A series of computer experiments was conducted for the self-propagating combustion synthesis process in the Nb-C system, based on the general theoretical model that was developed previously.1 A detailed and quantitative description was given for the various physical and chemical processes that take place during the combustion synthesis process. The results are presented at various length scales in order to provide an insight into understanding the mechanisms that are responsible for the self-propagating behavior. It was shown that a fundamental understanding and precise control of the process require a strong emphasis on the joint contributions of the rates of the various mass and energy redistribution processes that occur during the combustion synthesis process. A proper balance of each of the elementary process rates must be achieved to give rise to self-propagating behavior. This paper illustrates some of the capabilities of the general theoretical model in quantitatively describing the self-propagating combustion synthesis process.

In the combustion synthesis of TiC-Al2O3 composites from TiO2, Al, and C, different types of precursors, i.e., anatase and rutile TiO2, and graphite and carbon black were used. The measurements of combustion wave velocity showed that the reactivity was higher with graphite as a carbon source and anatase as a TiO2 source. This was also confirmed by analyzing the degree of conversion of reactants to products. For the precursors investigated, the wave propagation of the 3TiO2-4Al-3C system diluted by Al2O3 showed that the limit of wave propagation was more dependent on carbon source than on TiO2. The wave front of all the samples proceeded in unstable combustion mode. Temperature profiles in the reaction zone suggested that the heat-affected zone was broader in the case of graphite carbon source than carbon black. The microstructures and chemical composition of the products were also studied.

The epitaxial growth of yttria stabilized cubic zirconia produced via the solution precursor route deposited onto the basal and prismatic planes of sapphire was characterized. The evolution of the polycrystalline thin film was described with reference to two concurrent physical processes: abnormal grain growth due to the growth of grains with preferred orientations and a morphological instability which resulted in an uncovering of the substrate. X-ray diffraction, electron backscattering patterns (EBSP), and transmission electron microscopy (TEM) (plan- and cross-sectional view) were used to determine the epitaxial relation (normal and in-plane). The observed epitaxial orientations for the two substrate planes are listed in Table I. A computer search was used to determine the planar, near coincident site lattices (NCSL) for the observed normal epitaxial relations (c-plane: [001]ZrO2ll[0001]Al2O3; a-plane: [001]ZrO2‖[1210[Al2O3). The determined NCSL's did include all the observed epitaxial relations, but also included others not observed within the same range of misfit and coincident site density.

Crystalline thin films of silicon nitride have been grown on a variety of substrates by microwave plasma-enhanced chemical vapor deposition using N2, O2, and CH4 gases at a temperature of 800 °C. X-ray diffraction and Rutherford backscattering measurements indicate the deposits are stoichiometric silicon nitride with varying amounts of the α and β phases. Scanning electron microscopy imaging indicates β-Si3N4 possesses sixfold symmetry with particle sizes in the submicron range. In one experiment, the silicon necessary for growth comes from the single crystal silicon substrate due to etching/sputtering by the nitrogen plasma. The dependence of the grain size on the methane concentration is investigated. In another experiment, an organo-silicon source, methoxytrimethylsilane, is used to grow silicon nitride with controlled introduction of the silicon necessary for growth. Thin crystalline films are deposited at rates of 0.1 μm/h as determined by profilometry. A growth mechanism for both cases is proposed.

The morphologies of ZrSi2 on Si(111) and TiSi2 on Si(111) and (100) have been investigated, and the results compared and contrasted. Films were prepared by UHV deposition of Ti or Zr onto clean, reconstructed Si(100) or (111) substrates, and reacted by in situ annealing. The sheet resistivity of the ZrSi2 was measured and found to be 33-42 μΩ-cm. The morphologies were examined by transmission and scanning electron microscopy. In particular, the islanding properties were studied; both the temperature of the onset of islanding and the island characteristics were measured. The surface and interface energies have been determined from the contact angles of the silicide islands, according to a solid-state capillarity model. The system of ZrSi2 on Si(111) was found to have surface and interface energies lower than those of the system of TiSi2 on Si(100), but higher than those of the system TiSi2 on Si(111). ZrSi2 on Si(111) was found to island at a higher temperature than TiSi2 on either substrate, a result attributed to kinetic effects. Areal coverage of the islands was measured, and the results were consistent with the solid-state capillarity model. For both TiSi2 and ZrSi2, increasing faceted structure was observed with increasing anneal temperature. Preferred faceting planes were found to be of Si(111) and (100) type for TiSi2 islands and of Si(111) type for ZrSi2. Faceted islands were apparently epitaxial. As the solid-state capillarity model does not directly apply to islands with a faceted structure, an observation of the percentage of faceted islands produced by different annealing temperatures was used to suggest the processing conditions in which the model is applicable.

Silicon-aluminum oxynitrides and/or aluminum nitride were prepared by the reduction-nitridation of kaolin in graphite crucibles under hydrogen/nitrogen flow at temperatures in the range 1100 °C–1450 °C. Almost pure β'-SiAION was obtained in less than 24 h at 1200 °C. At high temperatures (e.g., 1450 °C) and for long reaction times (e.g., 10 h), β'-SiAION is fully reduced to AlN. In most preparations, β'-SiAION is formed together with small amounts of AlN. However, the formation of AlN can be limited by using short reaction times and/or by adjusting the reducing power of the atmosphere, i.e., the N2/H2 ratio. Compared to the carboreduction of aluminosilicates which always yields mixtures, the present method leads to almost pure products under appropriate conditions. The temperature for the formation of β'-SiAION is at least 200 to 400 °C lower than the temperatures used for the carboreduction of aluminosilicates or sintering of powders, respectively. A gaseous species formed by the reaction of hydrogen with graphite is suspected to be responsible for the nitriding reduction of kaolinite.

We present results of atom superposition and electron delocalization molecular orbital (ASED-MO) calculations of interactions of interstitial H with substitutional B and N in diamond. Nearest-neighbor and next-nearest-neighbor C atoms were relaxed in geometry depending on the cluster size, XC34H36 or XC70H60, respectively, where X = B or N and the H atoms saturate the surface dangling radical orbitals of the models. A small Jahn-Teller distortion occurs for interstitial B, a shallow acceptor which, in the B− state, sits in a tetrahedral lattice site. For interstitial N distortions are large, with a long C-N distance which stabilizes a ŝ∗ orbital that would otherwise be in the conduction band. This orbital has one electron in it and has its greatest amplitude on C; the bonding counterpart has its greatest amplitude on N and is similar to the N lone-pair orbital in amines. The calculations indicate that N is a deep donor and N+ relaxes to the tetrahedral lattice site. Interstitial H is a mid-band-gap donor and is possibly also an acceptor with a high 1.9 eV calculated activation energy barrier to migration. Interstitial H+ is expected to be very mobile, with a migration barrier of 0.1 eV. H− is predicted to be relatively immobile with an activation barrier for migration of 2.5 eV. The mobility of bond-inserted H around B in BH pairs should be high, with a calculated activation energy of 0.13 eV, but for N the comparable process has an activation energy of 2.50 eV. In NH pairs the interstitial H has formed a bond with the radical orbital on the C, so donation would be from the lone-pair orbital on N, which lies deep in the band gap; hence, the donor property is passivated.

The stability of BaFe12O19 and BaFe2O4 was studied by the oxygen coulometric titration technique between 700 °C and 1000 °C using a solid-state electrochemical cell. This temperature range is technologically important for the deposition of BaFe2O19 magnetic thin films. The thermodynamic information obtained from the titration measurements was corroborated with structural identification of phases prepared under electrochemically controlled conditions. Accordingly, a section of the Ba–Fe–O ternary phase diagram around the BaFe12O19 composition was constructed in this temperature range. The standard Gibbs free energy change for the decomposition of BaFe12O19 into BaFe2O4, Fe3O4, and O2 is given by the expression ΔG°[J/mol] = 7.23 × 105 −480T. In the oxygen pressure-temperature domain, the thermodynamic stability limits of BaFe12O19 and BaFe2O4 are given by the expressions In[Po2(atm)] = 69.37 −1.04 × 105 T−1 and In[Po2(atm)] = 27.68 −7.12 × 104 T−1, respectively. The stability limits determined here help define the process conditions for the successful synthesis of these phases.

Hollandite, [BaxCSy][Ti, Al)3+2x+y Ti4+8−2x+y]O16, has been subjected to aqueous chemical attack at 75 and 150 °C. The resultant surface alterations have been monitored by solution analysis, SEM/TEM investigations, and surface analysis. The evolution of the solid/solution system can be described by the following mechanisms: There is a near-instantaneous release of Cs and Ba from reactive surface sites, followed by relatively rapid dissolution of the first few monolayers. The release of Al drives the pH to low values as a result of Al-OH hydroxylation and precipitation of AIO(OH) and/or AI(OH)3 species. The rate of dissolution is reduced by an order of magnitude or more by the formation of a thin and continuous Al(Ti)-rich surface layer. The layer remains stable and protective even under extreme hydrothermal conditions. This description has implications for the use of hollandite as a ceramic phase for disposal of high level nuclear wastes.

We report on the formation of a new metastable phase of niobium-nitride (Nb-N) alloy powders. This nonequilibrium phase has been synthesized by milling elemental niobium (Nb) powders under purified nitrogen gas flow using a high energy ball mill at room temperature. The progress of the solid-gas reaction for the Nb-N system has been followed by means of x-ray diffraction, differential thermal analysis, transmission electron microscopy, and chemical analysis. After 720 ks of milling, the initial bcc-Nb is completely transformed to nonequilibrium-fcc-NbN containing about 50 at. % N. The lattice parameter of the end product fcc-NbN was calculated to be 0.433 nm. In addition, the particle and grain sizes of the completely reacted alloy powders are 3 μm and 5 nm, respectively. This transforms to a mixture of hcp-Nb4N3 and hcp-NbN at the high temperatures, as high as 1150 K.

A simple and convenient method for low-temperature synthesis of La0.84Sr0.16MnO3 powder is described. The technique involves autoignition of a carboxylate (citrate + acetate)-nitrate gel resulting from a thermally induced anionie oxidation-reduction reaction to yield an ash, which upon calcination produces the desired powder. The resulting powder is pure, homogeneous, and possesses ultrafine particle size of the order of 0.3 to 0.5 μm. The autoignition is restricted to a particular range of carboxylate to nitrate ratio in the gel. Attempts have been made to understand the ignition process with the help of Thermogravimetry (TG) and Differential Thermal Analysis (DTA) of the samples. The process appears to have a higher degree of reproducibility and a good material yield (more than 96%) suitable for large-scale production.

This paper concentrates on the microstructural features of steel containing 22 wt. % Cr, coated with Cr2O3 by laser processing. It turned out that after laser coating the Cr2O3 powder has completely transformed to Fe0.3Cr2.7O4 having the tetragonal distorted spinel structure. Dispersed in the coating are metallic particles with composition FeCr and a bcc structure. The phases in the coating can be explained from the Fe-Cr-O equilibrium phase diagram with the assumption that complete phase equilibrium is reached in the liquid state but not during solidification. The two equilibrium phases, L0 and Lm, that exist in the molten state solidify as two independent liquids. Addition of Si to the ceramic material, either from the steel matrix or from the ceramic powder, results in a dendritic solidification structure of Cr3O4 dendrites and a Si-containing glassy phase. The dendrites are oriented vertically in the coating, resembling the columnar microstructure that is also observed in ZrO2 thermal barrier coatings. This structure contains fewer microcracks parallel to the interface resulting in a mechanically more stable ceramic coating.

The effect of fiber orientation on the matrix plasticity and fracture behavior of SCS-6 fiber-reinforced Ti-15V-3Al-3Cr-3Sn composites was studied. The laminates used in this study were [0]6, [0/±45]s, and [90/±45]s. Three-point bending tests were conducted on chevron-notched specimens to determine the crack initiation energy, fracture toughness, and fracture strength as a function of notch length. The critical energy release rate was determined from the slope of the crack initiation energy versus notch length curve. The damage evolution and development of the matrix plastic deformation zone at the notch tip during the crack initiation and propagation as a function of fiber orientation were also determined. The relationships among the crack-tip matrix plastic deformation zone size, the critical energy release rate, and notch strength of the composites were discussed.

This paper is aimed at understanding the tribo-oxidation of a physical vapor-deposited TiN coating when sliding against a corundum ball. This is achieved through a compositional and structural analysis of the wear debris. Wear debris particles generated at three different sliding speeds were analyzed with micro-Raman spectroscopy, transmission electron microscopy, and electron probe microanalysis. The analysis showed that the wear debris when formed at the low and medium sliding speed consist of TiO2 with a nanocrystalline structure containing both anatase and rutile structural elements. Only rutile structural elements could be observed in the debris formed at the high sliding speed. These results on the characterization of the wear debris are interpreted with calculations of the flash temperature in the tribo-contact and with recent ball-on-disk results on the wear rate of TiN as a function of the sliding speed to propose a mechanistic view of the tribo-oxidation and wear process. The relation with previous and recent experimental results on the static oxidation of TiN is also given.

The electrical conductivity of 3Y-TZP ceramics containing SiO2 and Al2O3 has been investigated by complex impedance spectroscopy between 500 and 1270 K. At low temperatures, the total electrical conductivity is suppressed by the grain boundary glass films. The equilibrium thickness of intergranular films is 1-2 nm, as derived using the “brick-layer” model and measured by HRTEM. A change in the slope of the conductivity Arrhenius plots occurs at the characteristic temperature Tb at which the macroscopic grain boundary resistivity has the same value as the resistivity of the grains. The temperature dependence of the conductivity is discussed in terms of a series combination of RC elements.

Thin epitaxial films of β-SiC were grown by CVD on (100), (111), and (112) TiC substrates. TEM observations of the resulting interfaces revealed that island nucleation prevailed in the early stages of deposition for all three substrate orientations. Films grown on (111) and (112) TiC were monocrystalline, while SiC films deposited on (100) substrates were polycrystalline and not epitaxial, a phenomenon attributed to the poor match of atomic positions in SiC and TiC on their respective (100) planes. The (111) interface was abrupt and atomically flat, while the (112) interface exhibited {111} facets and steps. Simulated images of the stable (111) interface were calculated based on several possible atomic configurations, and the atomic structure of the interface was deduced from comparisons between the simulated images and phase-contrast TEM images.

The Al68Cu11Co21 decagonal phase was studied after annealing at 1000 °C for 24-760 h by transmission and scanning electron microscopy. The strong superstructure odd-n reflections in the [1,-2, 1, 0, 0] electron diffractograms were stable under annealing up to about 40 h. As a possible origin of the increased intensities of the odd-n reflections the formation of vacancy-ordered structures is discussed. The structure was modified by prolonged annealing. In several annealed samples a dense net of extra reflections overlapping the quasiperiodic reflections was observed. This observation was explained as an indication of the formation of metastable states during cooling. Differently ordered decagonal structures exhibited different transformation kinetics during cooling from high temperatures.

The reaction between the Ba-Cu-O melt (with a Ba to Cu ratio of 3-5 or 3-7) used for crystal pulling of Y-123 and several prospective crucible materials (MgO, Y2O3, Al2O3, and YSZ) has been investigated by dipping these materials into the melt at 985 °C and 1050 °C in air. For comparison, MgO, YSZ, LaAlO3, and SrTiO3 single crystals were also investigated. In the case of MgO single crystals, no wetting or reactive layer has been observed, although with increasing time an increasing amount of MgO has been dissolved into the melt. The contact angle between melt and MgO single crystal can be estimated to be about 40°, assuming a surface tension of 0.3 N/m. For all other materials, increasing time was accompanied by increasing wetting and/or an increasing reaction layer. For MgO polycrystals increased wetting with increasing time has been observed. For example, it takes about 30 min to wet the first 10 mm. The melt completely permeates samples with an apparent density of up to 85%. However, the permeated depth becomes very thin in samples with an apparent 95% density. It was found that Y2O3 is a better barrier to the melt at lower densities because even at 85% density the permeated layer is very thin, but the surface wetting occurs at a slightly faster rate for Y2O3 than for MgO.

The chemical stability of TiB2 and TiN in a silicon nitride matrix under various conditions of temperature and gaseous environments was investigated. The addition of TiB2 and TiN on the microstructure and mechanical properties was also studied. No trace of interactions between TiN and Si3N4 was noticed. The addition of TiB2 to Si3N4 enhanced conversion of the α to β phase of the Si3N4 matrix. Observations of BN and TiN indicated a possible reaction between TiB2 and Si3N4. The fracture toughness of Si3N4 was substantially enhanced with the addition of TiB2 or TiN, while the strength was decreased. Crack deflection was the major toughening mechanism in a Si3N4/TiB2 composite. Most of the microcracks passed through TiN particles and cleavaged along preferred orientations with large deflection angles.

Thermochemical calculations were carried out for the Self-propagating High-temperature Synthesis (SHS) of MoSi2, Mo5Si3, and their composites. Adiabatic temperatures and the amounts of liquid phases formed at the adiabatic temperature were calculated for different initial temperatures. These materials were prepared from elemental powders of Mo and Si by the thermal explosion mode of SHS. The products were characterized by x-ray diffraction, scanning electron microscopy, and microhardness. The larger particle morphology of MoSi2 and the MoSi2-Mo5Si3 composite and the finer morphology of Mo5Si3 can be explained in terms of the presence of transient liquid phase at the adiabatic temperature. The morphology of fracture surface of MoSi2 shows intergranular fracture, while Mo5Si3 and the MoSi2-Mo5Si3 composite show transgranular cleavage fracture.