(1) Non-aqueous Molecular Processing of Sulfide Ceramics. Sulfide ceramics belong to a class of compounds known as the chalcogenides. These are essentially compounds containing sulfur, selenium and tellurium. Chalcogenide compounds are characterized by metal-sulfur bonds which have strong vibrational frequencies. The materials therefore exhibit excellent transmission into the far infrared and have a number of applications. Sulfide compounds are semiconductors and are used in fabricating switching circuits. They also find application as phosphor materials in television picture tubes and also as cathode materials for advanced lithium batteries. These materials have always been synthesized using conventional high temperature melt solidification or aqueous precipitation techniques. Slow diffusion rates, volatilization of sulfur, inadequate mixing in the former and contamination due to oxygen in the latter leads to compositionally inhomogeneous sulfide ceramics. The problem is particularly severe in the case of reactive metals such as La, Al, Ti, Ga and Mo. Novel non-aqueous routes utilizing alkoxides in nonpolar non-aqueous solvents have been used to prepare compounds in the Ti-S system. Results to date have shown that titanium alkoxide reacts with organic and inorganic sulfidizing agents to form thioalkoxide precursors containing Ti-S bonds at room temperature. These precursors then transform to form crystalline TiS2 at 700C. The reaction mechanisms involved in the formation of the Ti- S bonds using acidic sulfide agents (H2S) and other reagents such as hexamethyl silyl sulfide, reagents that have (-S-S-) and (-S-S-S-) bonds have been studied using spectroscopic techniques such as FTIR and GC/MS. Recent results have indicated that sulfidization in solution occurs by a thiolysis reaction similar to the sol-gel reaction known for the formation of oxides. The thioalkoxide precursors transform to the crystalline sulfide. Control of heat treatment conditions result in the formation of unique microstructures ranging from monodispersed spheres to rosette patterned platelets of TiS2 with surface areas from 3 to 60 m2/gm. Cathodes made from these materials cycle extremely well and show efficiencies as high as 80%. The morphology of the crystalline sulfide has a significant impact on the electrochemical efficiency of the cathode. Thus, randomly oriented crystalline aggregates of the sulfide having large aspect ratios (defined as the ratio of the thickness of the basal plane direction to the prismatic plane) yield efficiencies as low as 60% while morphologies comprising of oriented platelets of the sulfide emerging from the monodispersed spheres with small aspect ratio result in efficiencies as high as 80%. This program is supported by the National Science Foundation. (2) A New solution coordination chemistry (SCM) approach to synthesizing high pressure phases in ternary sulfide systems. Sulfide compounds containing group IIB and group IIIA elements are covalently bonded materials that generally tend to undergo transformation to cubic polymorphs under high pressures and temperatures. One such compound, ZnIn2S4 in the Zn-In-S system has potential photovoltaic applications. At high pressures of 40 kbars, the equilibrium hexagonal phase undergoes a polymorphic modification to the cubic spinel form. We have shown that it is possible to synthesize nanocrystalline (400) powders of this high pressure modification at room temperature without application of any pressure. There is a strong indication of the coordination of the cations in solution affecting the formation of this phase. We are in the process of understanding the effect of the solution coordination of these ions on the cubic spinel formation using solution NMR and FTIR spectroscopy. Neutron Diffraction and high temperature Rietveld X-ray analysis have been conducted to analyze the cation coordination in this high pressure spinel phase. Results indicate that the synthesized form of ZnIn2S4 is indeed the high pressure cubic spinel phase with the Zn and In atoms occupying the tetrahedral and octahedral sites in the lattice. This program is supported by the National Science Foundation. (3) Synthesis of glasses and glass-ceramics for electronic packaging using the Modified oxide sol-gel (MOSG) and sol-precipitation (MOSP) techniques There is an increasing demand to fabricate high speed devices which calls for fabricating high density integrated circuits. As a result there is an ever increasing need to package these devices efficiently so that high speeds with minimum circuit delay are achieved. This calls for substrate materials to have very low dielectric constants. Ceramic materials have always been the choice for substrate application. In the last few years, the desire to have high performance fast switching devices with very quick rise times lead to the identification of glass-ceramic materials as candidate substrate materials. Glass-ceramics in the B2O3-SiO2-P2O5 system have shown considerable promise for packaging application because of their low dielectric values (3.8). We have synthesized glasses and glass-ceramics in this system using an inexpensive modified multicomponent sol-gel approach using oxides as starting materials as opposed to traditional metal alkoxides. The resultant gel powders have been characterized for their structure and morphology. using XRD and SEM. The powders have also been sintered using pressure and pressureless techniques. The sintered glasses and glass-ceramics contain the BPO4 phase which exhibit excellent dielectric constants in the range of 3.8- 4.3 @ 1MHz. Subsequent to the modified oxide sol-gel (MOSG) process a convenient and more efficient process known as the modified oxide sol precipitation approach was developed to overcome problems related to removal of carbon and the ensuing porosity. The resultant precipitated powders are clean and sinter much more easily using pressureless techniques. These glasses and glass-ceramics show a dielectric constant of 3.6 @ 1MHz. In order to lower the dielectric constant, the synthesized gels were treated with ammonia to incorporate nitrogen. Nitridation tends to result in delaying the crystallization temperatures and also lowering the propensity of the gels to undergo sintering. The structure of the gels were investigated using FTIR and the results clearly show the replacement of oxygen with nitrogen. The extent of replacement is seen to increase with increasing nitridation temperatures. The nitrided gels also show a lower dielectric constant of 4.006 @ 1MHz in comparison to values of 4.06 for the borosilicate gels. This program is supported by the National Science Foundation. (4) Processing of 3-D interconnected porous high thermal conductivity glass and ceramic composites for electronic packaging There is an overwhelming need to identify materials with optimum dielectric and thermal conductivity as substrates for high speed electronic packaging. This problem is even more severe in keeping with the current trends of increasing device densities and improved circuit configuration. Processing of a composite with optimum dielectric and thermal conductivity is an ideal solution to the problem. Unfortunately, there is no single material with ideal values for dielectric and thermal conductivity. Crystalline ceramics are present which have high thermal conductivity such as AlN and diamond. The dielectric constants are however quite high ranging from 7-10 at 1 MHz. Glasses on the other hand have low dielectric constants of 3-5 but are extremely poor thermal conductors. It was therefore decided to process a composite with interconnected channels of high thermal conductivity phase with corresponding interconnected porosity. The pores are then infiltrated with glass to form a hermetic seal. The research was initiated at Wright Patterson during the summer by myself as a visiting summer faculty. Preliminary results indicate that the composites can be fabricated with 28% porosity resulting in a three dimensional interconnected microstructure. The proposed ideas are being funded by an RIA from NSF and the Wright Patterson laboratories. Current results also indicate that the presence of surface oxide layer on the AlN particles hinders the effective thermal conductivity. Calculations indicate that ammonia treatments at 1000C result in lowering the oxide. Samples pretreated in ammonia prior to sintering indicate thermal diffusivity values which are 70% higher than samples which have the oxide layer. These results are indeed very promising for the fabrication of a composite displaying the optimum thermal conductivity and dielectric constant. This program is supported by the National Science Foundation and Wright Patterson Laboratory. (5) Chemical processing of binary and ternary transition metal nitrides Transition metal nitrides particularly, Mo2N, NbN, and W2N have important applications as catalyst in hydrodenitridation reactions. Ternary nitrides have also unique interesting magnetic applications. These nitrides have been traditionally synthesized using high temperature nitridation treatments of the oxides. The nitridation temperatures are very high (> 800- 1000C). We have synthesized molybdenum nitride precursors at room temperature using a solution reaction of inorganic salts of molybdenum with nitrogen containing reagents in non-aqueous solvents. The reaction is vigorous and control of the reaction parameters (temperature, concentration and rate of mixing) have resulted in nitrides with surface areas as high as 30 m2/gm. The concentration of the solutions, the method of addition and the use of coordinating agents in solution have a marked affect on the morphology of the resultant nitride. The present study is focused at understanding the effect of these parameters on the nitride powders. Recent results have indicated that the solvent coordinates with the initial starting salt in solution which then reacts with the nitrogen containing reagent to generate M-N bonds at room temperature. Prolonged reaction in solution results in side linked p-bond formation resulting in the formation of carbides in addition to the nitrides. Thus, the method has potential for synthesis of nitride and carbide composites. The original process has been modified to result in a oxynitride polymer which can be heat treated at moderate temperatures of 800C to generate corresponding binary and ternary nitrides. Another modification of this process is the reaction of inroganic transition metal halides with chelating agents to form an air stable nitrogen containing polymer. The polymer can be converted to the resulting nitride at temperatures as low as 900C. The technique is very powerful and can be used to synthesize multicomponent nitrides in the form of thin films and fine particles. At the present time we have successfully synthesized ternary nitrides in the Fe-Mo-N, Fe-W-N, Ti-Al-N, and Ni-Mo-N systems. This program is supported by the Air Force and National Science Foundation. (6) Hydrazide sol-gel processing of oxynitride glasses and ceramics Sol-gel processing has been extensively studied for the synthesis of oxide glasses, glass-ceramics and crystalline ceramics. Its application for the synthesis of nonoxide ceramics has hardly been researched into. In the present study we have shown that a highly refractory glass (crystallization temperatures > 1000C) have been synthesized at room temperature from a modified aluminum alkoxide solution. The resulting glass is thermally stable at temperatures in excess of 1000C. Spectroscopic studies reveal the presence of Al-O, Al-N and amino groups suggesting the formation of an oxynitride material. Crystallization of the material in nitrogen and ammonia results in the formation of the cubic spinel phase of aluminum oxynitride. FTIR, XRD and TEM have indicated the formation of a glass-ceramic with the ALON crystallites in the size range of 200. Subsequent treatment in ammonia even results in the transformation of the oxynitride to the hexagonal phase of pure AlN. The structure of the synthesized heat treated amorphous materials have been investigated using FTIR and Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR). The results indicate the formation of the cubic phase of ALON sugesting also the incorparation of nitrogen via the nitridation of the alkoxide at room temperature. Moreover, the reaction of unpolymrerized Al-sec butoxide with anhydrous hydrazine results in the formation of nanocrystalline ( 10-20 nm) AlN crystallites in the oxynitride glass matrix at 800C. This approach once again reveals the potential of the sol-gel process for synthesizing non-oxide ceramics. This program is supported by the National Science Foundation. (7) Chemical processing of transition metal oxides for lithium rechargeable batteries This area has been pursued via a corporate grant from Eveready Battery Company. The research includes science of chemical processing of transition metal oxides as cathodes for advanced lithium batteries. Research in the last quarter has involved developing novel low temperature aqueous and non- aqueous routes for the formation of the spinel compounds of the transition metal oxides. These processes have included chemical precipitation, aqueous and non-aqueous sol-gel processing resulting in the formation powders and gels. Results have indicated gel formation via a hydroxy condensation as well as a alkoxy-hydroxy condensation mechanism. The crystalline ceramic derived from each of the techniques result in novel microstructures and particles range from 0.2m to 10m depending on the processing conditions. Characterization of the powders for battery performance is being conducted at Eveready Battery Company. The resultant cathodes exhibit efficiencies in the range of 90-100% cathode utilization. Efforts are currently in progress to fabricate a one-step liquid decomposition system that will generate the crystalline transition metal oxide particles at temperatures as low as 300C. Preliminary results indicate that these particles depict cathode utilizations on the order of 95%. The novel colloidal based approach has formed the basis of a patent application. The current approach can easily be used to fabricate powders at the industrial scale. This program is supported by ARPA and a corporate funding from industry. (8) A New Ion-Exchange Process for the synthesis of transition metal nitrides for Catalytic Applications An ion-exchange process has been developed to synthesize transition metal nitrides supported on carbon as potential catalysts for hydrodenitrogenation reactions. The process involves use of a strong acid or weak acid resin and an inorganic salt of the transition metal. The ion-exchange reaction results in the loading of the transition metal on the polymeric skeleton comprising of carbon. The loaded resin can then be heat treated under suitable conditions to form the corresponding oxide or nitride. We have demonstrated this novel approach for synthesizing transition metal nitrides of Mo. Depending on the loading conditions and heat treatment, nitride composites of Mo can be synthesized. Preliminary results of surface area using BET and TEM characterization indicate 10 nm crystallites of Mo2N distributed in the carbon network exhibiting surface areas of the order of 900-1000 m2/g. The approach is very powerful and can be easily extended for the synthesis of a number of catalysts. This work is funded by the Air Force and NSF. (9) Chemical Synthesis of Fine Intermetallics in the Ti-Al System Titanium Aluminides are well known for their application in the aerospace industry. While these materials are being processed largely from powder metallurgy apporaches, opportunities exist for synthesizing them using novel low temperature chemical approaches. Accordingly, a new chemical approach based on solution chemistry has been developed to synthesize the titanium aluminide complex at room temperature. The reaction involves ametathesis reduction reaction of Ti and Al compounds using metallic lithium or the hydride. The resultant reaction is instantaneous and very vigorous resulting in the precursor. The precursors are now being heat treated and transformed to form the corresponding intermetallic. Preliminary reesults indicate that the precurso contains a non stoichiometric amount of the individual components. Further studies are currently being conducted. This research is supported by the Air Force. (10). Low temperature chemical routes for synthesizing Noble metal supported on oxide ceramics for sensor application Noble metal supported on tin oxides are used quite extensively as gas sensors to detect inflammable gases and also as humidity sensors. The sensitivity of these sensors are largely dependent on the stoichiometry, homogeneity of the material and more importantly, the surface area. All these attributes can be easily achieved using solution based approaches which provide the adequate mixing of the individual components at the molecular level. Thus, low temperature solution synthesis is a viable method for synthesizing these materials. A low temperature colloidal sol-gel based process has been developed for synthesizing noble metal tin oxide materials supported on different oxide materials. The powders are amorphous in the as-prepared state and can be subsequently heat treated to form the crystalline catalyst supported on the oxide matrix. The powders were observed under the SEM and are currently being tested for their sensor activity. This work is supported by a corporate grant.
Maintained by pk 12@andrew.cmu. edu