. The value of residual of these temperatures enlarges remarkably after start of reaction. The possibility of measurement of amount of evolving hydrogen allows to determinate both the start temperature of reaction of Al powder with water and fractional conversion of metal aluminum as result of reaction Fig.6. The scheme of apparatus for investigation of reactivity of Al powders + gel composition at linear heating in saturated water vapor up to 100 o C. 1- external liquid thermostat, 2-distilleted water, 3-internal liquid thermostat, 4-thermocooples, 5- sample (Al powder + gel composition), 6-reactor, 7-cooler, 8- connective tube, 9- volumeter, 10-thermometer 2 1

6 Fig.6. Temperature dependencies of volume of gaseous reaction products (mainly hydrogen) evolved in reaction of Al/H 2 O slurries heated linearly in atmosphere of saturated water vapour. Curve Gel pure 3% gel without Al powder. Aluminium carbide coatings were used for passivation of surface of Al particles. Table 1. The data characterizing the reactivity of Al/H 2 O slurry samples (3% polyacrylamide). Al powders surface passivated with: aluminum carbide coatings (lines 1-7); aluminum oxide coatings (lines 8-10); aluminum oxide and trimethylsilyl coating (lines 11-12). N Al powder T oini ( o C) T ofin ( o C) Al met 1(%) Al met O(%) (1) (2) (3) (4) (5) (6) 1* Al-CO2-1 72,3 83,3 ~100% * Al-CO2-1 72,5 80,9 59,6% * Al-CO2-1 72,3 78,2 55,00% Al-CO2-3 62,2 70,3 77,50% 80 5 Al-CO2-4 59,6 69,8 76,30% 80 6 Al-CO2-5 56,9 70,5 73,70% 91 7** Al-CO2-CO2 64,9 72,8 ~100% 13.6 (20) 8** Al-(N2-CO2-N2) 72,2 82,5 83,1% 75,8 9 Al2-OX 74, ,0% Al2-OX 75, ,10% Al2-OX 75,2 86,2 54,50% Al ,1 49,10% Al ,5 42,30% 83 * Sample Al-CO2-1 was investigated immediately after slurry preparation (line 1) and 4-5 hours after slurry preparation (lines 2 and 3). ** Al-CO2-CO2 and Al-(N2-CO2-N2) - powder prepared by the exploding wire method (Tomsk). Column (3) temperature of initiation of the first stage of reactions. Column (4) temperature of finishing of the first stage of reactions. Column (5) percentage of metal Al reacting in the first stage of reactions. Column (6) percentage of metal Al in aluminum powder.

7 Fig.7 shows the temperature dependencies of a volume of gaseous reaction products (mainly hydrogen) formed in reaction of Al/H 2 O slurries heated linearly in atmosphere of the saturated water vapour with the heating rate approximately 5 degree/minute. The samples composition was % UF Al powder and 68-70% of water gel (including 3 % of polyacrylamide). All Al powders were passivated using aluminium carbide coatings. The shape of curves of temperature dependencies testifies that the reaction between Al particles and gelled water passes at least through two stages. The start temperature of the first stage of reaction (57-72,5 о С) depends from the composition of a surface passivating coating. The admixture of aluminum oxide increases this value. The second stage of reaction is responsible for hydrogen evolution at the temperature near 100 о С. Tabl.1 summarizes the data characterizing the reactivity of slurry samples including samples covering with aluminum carbide coatings (lines 1-8) and oxide coatings (lines 9-13). Sample Al- CO2-1 was investigated immediately after slurry preparation (line 1) and after 4-5 hours of preliminary exposition of slurry at room temperature (lines 2 and 3). The distinction of values Al met 1 (percentage of metal Al reacting at the first stage of reaction (column 5)) for line 1 and for lines 2 and 3 evidences that the reaction between liquid water and Al particles covered with aluminum carbide is remarkable even at room temperature. It means that additional efforts are necessary to increase the time of life of slurries at near room temperatures. For example, surface hydrophobization of aluminum carbide coating may be used. For UF powders covered with aluminum oxide coatings the first stage of reaction with liquid water starts at temperatures 74,5-76 о С (lines 8-12). IV. Study of Al/H 2 O slurry samples heated linear in open system by derivatography. The samples of Al/H 2 O slurries were studied by method of derivatography in open system. Samples were placed in crucible plugged by the stopper restricting the rate of a water evaporation and heated linearly in inert argon flow up to 700 o C. The partial pressure of hydrogen near the crucible was measured by Hydrogen Analyzer (AH-1, AlfaBassence, Russia [9]). Samples of slurries were investigated immediately after preparation (mixture) and after 1, 2 and 3 days of slurry exposure at room temperature in atmosphere of saturated water vapor. Fig.7a. TG, DTA, T curves for Al / H 2 O slurry linear heating in inert argon flow up to 700 o C. Sample Al-CO2-1.

8 P H2 (mm Hg) 0,7 0,6 0,5 0,4 0,3 0,2 0, о С 108 о С Al-CO2, Fresh Fig.7b. Time dependence of hydrogen evolution for Al / H 2 O slurry linear heating in inert argon flow up to 700 o C. Sample Al-CO2-1. 0,0-0, Time (min) Fig.8a. TG, DTA, DTG, T curves for Al / H 2 O slurry linear heating in inert argon flow up to 700 o C. Sample Al-CO2-CO2. Fig.7a shows the shapes of TG, DTA, DTG curves for fresh prepared sample containing the aluminum powder Al-CO2-1 and Fig.7b shows the curve of hydrogen evolution in the same experiment. The passivative coating undergoes the collapse at temperatures 97 and 107 o C (maxima of DTA curve). The collapse is accompanied by the hydrogen evolution, which starts at temperature 97.5 o C and has maximum at 108 o C. Fig.8a shows the shapes of TG, DTA, DTG curves for fresh prepared sample containing the aluminum powder Al-CO2-CO2 and Fig.8b shows the curve of hydrogen evolution in the same experiment. The collapse of passivative coating starts at temperatures 76 o C. Maximum of heat evolution appears at 85 o C. The collapse of passivative coating is accompanied by hydrogen evolution. The hydrogen evolution appears at temperature 80.3 o C. Hydrogen concentration reaches maximum at 89 o C. Comparison of DTA, TG curves and curve of hydrogen evolution for the same samples lets to propose the existence of several types of active surface places ( hot points ) responsible for a passivating coatings. The reaction of these hot points with the water results in the collapse of metal Al oxidation accompanied by heat and hydrogen evolution. Both processes are in good agreement with each other. Hot points may be different for the different composition of passivating coatings and for UF Al particles with the different origin. The behavior of hot points

9 during the collapse of passivating coating can give information about surface composition of UF Al particles with different origin. 0,20 89 о С P H2 (mm Hg) 0,15 0,10 0, о С Al-CO2-CO2, Fresh Fig.8b. Time dependence of hydrogen evolution for Al / H 2 O slurry linear heating in inert argon flow up to 700 o C. Sample Al-CO2-CO2. 0, Time (min) 0,5 85 o C P H2 (mm Hg) 0,4 0,3 0,2 0,1 58 o C 68 o C Al-CO2-5, fresh 90 o C Fig.9a. Time dependence of hydrogen evolution for fresh prepared Al / H 2 O slurry sample linear heating in inert argon flow up to 700 o C. Sample Al-CO2-5. 0, Time (min) 0,5 77 o C P H2 (mm Hg) 0,4 0,3 0,2 0,1 33 o C 58 o C 70.7 o C Al-CO2-5, old 82 o C Fig.9b. Time dependence of hydrogen evolution for Al / H 2 O slurry sample ageing 24 hours at room temperature before linear heating in inert argon flow up to 700 o C. Sample Al-CO2-5. 0, Time (min)

10 The storage slurries at room conditions results in the change of slurry properties. The cause of this variation needs the special study. It may be both the reaction of aluminum carbide coating with liquid water or porosity of coating ensuring the contact of metal aluminum with gelled water. Fig.9 compares the hydrogen evolutions for fresh prepared slurry (Al powder Al-CO2-5) and the same sample after 24 hours storage in atmosphere of saturated water vapor at room temperature. It is clear that the ageing of sample leads to decrease the temperatures characterizing the process of hydrogen evolution. The total amount of evolving hydrogen increases with the ageing of the sample. Further ageing of the sample (more 30 hours) leads to the reverse result. It increases the temperatures characterizing the process of hydrogen evolution and decreases the amount of evolving hydrogen. V. Conclusion The properties of passivating coatings of Al UF particles designed on the basis of aluminum carbide were investigated. The preliminary results show that these coatings can provide the effective protection of a metal core of UF aluminum particles from the oxidation at their contact to atmospheric air at room temperature and natural humidity and can be used as the reactive passivating coating for energetic systems acting with participation of such oxidants as liquid water, water vapor and oxygen at high temperatures. The long-term (several hours) contact of Al UF particles with liquid water at room temperature in slurry composition can change the passivating properties of coatings. The additional investigation is necessary to permit the effective usage of aluminum carbide coatings in contact with liquid water. The comparison of DTA, TG curves and curve of hydrogen evolution for the same samples shows existence of several types of active surface places ( hot points ) responsible for the collapse of passivating coating and for the start of reaction of metal aluminum with liquid water. These hot points may be different for the different composition of passivating coatings and for Al UF particles with different origin. VII. Acknowledgment This work was supported partly by the Grant of the President of Russian Federation for support of young scientists and scientific schools SS and partly by the INTAS-Project Ref. Nr VIII. References 1. Michail N. Laritchev, Alexey N. Jigatch, Ilia O. Leipunsky, Michael L. Kuskov, «Aluminum Nanoparticles as the Energy Source for Multi-Sample Mars Return Mission», Pros. of International Conference - NanoTech "At the Edge of Revolution", AIAA Paper M.N.Laritchev, A.N.Jigatch, I.O.Leipunsky, M.L.Kuskov, P.A.Pshechenkov Aluminum Nanoparticles as a Basis for Fuel for Mars Conditions, Pros. of 10 International Workshop on Combustion and Propulsion, September 21-27, 2003, La Spezia, Italy. 3. S. Goroshin, A.J. Higgins, M.Kamel, Powdered Metals as Fuel for Hypersonic Ramjets, //37 th AIAA/ASME/SAE/ASEE Joint Pripaltion Conference and Exhibit, 8-11 July 2001, Salt Lake City, Utah, USA//, Pub. AIAA Jigatch A.N., Leipunsky I.O., Kuskov M.L., Pshechenkov P.A., Berezkina N.G., Laritchev M.N., Krasovsky V.G. //Khimicheskaya Fizika (in Russian)//, 2002, v.21, ¹4, P

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