Conversion of a Precursor Derived from Cage-Type and Cyclic Molecular Building Blocks into Al-Si-N-C Ceramic Composites

A precursor derived from (HAlN ⁱ Pr) _m and [MeSi(H)NH] _n , which mainly consisted of cage-type compounds and cyclic compounds, respectively, was converted into Al-Si-N-C ceramic composites via pyrolysis. A dehydrocoupling reaction between AlH groups and NH groups occurred at low temperatures (≤∼250°C), which mainly accounts for its high ceramic yield (69% up to 900°C). At high temperatures (≥∼250°C), the organic groups were decomposed. The product pyrolyzed at 1350° and 1500°C under Ar contained a 2H wurtzite-type compound and a β-Si₃N₄-type compound, while β-SiC was clearly detected in addition to these compounds in the product pyrolyzed at 1600°C under Ar. On the other hand, the product pyrolyzed at 800°C under NH₃ and subsequently at 1350°C under N₂ consisted of AlN and β-Sialon.     

Synthesis of Ultrafine β"-Alumina Powders via Flame Spray Pyrolysis of Polymeric Precursors

Flame spray pyrolysis of a polymeric precursor is used to prepare ultrafine powders that, when sintered, convert to essentially pure phase lithium-doped sodium β"-alumina. The precursor Na_1.67 Al_10.67 Li_0.33 [N(CH₂CH₂O)₃]_10.67-[OCH₂CH₂O]·x(HOCH₂CH₂OH) has been synthesized from stoichiometric amounts of metal hydroxides and tri-ethanolamine (N(CH₂CH₂OH)₃, TEA) in excess ethylene glycol. The precursor is dissolved in ethanol, and an atom-ized spray of the solution is combusted in a specially con-structed flame spray apparatus. Combustion occurs at ∼2000°C, followed by immediate quenching. This proce-dure provides for a measure of kinetic control over the process. The resulting nanopowder particles are 50–150 nm in diameter and exhibit powder X-ray diffractometry pat-terns similar to β"-alumina. Heating the nanopowder at 30°C/min to 1200°C with a 1 hisotherm converts it to pure β"-alumina. In preliminary sintering studies, green powder compacts (∼65% theoretical density) sintered at 1600°C for 12 min densify to 3.0 ± 0.1 g/cm 3 (∼92% theoretical density) with minimal loss of Na₂O. This procedure offers several processing and cost advantages over conventional β"-alumina syntheses.     

High-Temperature Strength of Liquid-Phase-Sintered SiC with AlN and Re2O3 (RE = Y, Yb)

Using AlN and RE₂O₃ (RE = Y, Yb) as sintering additives, two different SiC ceramics with high strength at 1500°C were fabricated by hot-pressing and subsequent annealing under pressure. The ceramics had a self-reinforced microstructure consisting of elongated α-SiC grains and a grain-boundary glassy phase. High-temperature strength up to 1600°C was measured and compared with that of the SiC ceramics fabricated with AlN and Er₂O₃. SiC ceramics with AlN and Y₂O₃ showed the best strength (∼630 MPa) at 1500°C, while SiC ceramics with AlN and Er₂O₃ the best strength (∼550 MPa) at 1600°C.     

Double Precursor Solution Coating Approach for Low-Temperature Sintering of [Pb(Mg1/3Nb2/3)O3]0.63[PbTiO3]0.37 Solids

Lead-based piezoelectric ceramics typically require sintering temperatures higher than 1000°C at which significant lead loss can occur. Here, we report a double precursor solution coating (PSC) method for fabricating low-temperature sinterable polycrystalline [Pb(Mg_1/3Nb_2/3)O₃]_0.63-[PbTiO₃]_0.37 (PMN–PT) ceramics. In this method, submicrometer crystalline PMN powder was first obtained by dispersing Mg(OH)₂-coated Nb₂O₅ particles in a lead acetate/ethylene glycol solution (first PSC), followed by calcination at 800°C. The crystalline PMN powder was subsequently suspended in a PT precursor solution containing lead acetate and titanium isopropoxide in ethylene glycol to form the PMN–PT precursor powder (second PSC) that could be sintered at a temperature as low as 900°C. The resultant d ₃₃ for samples sintered at 900°, 1000°, and 1100°C for 2 h were 600, 620, and 700 pm/V, respectively, comparable with the known value. We attributed the low sintering temperature to the reactive sintering nature of the present PMN–PT precursor powder. The reaction between the nanosize PT and the submicrometer-size PMN occurred roughly in the same temperature range as the densification, 850°–900°C, thereby significantly accelerating the sintering process. The present PSC technique is very general and should be readily applicable to other multicomponent systems.     

Interdiffusion in SiC–AlN and AlN–Al2OC Systems

AlN, Al₂OC, and the 2 H form of SiC are isostructural. Both SiC–AlN and AlN–Al₂OC form homogeneous solid solutions above 2000° and 1950°C, respectively. The kinetics of phase separation in the two systems, however, are quite different. Interdiffusion in both SiC–AlN and AlN-Al₂OC systems was examined in the solid-solution regime in an attempt to elucidate differences in the kinetics of phase separation that occur in the two systems when annealed at lower temperatures. Diffusion couples of (SiC)_0.3(AlN)_0.7/(SiC)_0.7(AlN)_0.3 and (AlN)_0.7(Al₂OC)_0.3/(AlN)_0.3(Al₂OC)_0.7 were fabricated by hot pressing and were annealed at high temperatures by encapsulating them in sealed SiC crucibles to suppress loss due to evaporation. Interdiffusion coefficients in (SiC)_0.3-(AlN)_0.7/(SiC)_0.7(AlN)_0.3 diffusion couples were measured at 2373, 2473, and 2573 K, and the corresponding activation energy was determined to be 632 kJ/mol. (AlN)_0.7(Al₂OC)_0.3/ (AlN)_0.3(Al₂OC)_0.7 samples were annealed at 2273 K. The interdiffusion coefficient measured in the AlN–Al₂OC system was much larger than that in the SiC–AlN system.     

Processing Aluminum Nitride-Silicon Carbide Composites via Polymer Infiltration and Pyrolysis of Polymethylsilane, a Precursor to Stoichiometric Silicon Carbide

AlN-SiC-particle-reinforced composites have been prepared at lessthan equal to1400°C using submicrometer AlN, -325 mesh alpha-SiC particles, and polymethylsilane (PMS; -(CH₃SiH) _n -) via a polymer infiltration and pyrolysis (PIP) process. PMS is an organometallic SiC polymer precursor that can be modified with 16 wt% cross-linking aid to provide mPMS. mPMS converts to nanocrystalline β-SiC with >80% ceramic yield (1000°C in argon) with some excess (<5 wt%) graphitic carbon. mPMS has been used successfully as a nonfugitive binder for AlN-SiC compacts. Densities of 2.5 versus 2.1 g/cm³ have been obtained after nine PIP cycles for disk-shaped compacts formulated with and without mPMS binder, respectively. alpha-SiC seeds crystallization of β-SiC derived from mPMS at temperatures as low as 1000°C. Some evidence suggests that AlN-SiC solid solutions form at particle/matrix interfaces.     

Phase Relations and Microstructural Development of Aluminum Nitride–Aluminum Nitride Polytypoid Composites in the Aluminum Nitride–Alumina–Yttria System

AlN–AlN polytypoid composite materials were prepared in situ using pressureless sintering of AlN–Al₂O₃ mixtures (3.7–16.6 mol% Al₂O₃) using Y₂O₃ (1.4–1.5 wt%) as a sintering additive. Materials fired at 1950°C consisted of elongated grains of AlN polytypoids embedded in equiaxed AlN grains. The Al₂O₃ content in the polytypoids varied systematically with the overall Al₂O₃ content, but equilibrium phase composition was not established because of slow nucleation rate and rapid grain growth of the polytypoid grains. The polytypoids, 24 H and 39 R , previously not reported, were identified using HRTEM. Solid solution of Y₂O₃ in the polytypoids was demonstrated, and Y₂O₃ was shown to influence the stability of the AlN polytypoids. The present phase observations were summarized in a phase diagram for a binary section in the ternary system AlN–Al₂O₃–Y₂O₃ parallel to the AlN–Al₂O₃ join. Fracture toughness estimated from indentation measurements gave no evidence for a strengthening mechanism due to the elongated polytypoids.     

Low-Temperature Synthesis of Aluminum Silicon Carbide Using Ultrafine Aluminum Carbide and Silicon Carbide Powders

The conditions for preparing α-aluminum silicon carbide (α-Al₄SiC₄) were examined by heating stoichiometric mixtures of ultrafine A1₄C₃ and SiC powders with sizes of <0.1 μm at and below 1600°C. The starting A1₄C₃ powder was obtained by the pyrolysis of trimiethylaluminum; the starting SiC powders were obtained by the pyrolyses of triethylsilane (3ES), tetraethylsilane (4ES), and hexamethyldisilane (6MDS). The reactivity of SiC with Al₄C₃ to form α-Al₄SiC₄ varies according to the kind of starting alkylsilane: 3ES > 4ES > 6MDS. The reaction of 3ES-derived SiC with A1₄C₃ produced α-Al₄SiC₄ at temperatures as low as 1400°C for 240 min, regardless of the presence of A1₄C₃ (trace). Only α-Al₄SiC₄ was formed at and above 1500°C for 60 min; the crystal growth was appreciable.     

Synthesis of Ba2NaNb5O15 Powders and Thin Films Using Metal Alkoxides

Transparent and highly oriented Ba₂NaNb₅O₁₅ (BNN) thin films have been prepared by using metal alkoxides. A homogeneous precursor solution was prepared by the controlled reaction of NaOC₂H₅, Nb(OC₂H₅)₅, and barium metal. The BNN precursor included a molecular-level mixture of NaNb(OC₂H₅)₆ and Ba[Nb(OC₂H₅)₆]₂ in ethanol. The alkoxy-derived powder crystallized to a low-temperature phase, and then transformed to orthorhombic BNN (tungsten bronze) at 600°C. BNN precursor films on substrates crystallized to orthorhombic BNN at 800°C via the low-temperature phase. Highly (002) oriented BNN films of tungsten bronze structure were successfully prepared on MgO (100) substrates at 700°C by using BNN underlayer.     

Preparation of Tungsten Cobalt Carbides and Oxide via an Organometallic Heterobimetallic Complex as a Single-Source Precursor

Tungsten cobalt carbides and oxides can be obtained via a single-step pyrolysis of an organometallic single-source precursor (eta⁵-C₅H₅)(CO)₃WCo(CO)₄ (1). Pyrolysis of 1 in an oxygen atmosphere produced WCoO₄ at 600°C. In a nitrogen atmosphere W₆Co₆C was obtained when 1 was heated at 700°C. However, under vacuum, the pyrolysis of 1 produced the other phase-W₃Co₃C-at 700°C. Both carbides were contaminated with graphitic carbon, as indicated by their ESCA spectra. Powders that were obtained by using these procedures had particle sizes of up to 100 µm. Micrography showed that the particles were porous, which indicated outgassing during pyrolysis.     

Some Properties of Aluminum Carbide Powder Prepared by the Pyrolysis of Alkylaluminu

Ultrafine aluminum carbide (Al₄C₃) powders with crystallite sizes of <40 nm were prepared by the pyrolyses of alkylaluminums, i.e., trimethylaluminum (Al(CH₃)₃: TMAL), triethylaluminum (Al(C₂H₅)₃: TEAL), triisobutylaluminum (Al(i-C₄H₉)₃: TIBAL) at a temperature between 950° and 1100°C. Although the pyrolysis of TMAL produced Al₄C₃ at 950°C, the pyrolysis temperature of TEAL to produce Al₄C₃ was raised up to 1100°C. The pyrolysis of TIBAL at 1100°C produced not only crystalline Al₄C₃ but also amorphous oxycarbide. The TEAL-derived powder had the highest true density (2.89 g^.cm⁻³ or 97% of the theoretical density) among the three kinds of powders.     

Aluminum Nitride Prepared by Nitridation of Aluminum Oxide Precursors

Aluminum nitride (AlN) powders were prepared from the oxide precursors aluminum nitrate, aluminum hydroxide, aluminum 2-ethyl-hexanoate, and aluminum isopropoxide (i.e., Al(NO₃)₃, Al(OH)₃, Al(OH)(O₂CCH(C₂H₅)(C₄H₉))₂, and Al(OCH(CH₃)₂)₃). Pyrolyses were performed in flowing dry NH₃ and N₂ at 1000°–1500°C. For comparison, the nitride precursors aluminum dimethylamide (Al(N(CH₃)₂)₃) and aluminum trimethylamino alane (AlH₃·N(CH₃)₃) were exposed to the same nitridation conditions. Products were investigated using XRD, TEM, EDX, SEM, and elemental analysis. The results showed that nitridation was primarily controlled by the water:ammonia ratio in the atmosphere. Single-phase AlN powders were obtained from all oxide precursors. Complete nitridation was not obtained using pure N₂, even for the non-oxide precursors.     

Electroceramics from Source Materials via Molecular Intermediates: PbTiO3 from TiO2 via [Ti(catecholate)3]2-

The precursor [NH₄]₂[Ti(catecholate)₃] · 2H₂O is known to react with Ba(OH)₂· 8H₂O in an acid/base process that generates Ba[Ti(catecholate)₃] · 3H₂O, a compound which undergoes low-temperatue calcination to produce BaTiO₃ powder. Attempts to develop similar routes to PbTiO₃ have been frustrated, since lead(II) hydroxide does not exist. The amphoteric yellow PbO and the basic oxide, Pb₆O(OH)₆⁴⁺, are both insufficiently basic to react with [NH₄]₂[Ti(catecholate)₃] · 2H₂O. Based on the large sizes of both the [Ti(catecholate)₃]^2- anion and the Pb²⁺ cation, a precipitation method has been developed in which lead nitrate and [NH₄]₂[Ti(catecholate)₃] · 2H₂O are added together in an aqueous medium causing precipitation and leaving only NH₄NO₃ in solution. The lead-titanium-catecholate complex that forms in this manner undergoes low-temperature pyrolysis to produce PbTiO₃. SEM indicates a submicrometer ultimate crystallite size.     

Hydrolytic Condensation of Tin(IV) Alkoxide Compounds to Form Particles with Well-Defined Morphology

The sol-gel-type condensation of tin(IV) ethoxide [Sn(OEt)₄] n (where OEt is ethoxide) under basic conditions produced spherical, submicrometer-sized tin(IV) oxide (cassiterite) particles. Transmission electron microscopy and powder X-ray diffraction data indicated that the grain size was approximately 20 to 30 Å (2 to 3 nm). The mixed-metal alkoxide compound [ZnSn(OEt)₆] was hydrolyzed under analogous conditions to give either spherical or octahedral submicrometer-sized crystalline particles of ZnSn(OH)₆ depending on the solvents used. These data demonstrated that the stoichiometry of the mixed-metal alkoxide precursor was retained during condensation. Thermal treatment of ZnSn(OH)₆ resulted in crystallization of ZnSnO₃ at approximately 676°C. At neutral pH, hydrolysis of [ZnSn(OEt)₆] resulted in formation of a high surface area (261 m²/g) amorphous powder.     

Oxidation Kinetics and Composite Scale Formation in the System Mo(Al,Si)2

The oxidation kinetics of hot-pressed Mo(Al_0.01Si_0.99)₂ and Mo(Al_0.1Si_0.9)₂ were measured at 480°C, and between 1200° and 1600°C. The qualitative oxidation of arc-melted Mo(Al_0.1Si_0.9)₂, Mo(Al_0.3Si_0.7)₂, Mo(Al_0.5Si_0.5)₂, and Mo₃Al₈ was examined after 600°C for 1000 h in air. At all temperatures, the compositional difference between the materials yielded very different oxidation rates and scale microstructures. At 1400° and 1500°C, microstructural evolution of the oxide scales resulted in improved oxidation resistance at long times (>400 h). At these temperatures, a significant reduction in the long-time oxidation kinetics was correlated with the in situ formation of an inner mullite scale. At 480° and 600°C, oxidation resistance improved significantly with increasing aluminum concentration. Contrary to the behavior of MoSi₂, samples of Mo(Al_0.01Si_0.99)₂ did not demonstrate catastrophic oxidation, and samples of Mo(Al_0.1Si_0.9)₂ were very oxidation resistant.     

Spark Plasma Sintering of Nano-Sized TiN Prepared from TiO2 by Controlled Hydrolysis of TiCl4 and Ti(O-i-C3H7)4 Solution

Nano-sized TiO₂ powders were prepared by controlled hydrolysis of TiCl₄ and Ti(O-i-C₃H₇)₄ solutions and nitrided in flowing NH₃ gas at 700°–1000°C to form TiN. Nano-sized TiN was densified by spark plasma sintering at 1300°–1600°C to produce TiN ceramics with a relative density of 98% at 1600°C. The microstructure of the etched ceramic surface was observed by SEM, which revealed the formation of uniformly sized 1–2 μm grains in the TiCl₄-derived product and 10–20 μm in the Ti(O-i-C₃H₇)₄-derived TiN. The electric resisitivity and Vickers micro-hardness of the TiN ceramics was also measured.     

Alternative Route for Synthesis of Barium Titanyl Oxalate: Molecular Precursor for Macrocrystalline Barium Titanate Powders

An important molecular precursor to barium titanate, namely, barium titanyl oxalate [BaTiO(C₂O₄)₂.4H₂O], has been synthesized by an alternative route. An alcoholic solution containing 1 mol of butyl titanate monomer [(C₄H₉O)₄Ti] is reacted with alcoholic solution containing 2 mol of oxalic acid (H₂C₂O₄:2H₂O) to form an intermediate soluble oxalotitanic acid [H₂TiO(C₂O₄)₂.nH₂O]. The oxalotitanic acid in alcoholic medium is subjected to cation exchange reaction with aqueous solution containing equimolar barium acetate to form an insoluble barium titanyl oxalate (BTO) in yields of 80–85% at room temperature. The pyrolysis of BTO in air at T .750°C/5 h produced barium titanate (BT) powders.     

Pyrolysis of Poly(isopropyliminoalane) to Aluminum Nitride

The pyrolysis processes of poly(isopropyliminoalane) ((HAlN ⁱ Pr) _n ) were investigated, using mass spectrometry to analyze the gases and infrared spectroscopy to analyze the residual solids. The major mass loss (in the temperature range of 240°–540°C) consisted of two different pyrolysis stages. At the first stage (240°–320°C), (HAlN ⁱ Pr)₆ was detected continuously as a gas, and the precursor was converted to a cross-linked structure. A polymerization mechanism without a release of organic compounds has been proposed, and the formation of (HAlN ⁱ Pr)₆ during polymerization (besides its evaporation) has been suggested. The second stage (320°–560°C) involved the formation of various organic compounds, and radical processes for their formation were proposed.     

REVISION OF CRYSTALLINE PHASES IN THE SYSTEM ZRO2-P2O5

Crystalline phases in the system ZrO₂-P₂O_s were reexamined. Samples of varied composition were prepared by the sol-gel route and calcining, as well as by reacting mixtures ofZrO₂ and NH4H2PO4, at temperatures from 200° to 1400°C. Confusion regarding compositions and structures was clarified, and the phase Zr₃(PO₄)₄ was confirmed. The latter can be written Zro.₂sZr₂(PO₄)₃ and is iso-structural withNaZr₂P₃O₁₂. The role of mineralizers and the influence of zirconium salts can now be understood.     

Preparation of Mesoporous Silicon Nitride via a Nonaqueous Sol–Gel Route

A silicon diimide gel Si(NH) _x (NH₂) _y (NMe₂) _z was prepared by an acid-catalyzed ammonolysis of tris(dimethylamino)silylamine. Pyrolysis of the gel at 1000°C under NH₃ flow led to the formation of an amorphous silicon nitride material without carbon contamination. All of the gel and pyrolyzed products exhibited a mesoporous structure with a high surface area and narrow pore-size distribution. The effective surface area of the pyrolyzed silicon nitride residues decreases with increasing temperature, but the heating rate during pyrolysis has little influence on the surface area and pore-size distribution of the final mesoporous ceramic Si₃N₄ products because of the highly cross-linked structures of the precursor silicon diimide gel.