Formation and Stability of Ferroelectric BaTi2O5

BaTi₂O₅ (BT₂) is thermodynamically stable over a very narrow temperature range between 1220° and 1230°C: a modification to the BaO–TiO₂ phase diagram is proposed. This thermodynamic stability was shown by constructing a time–temperature transformation diagram for the decomposition of BT₂. Once formed, BT₂ appears to be stable indefinitely at 1220°–1230°C; at higher temperatures, the decomposition rate increases with temperature; at lower temperatures, the decomposition rate increases with decreasing temperature and passes through a maximum at ∼1200°C; below ∼1150°C, BT₂ has long-lived kinetic stability. Kinetic considerations show a nucleation and growth mechanism for decomposition, with a nucleation induction period that is very temperature dependent. BT₂ can be prepared by various routes, including solid-state reaction of oxides below ∼1100°C; because it is metastable at all temperatures other than 1220°–1230°C, its formation is an example of Ostwald's rule of successive reactions. Discrepancies in the literature concerning the reported stability range of BT₂ can be explained by the complex dependence on temperature and time of both its formation and decomposition, for both of which, the nucleation stage is rate limiting.     

Synthesis and Characterization of Nanocrystalline Niobium Nitride Powders

Nanocrystalline NbN powder was synthesized by the direct nitridation of amorphous Nb₂O₅ powder with high BET surface area. X-ray diffractometry analysis indicated that the cubic-phase NbN powder could be obtained by nitridation at 650°–800°C for 3–8 h. Transmission electron microscopy images showed that the particle sizes were in the range of 15–40 nm. The effect of the nitridation temperature and holding time on the powder properties was discussed.     

Stability of Molybdenum Disilicide in Combustion Gak Environments

The stability of MoSi₂ in combustion gas at 1370° and 1600°C was evaluated using SOLGASMIX-PV thermodynamic modeling, periodic weight measurements, and characterization via XRD, SEM, EDS, and image analysis. Passive oxidation occurred at both temperatures. During an initial stage of exposure, specimen surfaces oxidized to form MoO_3(g) and amorphous SiO₂ via reduction of CO₂ and H₂O. After a short time (<6.5 min at 1370°C, <1 min at 1600°C), the oxidation mechanism switched; Mo₅Si₃ and amorphous SiO₂ formed as oxidation products. The first mechanism esulted in the formation of 46.1 vol% at 1370°C and 42.6 vol% at 1600°C of the amorphous silica surface coating. The attainment of a near-terminal weight gain implied silica formation was limited by H₂O and CO₂ diffusion through the silica coating.     

Synthesis and Mechanism of Nanosized AlN from an Aluminum Oleic Emulsion Using Carbothermal Reduction at Low Temperatures

Nanosized Al₂O₃ particles homogeneously dispersed in a matrix of amorphous carbon (a-C) were prepared by decomposition of an aluminum oleic emulsion at 600°C in Ar. Nanosized aluminum nitride (AlN) grains were prepared by carbothermal reduction and nitridation (CRN) of this Al₂O₃–a-C mixture in NH₃ using graphite, BN, and alumina crucibles or boats. The phases formed by CRN were identified by X-ray diffraction analysis. The morphology and grain size of the AlN were determined by transmission electron microscopy. The formation of single-phase AlN was achieved at temperatures as low as 1150°–1200°C in NH₃ using a cylindrical graphite crucible with holes in its two flat faces. Mass spectroscopy (MS) showed that a significant amount of HCN and a minor amount of C₂H₂ are formed at 500°C by reaction of NH₃ with carbon at the decomposition temperature of NH₃. A most probable formation mechanism of the AlN from nanosized Al₂O₃ and a-C in NH₃ is discussed on the basis of MS results and thermodynamic considerations.     

Synthesis of Nanocrystalline Cubic Tantalum(III) Nitride Powders by Nitridation–Thermal Decomposition

Tantalum(V) nitride, prepared by nitridation of nanosized Ta₂O₅ at 800°C for 8 h under ammonia flow, was thermally decomposed to cubic nanocrystalline TaN at a temperature of 1000°C for 3 h under argon atmosphere. The resulting powders have been characterized using X-ray diffraction (XRD) and transmission electron microscopy. XRD-pure cubic TaN nanoparticles with a diameter of 50–100 nm can be obtained by the process. The decomposition process was found to depend on the temperature. Mechanisms that account for the decomposition of Tantalum(V) nitride are discussed. The results indicate that the method can permit formation of cubic-phase Tantalum(III) nitride under ambient pressure and moderate temperatures.     

Synthesis of Oxynitride Powders via Fluidized-Bed Ammonolysis, Part I: Large, Porous, Silica Particles

Reaction of silica (SiO₂) with triethanolamine (TEA, N(CH₂CH₂OH)₃) and ethylene glycol (EG) under conditions (∼200°C) where byproduct water is removed resulted in the formation of the neutral silatrane glycolate complex, N(CH₂CH₂O)₃SiOCH₂CH₂OH (or TEASiOCH₂CH₂OH) in essentially quantitative yield. Solutions of this neutral precursor in EG, when rapidly pyrolyzed and then oxidized at 500°C, formed porous ceramic powders with high specific surface areas (>500 m²/g). These powders were nitrided via ammonolysis in a fluidized-bed reactor at temperatures of 700°-1000°C. The resulting nitrided powders were characterized by thermal and chemical analyses, diffuse reflectance infrared spectroscopy, gas sorption, and X-ray photoelectron spectroscopy. The apparent activation energy for the nitridation process was determined to be 54 kJ/mol. Following nitridation, the powders were amorphous and had nitrogen contents as high as 21 wt% with retained surface areas >300 m²/g at 1000°C. Under the nitridation conditions used, the extent of nitrogen incorporation correlated linearly with increases in material density. This linearity suggested that the change in density occurred primarily because of changes in coordination that occurred as trivalent nitrogen replaced divalent oxygen in the glass structure and nominally because of viscous flow. The linear density increase also suggested that pore trapping did not occur under these processing conditions. This work serves as a model for ongoing studies on the nitridation of high-surface-area ceramic powders produced by the rapid pyrolysis of mixed-metal TEA alkoxides.     

Preparation of Nanosized Perovskite-Type LaMnO3 Powders Using the Thermal Decomposition of a Heteronuclear Complex, LaMn(dhbaen)(OH)(NO3)(H2O)4

The heteronuclear LaMn(dhbaen)(OH)(NO₃)(H₂O)₄ complex was synthesized and perovskite-type hexagonal LaMnO₃ was obtained by its thermal decomposition at approximately 700°C. The complex and its decomposition products were analyzed using simultaneous thermogravimetric and differential thermal analysis (TG/DTA), X-ray diffraction (XRD) analysis, Fourier-transform infrared (FTIR) spectroscopy, Auger electron spectroscopy (AES), transmission electron microscopy (TEM) characterization, and specific surface area measurements. Although XRD analysis did not show the peaks of LaMnO₃ for the sample sintered at 600°C, the presence of polycrystalline LaMnO₃ together with an amorphous phase was confirmed by TEM-selected area diffraction. Particle sizes of the samples decomposed at 600° and 700°C were 20 and 50 nm, respectively. For the conventional solid-state reaction method, XRD results showed the formation of a LaMnO₃ single phase for the samples fired above 1000°C. However, AES showed that the elemental distributions of La, Mn, and O on the surface were not homogeneous even for the sample sintered at 1200°C. The thermal decomposition of the heteronuclear complex at low temperatures allows the synthesis of single-phase hexagonal LaMnO₃ powders having nanosized particles, homogeneous and free of intragranular pores, which are suitable for electroceramics applications.     

Synthesis and Characterization of Amorphous Si2N2O

Amorphous silicon oxynitride powder was synthesized by nitridation of high-purity silica in ammonia at 1120°C. The resulting material was X-ray amorphous, and its chemical characteristics were determined by X-ray photoelectron spectroscopy (XPS) and ²⁹Si nuclear magnetic resonance (NMR). The XPS analysis showed a shift to lower binding energies for the Si₂ p peak with increasing nitrogen content. Upon initial nitridation, the full width at half maximum (FWHM) of the Si₂ p peak increased, but decreased again at higher nitrogen contents, thus showing the formation of a silicon oxynitride phase with a single or small range of composition. The ²⁹Si NMR analysis showed the formation of (amorphous) Si₃N₄ (Si–N₄) and possibly two oxynitride phases (Si–N₃O, Si–N₂O₂). It is concluded that while XPS, FT-IR, and nitrogen analysis may show the formation of an homogeneous, amorphous silicon oxynitride (Si₂N₂O) phase, the formation of phase–pure, amorphous Si₂N₂O is extremely difficult via this route.     

Surface Chemistry and Structure of Ultrafine Silicon Carbide: An FT-IR Study

Using transmission FT-IR, surface characterizations are performed on ultrafine SiC powders produced by a laser-driven method. The surface species sensitive to thermal treatments (OH, CH _{x ,}, C = O, SiH _x ) are identified on samples evacuated at various temperatures. Absorption bands attributed to overtones of the fundamental Si—C modes are also present in the IR spectra and remain unchanged after treatment. The reaction of SiC with oxygen and water vapor produces a layer of silica on the sample and gaseous CO₂; the reaction with ammonia results in a partial nitridation of the surface, with the formation of NH _x groups that apparently increase the stability of the SiC against oxidation; and reaction with hydrogen produces methane in the gas phase and causes the disappearance of the bands due to surface CH _x groups.     

Outstanding Problems in the Kaolinite-Mullite Reaction Sequence Investigated by 29Si and 27Al Solid-state Nuclear Magnetic Resonance: 11, High-Temperature Transformations of Metakaolinite

Solid-state ²⁹Si and ²⁷Al NMR spectra of kaolinite fired at 800° to 1450°C, interpreted in light of a newly proposed metakaolinite structure and complementary X-ray diffraction results, lead to the following conclusions about the hightemperature reactions: (1) Removal of the final residual hydroxyl radicals of metakaolinite at ∼9707deg;C triggers the separation of a considerable amount of amorphous free silica and the formation of poorly crystalline mullite and a spinel phase. (2) Mullite and spinel form in tandem, the former originating in the vicinity of AI-0 units of regular octahedral and tetrahedral symmetry randomly distributed throughout the metakaolinite structure. (3) The initially formed mullite is alumina-rich but at higher temperatures progressively gains silica, approaching the conventional 3Al₂O₃· 2SiO₂ composition. (4) The spinel phase contains insufficient Si to be detected by ²⁹Si NMR but has a ²⁷Al NMR spectrum consistent with γ-Al₂O₃. On further heating, the spinel is converted to mullite by reaction with some of the amorpholls silica, the balance of which eventually becomes cristobalite.     

Formation of Nanocrystalline Transition-Metal Ferrites inside a Silica Matrix

Nanocrystalline transition-metal ferrites were synthesized inside an amorphous silica matrix by the sol–gel method. The formation of spinel ferrites began above 400°C, giving fine particles of about 10 nm at 800°C. This is associated with a specific role of the silica matrix, which facilitates the diffusion of the reacting cations, enhancing the ferrite formation. Above 1000°C the MnFe₂O₄ and CuFe₂O₄ nanoparticles lost their fine nature. The dried gels and crystalline materials were characterized by X-ray diffraction, thermal, FTIR, and BET analyses as well as by high-resolution scanning transmission electron microscopy.     

Mullite Joining by the Oxidation of Malleable, Alkaline-Earth-Metal-Bearing Bonding Agents

Metallic Ba-Al-Si bonding agents have been used to produce all-ceramic, BaO-Al₂O₃-SiO₂ bonds between plates of mullite (Al₆Si₂O₁₃). Ba-Al-Si tapes (200 (μm thick) were fabricated by compaction and rolling of mechanically alloyed powder. The Ba-Al-Si tapes were placed between mullite plates and then oxidized by heating to a peak temperature of 1230°C in air. The oxidized tapes strongly adhered to the mullite plates at 25° and 1000°C, as indicated by the fracture morphologies obtained from compressive shear tests. Electron microscopy (EPMA, TEM) revealed that the bulk of the oxidized Ba-Al-Si tapes (away from the interfaces with mullite) consisted largely of the compound BaAl₂Si₂O₈, along with some BaSiO₃ and an amorphous, barium-rich aluminosilicate. The interface between the oxidized bonding agent and bulk mullite consisted of a mixture of BaAl₂Si₂O₈, Al₆Si₂O₁₃, A1₂O₃, BaAl₂O₄, and an amorphous, barium-bearing aluminosilicate.     

Reactions between Hot-Pressed Calcium Hexaluminate and Silicon Carbide in the Presence of Oxygen

The reactions between hot-pressed calcium hexaluminate (CaAl₁₂O₁₉, hibonite) and silicon carbide (SiC) at 1100°-1400°C in air and nominal argon atmospheres were investigated. In inert atmospheres, there was no evidence of reaction at temperatures up to at least 1400°C. In air, the oxidation of SiC produced a layer of silica or a multicomponent amorphous silicate (depending on impurities) that reacted with CaAl₁₂O₁₉. At temperatures below 1300°C, the reaction resulted in the stratification of two distinct interfacial layers: a partially devitrified CaO-Al₂O₃-SiO₂ glass adjacent to SiC and a CaAl₂Si₂O₈ (anorthite) layer adjacent to hibonite. At 1400°C, a large amount of liquid was formed, the majority of which was squeezed out from between the reaction couple. No distinct layer of anorthite was present; instead, the anorthite was replaced by a layer of alumina between the glass-rich layer and hibonite. An activation energy of 290 kJ/mol was determined for the reaction, which is consistant with oxygen diffusion through a calcium aluminosilicate glass. The reaction between rare-earth hexaluminates and SiO₂ was predicted to produce a more-viscous glass than CaAl₁₂O₁₉ and SiO₂ and, therefore, have slower reaction kinetics, because of lower mass transport in the glass.     

Reaction and Formation of Crystalline Silicon Oxynitride in Si–O–N Systems under Solid High Pressure

Oxidized amorphous Si₃N₄ and SiO₂ powders were pressed alone or as a mixture under high pressure (1.0–5.0 GPa) at high temperatures (800–1700°C). Formation of crystalline silicon oxynitride (Si₂ON₂) was observed from amorphous silicon nitride (Si₃N₄) powders containing 5.8 wt% oxygen at 1.0 GPa and 1400°C. The Si₂ON₂ coexisted with β-Si₃N₄ with a weight fraction of 40 wt%, suggesting that all oxygen in the powders participated in the reaction to form Si₂ON₂. Pressing a mixture of amorphous Si₃N₄ of lower oxygen (1.5 wt%) and SiO₂ under 1.0–5.0 GPa between 1000° and 1350°C did not give Si₂ON₂ phase, but yielded a mixture of α,β-Si₃N₄, quartz, and coesite (a high-pressure form of SiO₂). The formation of Si₂ON₂ from oxidized amorphous Si₃N₄ seemed to be assisted by formation of a Si–O–N melt in the system that was enhanced under the high pressure.     

Thermal Decomposition of Titanium Alkoxide and Silicate Ester in Organic Solvent: A New Method for Synthesizing Large-Surface-Area, Silica-Modified Titanium(IV) Oxide of High Thermal Stability

Silica-modified titanium oxide (S-TiO₂) powders that have an anatase structure were synthesized via the thermal decomposition of mixtures of titanium(IV) isopropoxide (TIP) and tetramethyl orthosilicate (TMOS) in toluene at 300°C. These S-TiO₂ materials had high rutile-transformation temperatures and maintained large surface areas at elevated temperatures (550°–1000°C). For example, the product that was prepared from a 9:1 TIP:TMOS mixture transformed to rutile at ∼1100°C and possessed a surface area of 160 m²/g, even after calcination at 800°C for 1 h.     

Hot Isostatic Pressing and Characterization of Sol-Gel-Derived Chromium(III) Oxide

Chromium (III) oxide (Cr₂O₃) crystallizes at low temperatures from an amorphous material prepared by adding hydrazine monohydrate to an aqueous solution of Cr(NO₃)₃-9H₂O. Individual particles of Cr₂O₃ tend toward a hexagonal morphology above 800°C. Well-densified Cr₂O₃ pellets (98.8% of theoretical density) have been fabricated by hot isostatic pressing for 2 h at 1100°C and 196 MPa. Their fracture toughness is 4.4 MPa.m^1/2. The sample annealed in air for 12 h at 1300°C exhibits a high electrical conductivity of 3.6 Ω^-1.m^-1at 700°C.     

Mechanochemical Activation-Assisted Low-Temperature Synthesis of CaZrO3

Calcium zirconate (CaZrO₃, CZ) was prepared using a solid-state reaction with mechanochemical activation through vibro-milling, aiming at completing the reaction CaO+ZrO₂=CaZrO₃ at relatively low calcination temperatures. Changes in the crystallite size and homogeneity of the mixed components CaO and ZrO₂ in the starting mixtures were observed with different milling times. The influence of milling on the incipient temperature of CZ formation and completion of CZ formation was investigated. It is concluded that milling of the reactants for 20 h lowered the incipient temperature of CZ formation from 800° to 600°C, and the temperature of complete CZ formation from above 1100° to 800°C.     

Thermochemical Reactions Between PZT and Ag/Pd Powders: Relevance to Cofiring of Multilayer Actuators

Mixtures of Ag_{1− x} Pd _x ( x =0.2, 0.3) and doped PZT ceramic powders have been heat treated in air and the resulting phase content has been analyzed by X-ray diffraction and transmission electron microscopy. Beginning around 400°C, a phase similar to PbPdO₂ is formed on the surface of the Ag_{1− x} Pd _x particles and subsequently decomposes at temperatures <700°C. Consequently, the remaining Ag_{1− x} Pd _x powder becomes significantly silver-rich while the reaction progresses. After decomposition the Pd appears to realloy and the initial Ag_{1− x} Pd _x composition is recovered. We show that the reaction is all but eliminated in a nitrogen atmosphere. The occurrence of this reaction was also investigated in PZT multilayer actuators cofired with Ag_0.7Pd_0.3 electrodes. Transmission electron microscope analysis revealed the presence of distinct crystallites at the electrode–ceramic interface, most likely nucleated from a PbO liquid phase arising from the decomposition of PbPdO₂.     

Influence of Processing Conditions on the Electrical Properties of CaCu3Ti4O12 Ceramics

The electrical properties of a series of CaCu₃Ti₄O₁₂ ceramics prepared by the mixed oxide route and sintered at 1115°C in air for 1–24 h to produce different ceramic microstructures have been studied by Impedance Spectroscopy. As-fired ceramics are electrically heterogeneous, consisting of semiconducting grains and insulating grain boundaries, and can be modelled to a first approximation on an equivalent circuit based on two parallel RC elements connected in series. The grain boundary resistance and capacitance values vary as a function of sintering time and correlate with the ceramic microstructure based on the brickwork layer model for electroceramics. The large range of apparent high permittivity values for CaCu₃Ti₄O₁₂ ceramics is therefore attributed to variations in ceramic microstructure. The grain-boundary resistance decreases by three to four orders of magnitude after heat treatment in N₂ at 800°–1000°C but can be recovered to the original value by heat treatment in O₂ at 1000°C. The bulk resistivity decreases from ∼80 to 30 Ω·cm with increasing sintering time but is independent of heat treatment in N₂ or O₂ at 800°–1000°C. The origin of the bulk semiconductivity is discussed and appears to be related to partial decomposition of CaCu₃Ti₄O₁₂ at the high sintering temperatures required to form dense ceramics, and not to oxygen loss.     

Mesoporous Silicon Nitride Synthesis Via the Carbothermal Reduction and Nitridation of Carbonized Silica/RF Gel Composite

Silicon nitride (Si₃N₄), a high-temperature structural ceramic, was synthesized in a mesoporous form by a simple approach. At first, silica/resorcinol–formaldehyde (RF) gel was formed via sol–gel polycondensation process, using resorcinol and formaldehyde as sources for porous RF structure and amino propyl trimethoxysilane as a precursor for silica. Pyrolysis of the dried gel at 250°C for 2 h following by at 750°C for 4 h resulted in silica/carbon composite that could be converted into mesoporous Si₃N₄ or Si₃N₄/silicon carbide composite via the carbothermal reduction and nitridation process at 1450°C. Significant increase in surface area of the products, comparing with that of the conventional Si₃N₄ granules, was observed. The content of silica in the starting composite was found to be a critical factor influencing both phase and porosity of the obtained product.