Environ. Sci. Technol. 1997, 31, 261-265
Selective Mechanochemical Dehalogenation of Chlorobenzenes over Calcium Hydride STEVEN LOISELLE, MARIO BRANCA, GABRIELE MULAS, AND GIORGIO COCCO* Dipartimento di Chimica, Universita’ di Sassari, Via Vienna 2, I-07100 Sassari, Italy
We have shown that in the presence of a reactive substrate, chlorinated organic compounds can be dehalogenated by mechanical treatment, and a specific reaction product can be obtained. We have used a ball milling process at low temperature and atmospheric pressure to produce a dechlorination of up to 100% for both liquid and solid chlorinated compounds. The products of the completed reaction for trials with hexachlorobenzene and chlorobenzene were both principally restricted to benzene and chloride salts. The use of CaH2 as a source of active hydrogen produces a much more specific reaction in significantly less time in comparison with CaO and MgO substrates reacted under hydrogen atmosphere. The process was found to depend on the injected mechanical energy and the collision frequency. In the case of hexachlorobenzene, an explosivetype reaction was observed to occur at specific milling times as a function of the kinetic energy employed.
Introduction The presence of hazardous chlorinated organic compounds in our environment, PCBs in particular, is a growing technological and public relations dilemma (1, 2). The development of treatment processes (3) for these health-threatening compounds is a high priority in present related research. Current oxidative remediation methods such as incineration do not work well since carbon bonds in aromatic rings with halogen substitution are highly resistant to oxidative degradation. At higher temperatures, reactions can be accompanied by toxic cogeners and unwanted byproducts, for example, dioxin (4, 1). Recent approaches to reductive remediation methods have included the use of microbial degradation (5), homogeneous catalysis (6), or electrolysis (7). Recently, the destructive adsorption of chlorinated benzenes on ultrafine particles of magnesium oxide and calcium oxide has been reported by Li et al. (8). Total decomposition was observed at 900 °C under hydrogen flow, and MgCl2 or CaCl2 was obtained as the major chlorinated product. The authors stress the need for high surface area substrates to favor extensive decomposition. When a lower surface area and temperature or an air gas flow were used, dibenzo-pdioxin and a monochloro derivative were observed in small quantities. Calcium oxide and Ca- or Mg-powdered metals have also been employed as reactive substrates in the mechanochemical destruction of organochlorides including chlorobenzene, PCB, and DDT (9). The authors observed a virtually complete breakdown of the toxic molecules into harmless byproducts such as carbon, calcium hydroxide, and calcium chloride. * Corresponding author e-mail address:
[email protected].
S0013-936X(96)00398-7 CCC: $14.00
1996 American Chemical Society
The method has attracted much attention (10), even if details of the patented process were scanty. The use of mechanical energy to sustain difficult reactions is not new. Mechanical treatments under reactive atmospheres have been used for a number of years to speed up the interaction between a solid and its environment (11). The disruptive mechanical action largely increases the specific surface area of friable materials that can reach an ultrafine nanostructure. It is therefore possible to find a link between the structural conditions that CaO/MgO develop under milling and the high surface area of reactants used by Li et al. (8) in the thermal destruction of chlorinated compounds. However, beyond the increase of the surface area, mechanically sustained processes show significant differences in respect to the thermally activated ones. Along these lines, we have recently reported mechanochemical hydrogenation reactions via transition metal and/ or intermetallic hydrides (12). Sustained by the enhanced mobility of the “hydride” hydrogen under high-energy milling, carbon monoxide, graphite, and ethylene were readily converted to hydrocarbons with a high selectivity to methane and, in the latter case, to ethane. The dynamic role of hydrogen to steer the end products of the milling processes was demonstrated by using hydrogen either as a gaseous reagent or released in active form from metal hydride lattice (13). Within this framework, it is feasible to examine the mechanochemical dechlorination of organochlorines over ionic hydrides. The reactivity of these compounds strongly depends on their state of dispersion (MgH2 is a striking example) (14), and a high reactivity is therefore expected under milling. Among the alkaline earth dihydrides, we selected CaH2 which, interestingly enough, is used as a hydrogen source in industrial scale reduction processes. We used liquid chlorobenzene and solid hexachlorobenzene as test compounds. For the sake of comparison, runs were also performed over calcium and magnesium oxides.
Experimental Section A modified Spex 8000 ball mill was employed for the majority of the trials. The milling vial consists of a hollow hardenedsteel cylinder that fits into a holding device that swings along a three-dimensional path. Under standard conditions, this motion occurs with a frequency of 875 cycles/min. The balls, loose inside the reactor, collide with the cylinder wall and with one another, transferring their kinetic energy to the trapped powder during the impact. The number of collisions per second (hits/s) and the impact energy (J/hit) vary widely; the former parameter depending on the speed of the motor as well as on the number and size of the employed balls; the latter depending on the mass and the relative velocity of the colliding balls. Steel or tungsten carbide balls of different size and mass were employed, the corresponding impact energy spanning from about 0.01 to 0.3 J/hit calculated as 1/2mv2 where m is the mass of the ball and v is its relative velocity at the collision event. A collision frequency of 29 or 43 hit/s was measured in single ball experiments performed with the mill operating at 870 or at 1240 cycles/min. Full details of the experimental procedures we have developed to evaluate the collision energy and the impact frequency in milling processes are given in refs 13 and 15. Chlorobenzene (99.9% HPCL grade) and hexachlobenzene (99%) supplied by Aldrich were used, both of which were analyzed and not found to contain traces of the reaction products as impurities. Solid substrates, CaO (99.9%), MgO (99%, -325 mesh), and CaH2 (95%,-4+40 mesh) were also obtained from Aldrich.
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Solid reactants were loaded into the milling reactor under purified argon in a glove box (residual humidity, oxygen and nitrogen < 5 ppm). Degassed chlorobenzene and any nonargon atmospheres were injected through septa-filled injection ports and leak-proof valves, which were also used for gas sampling. As a rule, inorganic substrates were employed in excess, typical Ca/Cl ratios ranged from 15:1 for chlorobenzene to 2:1 for hexachlorobenzene for a total vial load of about 9 g. The mechanical activation process was run continuously from 1 to 12 h depending upon the milling operative conditions. At the completion of the milling procedure, gaseous products, sampled from the reactor’s head space, were directly monitored by a PE 8500 GC equipped with a flame ionization detector and a GC-Q column (J&W). Organic products were first solubilized by adding established quantities of n-pentane to the reacted mixture, and liquid samples were analyzed by using a Fisons HRGS 5300 GC and a Finnigam Tracker mass spectrometer GC/MS PE 8420 equipped with a ion trap detector and a HP PONA column. For runs on MgO and CaO, special attention was also paid to detect dioxin products. Mercury nitrate titration procedures (16) were also adopted for a routine analysis of the produced inorganic chlorides. On the basis of the analytical data, chloride mass balancings were performed, the results fitting within (3%. Unless otherwise specified, conversion data are reported as total percent organic chlorine converted to inorganic chloride. Reagents and product powders were subjected to standard X-ray diffraction analysis. Safety. It must be noted that possible byproducts of an incomplete reaction are toxic. In addition, benzene, the principle product of the completed reaction, is a known carcinogen. All procedures must be performed with attention and in a controlled environment. Hydrides will burn when in contact with water and oxygen. Furthermore, the hydride in our process is highly activated and therefore much more reactive.
FIGURE 1. Mechanochemical conversion of chlorobenzene observed at different ball collision energy over CaH2 (squares), CaO (full circles), and MgO (open circles) substrates. Conversion values refer to a constant milling time of 12 h. Powder load, 11.2 g; impact frequency, 29 hits/s; ball mass, 1-18 g.
Results and Discussion Our initial concern focuses on chlorobenzene as the chlorinated compound and CaH2 as the substrate. A control experiment was performed first by adding the chlorobenzene reactant to the as received coarse-grained CaH2 or to premilled finely divided CaH2, and the reaction was allowed to proceed for 12 h by agitating the reaction vessel only. No conversion to inorganic chloride was observed in the former case and in the latter never exceeded 1%. Dechlorination became noticeable only when reactant and substrate were milled together. This suggests that the reaction, while surely dependent on the structural conditions of the substrate, is strongly hindered when the substrate surface is not highly activated and continually recleaned. Chlorobenzene conversion was then followed as a function of the milling time and at different impact regimes. As a general trend, an induction time is needed before the dehalogenation reaction starts, thereafter increasing quantities of products form as a function of milling time. The induction period, during which the comminution process is in an incomplete stage preventing the intimate contact of the reactants, is shortened by increasing the relative velocity or the mass of the colliding balls, i.e., the kinetic energy at the impact. In our experiments, in all but a few noted cases a single ball was utilized. Furthermore, by increasing this parameter, higher conversion levels are achieved in significantly less time. The conversion values are presented in Figure 1 (squares) calculated as a function of the hit energy after a constant milling time of 12 h. For instance, conversion percentage passes from 0.2 at 0.011 J/hit to about 33 at 0.088 J/hit whereas more than 95% of chlorobenzene is transformed beyond an impact energy of 0.25 J/hit.
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FIGURE 2. CG/MS analyses at intermediate stage of the mechanochemical reaction between C6H5Cl and CaH2 performed at 1240 (upper trace) and 870 rpm (lower trace). The stronger signal pertains to n-pentane used as solvent, whereas peak 1 is due to benzene, peak 2 is due to residual chlorobenzene, and peak 3 is due to biphenyl as shown by the mass spectra quoted in the insets. As for degradation products, we found consistent concentrations in both headspace and liquid phase. In neither phase did we find chlorinated organo compounds besides chlorobenzene. Likewise, the reductive process was found to be selective, meaning that within a certain impact energy range, the final reduction products were restricted to benzene. In this regard, it must be noted that biphenyl was observed at the lower impact energy and in the initial stages of the transformation: typical GC/MS results are reported in Figure 2. Methane and ethane formed at the higher end of the energy range. The X-ray diffraction patterns of the reacted powders were dominated by the reflections of residual CaH2 loaded in a large stoichiometric excess. Possible signals arising from the expected CaCl2 product were not easily assignable, being only perceivable as faint peaks or shoulders superimposed over the main hydride profiles. Milling trials were also performed by changing atmosphere (hydrogen, air), internal pressure (0.1-0.5 MPa), and ambient temperature (273-373 K). We found that the reaction conversion was independent of these parameters. More importantly, the percent conversion did not change appreciably when gaseous hydrogen was used as the milling environment. This suggests that the organic chloride reduc-
TABLE 1. Comparison of Substrate Reactivitya,b MgO main products methane ethane benzene
0.3 0.9
CaO 1.2 0.1 1.4
CaH2 4.2 0.5 62.7
a
Results refer to the molar ratio of products to chlorobenzene in the gaseous phase. b Milling conditions: 6-h elapsed time; H2 atmosphere, initial substrate, chlorobenzene (liquid) molar ratio 40:1, temperature of the vial reactor 308 K.
tion occurs via the “hydride” hydrogen promoted to a high activation level in the finely divided and structurally destabilized hydride phase. This point will be further demonstrated later. As for a possible reaction mechanism, it is generally maintained that a large amount of excess energy is accumulated in a solid subjected to intensive milling in the form of structural defects and in the extended network of intergranular boundaries. These defects exert a strong influence on the migration of the reactant in the stressed lattice surface. With continued milling, the specific surface area increases, local states develop, and intermixing is forced to a molecular level. Chemically active non-equilibrium states are formed, and the intensification of the interdiffusion process facilitates the breakup of chemical bonds. It has been reported that the breaking up of bonds under mechanical stresses primarily results in the development of radical centers that are capable of reacting into subsequent multistage processes (17, 11). On the other hand, it is well-known that aryl radicals are formed by the reduction of aryl chlorides. This process occurs by the transfer of an electron from some reducing agent to the aryl halide, giving rise to the radical anion that fragments to give inorganic chloride and aryl radicals. Whatever the possible mechanism is, the formation of organic radicals was observed, on a qualitative level by EPR analysis when a small quantity of chlorobenzene was added to a CaH2 sample milled in a ceramic container. Likewise the noticeable formation of biphenyl at incomplete stages of the process, observable in Figure 2, is in line with these findings. Even if this point requires further inquiry, a free radical mechanism can be surmised from this preliminary on-off test. The following results concern the use of MgO and CaO substrates activated under gaseous hydrogen. In comparison with calcium hydride, a decrease in the reactivity was revealed in these tests, and some of the results are presented in Figure 1. For instance, after 12 h milling at a low energy transfer regime (0.088 J/hit), the 33% overall conversion observed over CaH2 decreased to 9% and to 11% in those tests with MgO and CaO, respectively. Data analysis relevant to the volatile products in the headspace of the reactor, compared in Table 1, are more significant, demonstrating the hydride’s higher efficiency. It is worth noticing that by using CaH2 there is a more favorable thermodynamic condition for reduction. The available pertinent data are quoted in Table 2. When 2 mol of monochlorobenzene are considered, the free energy gains due to the formation of MgCl2 or CaCl2 from the parent oxides are -195.8 and -318.4 kJ/mol, respectively. In the case of CaH2, the values above compare to -541.4 kJ/mol if CaCl2 is taken into account as the final product of the straight decomposition route. As a further advantage, the oxide substitution with hydrides allows a reductive path to be realized in which oxygen is totally absent. We finally note that, using CaO or MgO, the chlorobenzene conversion never goes to completion within the explored range of the impact energies. After 48 h of milling, a noticeable amount of the organic compound is still present (about 3%),
TABLE 2. Thermodynamic Data Relevant to Studied Compounds compd
∆H °f (kJ/mol)
ref
S ° (JK/mol)
ref
MgO CaO CaH2 MgCl2 CaCl2 CaHCl C6H6 (l) C6H5Cl (l) C6Cl6
-601.6 -634.9 -181.6 -641.4 -795.0 -504.2 48.9 10.5 -141.8
18 18 18 18 18 19 20 21 22
26.9 38.1 41.8 89.6 108.4
18 18 18 18 18
173.3 197.5 260.2
23 23 23
FIGURE 3. Hexachlorobenzene conversion as a function of the collision energy after a constant milling time of 6 h. A trend discontinuity is observed at about 0.07-0.08 J/hit. The present trials were performed by employing a single ball of different size and mass (stainless steel or tungsten carbide ranging between 1 and 32 g) or by adjusting the mill speed (we used a series of expanding motor pulley). which differs substantially from the results presented in ref 9b where the remaining chlorobenzene, after being milled for 12 h with CaO, was found to be 7 ppm. A definitive comparison is, however, difficult due to the lack of details in ref 9. In the following, we deal with the hexachorobenzene dehalogenation. Due to the above results, tests were performed using only CaH2. Relevant data are presented in Figure 3 where the C6Cl6 conversion is plotted against the impact intensity. These data now refer to milling runs performed up to 6 h. Although low degrees of conversion were observed at the lowest energy regimes, a sharp increase up to 100% was seen, crossing an energy threshold at around 0.07 J/hit. This behavior differs from the gradual increase of product observed for chlorobenzene, for which the conversion rate was directly proportional to the impact energy. In the case of the hexachloro, a small increase in the impact energy across the energy threshold (and the corresponding amount in the energy absorbed by the system) is enough to drive the reaction to completion. A new mechanism therefore must be responsible for the observed steep conversion up to 100%. Further evidence of this was gained by calorimetric tests. The milling vial was insulated to maintain the measured temperature close to that of the vial and to reduce the effect of the surrounding environment. The temperature was continuously monitored during the milling by using a Pt lamellashaped thermometer fixed on the external wall under the cover. Typical trends are presented in Figure 4. After a certain milling time, which is shorter for larger impact energies, a sudden (< 1 s) temperature increase was observed. It is worth noticing that the initial temperature rise is due to the thermally dissipated kinetic energy and that the thermal gradient at the beginning is proportional to the impact energy (13, 17). A steady state is then encountered in which the temperature stabilizes, pointing out the nonperfect adiabatic conditions
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FIGURE 4. Temperature trace of the vial during the mechanochemical dehalogenation of hexachlorobenzene. A thermal spike is evident above the steady temperature level. Test performed at 0.16 J/hit. of the insulating cover. In spite of this however, the thermal shocks observed can be considered adiabatic due to the time scale of the event. Moreover, roughly constant temperature jumps were found in repeated trials, irrespective of the impact energy used. GC and GC/MS analysis performed on products of a partially completed reaction, i.e., before the adiabatic shocks, revealed only traces of intermediate organic products such as chlorobenzene, o-dichlorobenzene, 1,2,3-trichlorobenzene, 1,2,4,5- and 1,2,3,4-tetrachlorobenzene, and pentachlorobenzene. Conversely only benzene, apart from methane, was found as the organic product when the reaction was interrupted immediately after the temperature spike. These results are confirmed by the X-ray diffraction analysis. To avoid hygroscopicity effects, which prevent the observation of the hydride and chloride phases, X-ray scans were performed under helium in a special cell with mylar windows. Illustrative patterns are presented in Figure 5 together with those of CaH2 and C6Cl6 reported here as a reference. In addition to residual hydride (JCPDS, card 13384) (24), the peak analysis of the reacted mixture demonstrated the formation of CaCl2 (JCPDS, 12-56), CaHCl (JCPDS, 14-167), and carbon graphite (JCPDS, 26-1079), whereas signals of the reagent compounds are only discernible before the registered sharp rise in the vial temperature. Moreover a visual comparison of the peak intensities indicates the prevalence of the CaHCl phase over the expected CaCl2 product even if it is questionable whether CaHCl forms directly during the dechlorination process or if CaCl2 reacts later with the excess of CaH2. In concluding this analysis, we also note the remarkable peak shifts and the severe line broadening (25) that characterize the patterns of milled powders (we have already mentioned this behavior) demonstrating the high level of structural and chemical disorder (26) accumulated in the process. An explosive-type reaction clearly emerges from the results. Further evidence was obtained from the thermal data. The measured values of the temperature increases at the edge yielded an average of 51 ( 6 °C which, considering the calorimetric characteristics of the vial reactor (treated as a calorimetric bomb), corresponds to a total released heat of -20.23 kJ ( 10%. Taking into consideration the amount of reactant powders (5.58 g of CaH2 and 3.2 g of C6Cl6 corresponding to 0.011 mol), the heat of reaction is estimated to be -1836 ( 40 kJ/mol. From the data reported in Table 2, a ∆H° of -1728 kJ/mol of hexachorobenzene is foreseen
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FIGURE 5. Cobalt-Kr X-ray diffraction patterns for CaH2 powders and for some of the reacted mixtures. The upper traces, which are shown as a reference, refer to CaH2 powders “as received” (a) and after a milling treatment of 6 h (b) and to the as, received hexaclorobenzene. The other patterns correspond to the C6Cl6 + CaH2 mixtures immediately before and after the sudden increase in the temperature vial. In the latter, the prevalence of CaHCl over CaCl can be seen. Bars at the bottom of the figure mark peak positions quoted according to JCPDS data (24). for the reaction C6Cl6 + 6CaH2 f 6CaHCl + C6H6 and of -1649 kJ/mol if CaCl2 is taken as the final product instead of the hydride-halide salt. Moreover further -49 kJ/mol must be taken into consideration for the decomposition of benzene to carbon and hydrogen. Whatever the CaHCl/CaCl2 relative amounts in the reacted mixture, the experimental heat of reaction stands remarkably close to the range of quoted values. However, such estimates need further refining before a definite correlation can be established. Some considerations are now possible. It is known that high-energy mechanical treatment of highly exothermic systems can induce a self-sustaining reaction (27). Usually this stage follows a period during which a deformation intermixing of the solid components occurs on a molecular or cluster level. When a supersaturation of critical defects is reached, reaction propagates spontaneously. Combustion or quasi-explosive processes have been observed during ball milling of metal oxides or halides with strong reducing elements such as Ca and Mg (28). A similar phenomenon occurs in our dehalogenation reaction. Discrete amounts of energy are progressively absorbed by the reacting particles as microstructural defects in the reactant particles, bringing the excess energy nearer to the activation energy required for the reaction. A self-propagating transformation is then observed, sustained by the high reaction heat. Some aspects of the studied reactions require further insights; in particular the amount of the mechanical energy actually absorbed by the substrates during the process needs to be quantified as well as its correlation to the consequent structural and electronic conditions that are progressively achieved. The evaluation of these parameters is not a simple task due to the lack of detailed experimental procedures concerning the reactivity of nonstatic surfaces. In addition, we used chlorobenzene and hexachorobenzene in their pure state in an attempt to understand the chemistry involved in the process. Of course, any real world application would involve a mixture of compounds, complicating the eventual end products. Trials using different
chlorinated compounds are under way. The advantages of mechanical treatments over incineration are obvious in that they are low-temperature and low-pressure reactions with controllable end products. The absence of oxide compounds among the reactant species avoids the formation of hazardous and un-intended byproducts, toxic dioxin in particular. Moreover, in addition to the significant advantages of a milling treatment such as the extension of the available surface area and the continued recleaning of the available surfaces (thus preventing the reaction products from slowing the reaction mechanism), the specific use of hydride phases allows the reductive process to be carried out along a neat and quite straightforward transformation path. Compared to the more conventional reduction method under hydrogen flow, the effectiveness of metal hydrides as active hydrogen carriers is manifest when the self-sustaining dechlorination of C6Cl6 is considered. Future work should focus on different milling techniques including rod milling, which was not touched upon in our study. Different techniques might have advantages over ball milling in efficiency and scaling up to industrial size processes. Further investigation should also be done on other chlorinated cogeners. There is every reason to believe that this process can be used for the treatment of fluorinated or brominated compounds as well. In short, while mechanochemical processes in their present state of development are not easily transferable to a wide range of environmental waste treatments, the prospects for their use after further study are attractive.
Acknowledgments We thank the USL Chemical Laboratories and the Sardinia Service, Tecnologie Avanzate per l’Ambiente srl, for skillfull technical assistance during the GC/MS analysis. This research has been supported by CNR Rome under a PS contract and by a MURST project.
Literature Cited (1) (a) Safes, S. Hazards, Decontamination, and Replacement of PCB; Crine, J., P. Ed.; Plenum Press: New York and London, 1988. (b) PCBs and the environment; Waid, J. S., Ed.; CRC Press: Boca Raton, Fl. 1986. (c) PCBs: Human an Environmental Hazards, D’Itri, F., Kamrin, M. A., Eds.; Butterworth Publishers: Boston, 1983. (2) Borlakogler, J. T.; Dils, R. R. Chem. Brit. 1991, 815. (3) Murena, F.; Famiglietti, V.; Gioia, F. Eng. Prog. 1993, 12, 231. (4) Evans, D. H.; Pirbazari, M.; Benson, S. W.; Tsotsis, T. T.; Devinny, J. S. J. Hazard. Mater. 1991, 27, 253. (5) Ukrainczyk, L.; Chibwe, M.; Pinnavaia, T. J.; Boyd, S. A. Environ. Sci. Technol. 1995, 29, 439.
(6) Liu, Y.; Schwartz, J.; Cavallaro C. L. Environ. Sci. Technol. 1995, 29, 836. (7) Zhang, S.; Rusling, J. F. Environ. Sci. Technol. 1995, 29, 1195. (8) Li, Y. X.; Li, H.; Klabunde, K. J. Environ. Sci. Technol. 1994, 28, 1248. (9) (a) Australian patent Application PL6474, 1992. (b) Rowlands, S. A.; Hall, A. K.; McCormick, P. G.; Street, R.; Hart, R. J.; Ebell, G. F.; Donecker, P. Nature 1994, 367, 223. (10) Borman, S. Chem. Eng. News 1993, 11, 5. (11) Heinicke, G. Tribochemistry; Akademie-Verlag: Berlin, 1984. (12) (a) Cocco, G.; Mulas, G.; Pintore, M.; Piliu, G.; Schiffini, L. Proceedings of the International Conference on Structural Application of Mechanical Alloying; deBarbadillo, J. J., Froes, F. H., Schwartz, R. B., Eds.; ASM International: Materials Park, OH, 1993; p 425. (b) Cocco, G.; Mulas, G.; Schiffini, L. Mater. Sci. Forum 1995, 179-181, 281. (13) Cocco, G.; Mulas, G.; Schiffini, L. Mater. Trans., JIM 1995, 36 (2), 150. (14) Mackay, K. M. In Comprehensive Inorganic Chemistry; Bailar, J. C., Emeleus, H. J., Nyholm, R., Trotman-Dickenson, A. F., Eds.; Pergamon Press: Oxford, 1973; p 28. (15) Mulas, G.; Schiffini, L.; Cocco, G. Proceedings of the International Symposium on Metastable, Mechanically Alloyed and Nanocrystalline Materials, ISMANAM 95, July 24-28, Quebec Canada. Mater. Sci. Forum. 1996, 225-227, 237. (16) Official Methods of Analysis of the AOAC, 15th ed.; Helrich, K., Ed.; The Association of Analytical Chemists, Inc: Arlington, VA, 1990. (17) Butyagin, P. Yu. Sov. Sci. Rev. B. Chem. 1989, 14, 1. (18) Materials Thermochemistry, 6th ed.; by Kubaschewski, O., Alcock, C. B., Spencer, P. J., Eds.; Pergamon Press: Oxford, 1993. (19) Ehrlich, P.; Peik, K.; Koch, E. Z. Anorg. Allgem. Chem. 1963, 324, 113. (20) Wadso, I. Acta Chem. Scand. 1968, 22, 2438. (21) Techniques of Chemistry, Vol. II: Organic Solvent, Physical Properties and Methods of Purification; Riddick, J. A., Bunger, W. B., Sakano T. K., Eds.; John Wiley & Sons: New York, 1986. (22) Platonov, V. A.; Simulin, Yu. N. Zhur. Fiz. Khim. 1983, 57, 1387. (23) Domalski, E. S.; Hearing, E. D. J. Phys. Chem. Ref. Data 1993, 22 (4), 805. (24) JCPDS International Centre for Diffraction Data, 1601 Parke Lane, Swarthmore, PA, 1987. (25) Klug, H. P.; Alexander, L. E. X-ray Diffraction Procedures; John Wiley & Sons: New York, 1974. (26) Waseda, Y. Prog. Mater. Sci. 1981, 26, 1. (27) Tchakarov, Chr.; Gospodinov, G. G.; Bontsev, Z. J. Solid State Chem. 1982, 41, 244. (28) Schaffer, G. B.; McCormick, P. G. Mater. Sci. Forum 1992, 8890, 779.
Received for review May 6, 1996. Revised manuscript received August 7, 1996. Accepted September 3, 1996.X ES960398S X
Abstract published in Advance ACS Abstracts, November 1, 1996.
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