Variations in sintering kinetics and microstructure of alumina with chromium and molybdenum additions

Chromium and molybdenum were introduced into alumina through gel synthesis. Both elements are soluble in aluminium hydroxide and low-temperature (<1400°C) aluminium oxide. In the course of transformation from hydroxide to oxide, the evaporation-condensation of vapour phase (below 1000° C) causes shrinkage of the porous compact due to particle rearrangement and growth through capillary drag and adsorption. Subsequently, differences in agglomerate structure, grain growth, solute atmosphere and densification arise because of variations in the crystal lochemical behaviour of chromium and molybdenum. The solute atmospheres of chromia-vacancy and molybdenum metal reveal sub-grain boundaries and dislocations. In chromia-alumina solid solution these defects annihilate and contribute to densification (98%th) with increase in temperature to 1600° C. However, with further rise in temperature to 1700° C, the solid solution desinters to 85%th and creep cavities show transition from grain boundary to lattice creep. In Mo-Al₂O₃ composite the defects are locked by molybdenum solute, and as a result there is an insignificant increase in densification (30%th at 1600°C).     

Synthesis of dense forms of B-C-N system using chemical-vapor-deposition/high-pressure process

Composites made up of boron, carbon, and nitrogen were studied using a combination of chemical vapor deposition (CVD) and high pressure (HP) techniques. The CVD/HP process comprised deposition of C_x (BN)_1–x composites where x = 0.03 or 0.8, followed by heat treatment of the deposited samples under pressures from 6 to 10 GPa using an octahedral-anvil apparatus. Also a sample prepared by nitriding of boron and carbon to compose x = 0.5 was studied up to 16 GPa. A diamond-type solid solution was obtained after the C_0.8(BN)_0.2 sample was treated at 10 GPa and 1600°C or at 8 GPa and 1700°C whereas the C_0.03(BN)_0.97 and C_0.5(BN)_0.5 samples exhibited decomposition into carbon and BN under all the P, T conditions studied. Sintered compacts consisting primarily of the cubic BN and carrying hardness with a maximum value of 54 GPa were obtained from C_0.03(BN)_0.97.     

Two-step sintering of a TiB2-Ni cermet

The effect of atmosphere control on the sintering behaviour of TiB₂-20 wt% Ni ceramics was investigated. Sintering was performed at 1700 °C for 1h with commercial TiB₂ powders produced by the earbothermic reaction. Under a fixed single atmosphere such as vacuum or Ar, densification was limited and large pores remained. However, a sintered density above 99% of theoretical could be achieved by a two-step sintering procedure in which the atmosphere was changed from vacuum to Ar at 1600 °C during the heat-up with subsequent isothermal sintering being performed at 1700 °C for 1h. Qualitative considerations about these phenomena are discussed.     

Sintering and microstructural studies in the system ZrO2 · TiO2 · CeO2

Tetragonal zirconia polycrystals (TZP) in the system ZrO₂ · TiO₂ · CeO₂ have been prepared from titanium and zirconium alkoxides and cerium nitrate precursors. The change in microstructure with sintering temperature in the range 1300 to 1600° C has been characterized. A fully tetragonal structure with theoretical final density has been achieved after liquid-phase sintering in the range 1350 to 1400° C for 2h. Sintering at temperatures above 1450° C resulted in a loss of stabilizer from the matrix, by the formation of zirconium titanate and to a cerium-, titanium-rich liquid. The loss of stabilizer was such that in the temperature range 1500 to 1600° C, extensive transformation to monoclinic zirconia occurred spontaneously on cooling. The tetragonal zirconia formed after sintering at 1350° C was found to be very stable. Thec/a ratio of the tetragonal phase in this system is higher than in any of the binary TZP systems reported in the literature. The stability of the tetragonal phase is believed to be associated with this highc/a ratio.     

Microstructural developments in Mg-Ti-PSZ systems

The microstructure of titania-added Mg-partially-stabilized zirconia (PSZ) is dramatically influenced by thermal treatments. Effects of various sintering, heat-treatment and thermal shock cycling parameters on the microstructure of the Mg-Ti-PSZ system are described. Conditions favourable for the growth of needle-like Ti-rich reinforcements in highly thermalshock-resistant Mg-Ti-PSZ ceramics are identified. TiO₂ seems to play a catalytic role in the formation of Zr-rich networks during high-temperature (1700°C) sintering of the Mg-Ti-PSZ system, quite similar to those found in Mg-PSZ, heat-treated above 1300 °C.     

Improvements in refractoriness and properties of Nicalon fibres by high-temperature heat-treatment

Nicalon SiC fibres were heat-treated in various atmospheres and at various pressures. Initially CO, nitrogen and air were used as the heat-treatment environment at one atmosphere pressure. Microstructural changes and any related strength degradation or improvement were measured for the heat-treated fibres. After heat-treatment in the temperature range 1000°C–1600°C, each sample showed different weight changes. Thus, in air, a weight gain was observed with increasing temperature, whereas in CO and N₂, weight losses were observed but with a smaller weight loss observed for CO. Moreover, carbon monoxide had a significant effect on the strength retention of the fibres. Since the lowest weight loss was observed after heat-treatment in CO at one atmosphere, high pressure CO gas was used to heat-treat Nicalon fibres between 1000°C and 1700°C and the resulting fibres were analysed by single-filament strength testing, scanning electron microscopy, and X-ray diffraction. The results were completely different compared with those in one atmosphere of CO. As the temperature increased, weight and strength increased whereas at one atmosphere pressure, both weight and strength had decreased. The weight increase was because of surface reaction between the CO atmosphere and the SiC fibre and/or because of deposition of carbon from the pressurised CO gas, giving the fibre a surface carbon coating. Carbon coating of a fibre is a beneficial property for CMCs since it provides a weak interface which facilitates pull-out during fracture.     

Low temperature sintering of aluminum nitride with millimeter-wave heating

Rapid sintering of Yb₂O₃-added AlN was performed by applying the 28 GHz millimeter-wave heating method. It was found that full densification over 97%T.D. was attained by sintering at 1600°C for 20 min. The densification temperature was decreased by about 300°C, compared with those in the conventional method by an electric furnace. A high thermal conductivity over 180 W/(m · K) was obtained in the sample sintered at 1700°C for 40 min, even under non-reducing atmosphere. The main factor resulting in the rapid and low temperature sintering was attributed to efficient selective absorption of the millimeter-wave into Yb₂O₃ additive.     

Synthesis of BaZrO3 by a solid-state reaction technique using nitrate precursors

High phase purity barium metazirconate powders have been synthesized from a modified solid-state reaction. Reactive powders consisting of submicron particles and narrow particle size distribution were obtained by heating a 1:1 molar mixture of barium nitrate and zirconyl nitrate at 800°C up to 8 h. Simultaneous thermal analysis (TG-DTA) assisted in elucidating the probable reaction pathways leading to the formation of the target compound in the BaO-ZrO₂ system. Systematic structural and microstructural characterization on the green powders and the compacts sintered up to 1700°C were carried out. A two-stage sintering schedule consisting of a 6 h soak at 1600°C followed by slow heating up to 1700°C with no dwell, led to highly dense microstructural features.     

Synthesis and structure of chemically vapour-deposited boron nitride

Chemically vapour-deposited boron nitride (CVD-BN) plates have been synthesized on a graphite substrate by the reaction of the BCl₃-NH₃-H₂ gas system in a deposition temperature (T _dep) range from 1200 to 2000° C, with a total gas pressure (P _tot) which was varied from 5 to 60 torr. The effects ofP _tot andT _dep on the crystal structure and the microstructure of the CVD-BN plate were investigated. Turbostratic BN(t-BN) was deposited above 10 torr, at anyT _dep in the range investigated. The interlayer spacing (c ₀/2), the crystallite size (Lc) and the preferred orientation (PO) were strongly affected byT _dep. The t-BN obtained at lowT _dep had largec ₀/2 and smallLc andPO. AsT _dep increased,c ₀/2 tended to decrease whereasLc increased and thec-plane of the crystallites became oriented parallel to the deposition surface. At aP _tot of 5 torr, a mixture of t-BN and h-BN (hexagonal BN) was deposited at anyT _dep above 1700° C, and two kinds of t-BN different inc ₀/2 co-deposited at aT _dep below 1600° C. Moreover, it was indicated that r-BN (rhombohedral BN) was included in the deposits obtained at aP _tot of 5 torr and aT _dep of 1500 to 1600° C.     

An investigation of thermal decomposition of β-FeOOH nanowire arrays assembled in AAO templates

β-FeOOH nanowire arrays were assembled into porous anodized aluminum oxide (AAO) templates by electrochemical deposition in the mixture solution of FeCl₃ and (NH₄)₂C₂O₄. In order to obtain well-crystallized α-Fe₂O₃ and other iron oxides nanowires, β-FeOOH nanowire arrays with amorphous crystal structure were heat-treated at different temperatures from 200 to 600 °C. The decomposition products were characterized by DTA, XRD, FTIR, and Mössbauer spectroscopy. When heat-treated at 200 °C, only 65% of β-FeOOH decomposed, whereas, when the temperature was up to 300 °C, it was completely decomposed and formed poorly crystallized β-Fe₂O₃. This transition temperature is higher than the 200 °C obtained on other β-FeOOH materials. However, when heated above 300 °C, the main products are characterized as poorly crystallized α-Fe₂O₃ nanowires, whereas, well-crystallized α-Fe₂O₃ nanowire arrays can be formed when the temperature was up to 600 °C, and this temperature is also higher compared with those temperatures observed on other β-FeOOH materials. From Mössbauer results, the α-Fe₂O₃ nanowires were composed of fine particles in which 66% of the particles are superparamagnetic.     

Mechanical behavior of alumina and alumina-feldspar based ceramics in an acetic acid (4%) environment

This study investigates the mechanical properties of alumina-feldspar based ceramics when exposed to an aggressive environment (acetic acid 4%). Alumina ceramics containing different concentrations of feldspar (0%, 1%, 5%, 10%, or 40%) were sintered at either 1300, 1600, or 1700 °C. Flaws (of width 0%, 30%, or 50%) were introduced into the specimens using a saw. Half of these ceramic bodies were exposed to acetic acid. Their flexural strength, K_IC, and porosity were measured and the fractured samples were evaluated using scanning electronic- and optical microscopy. It was found that in the ceramic bodies sintered at 1600 °C, feldspar content up to 10% improved flexural strength and K_IC, and reduced porosities. Generally, it was found that acetic acid had a weakening effect on the flexural strength of samples sintered at 1700 °C but a beneficial effect on K_IC of ceramics sintered at 1600 °C. It was concluded that alumina-based ceramics with feldspar content up to 10% and sintered at higher temperatures would perform better in an aggressive environment similar to oral cavity.     

Specific Features of Phase Transformations in Lanthanum–Nickel–Aluminum Alloys under the Action of Hydrogen

By the methods of differential thermal and X-ray phase diffraction analyses, we study the interactions in LaNi_{5 – x} Al _x –H₂ systems with x = 1.0 and 1.5 at temperatures varying from the room temperature to 920°C under an initial pressure of hydrogen of up to  5.0 MPa. The temperature of disproportionation is equal to 555 and 535°C for the compounds with aluminum content x = 1.0 and 1.5, respectively. The original phase decomposes into LaH _x and Ni₃Al. At temperatures above 800°C, we observe the onset of recombination of the original phase leading to the formation of a compound with double-spaced structure. The products of disproportionation of the LaNi₄Al compound, i.e., La(Ni, Al)₅, LaH _x , Ni₃Al, and an unknown phase, recombine in a vacuum at 215°C with hydrogen release and the formation of one or several unknown phases. The products of disproportionation of the LaNi_3.5Al_1.5 compound, i.e., LaH _x , Ni₃Al, and an unknown phase, recombine in a vacuum at 150, 230, and 580°C with hydrogen release and the formation of the original phase.     

Influence of Titanium on Hydrogen-Induced Transformations in the Laves Phases Based on Zirconium

By the methods of differential thermal, and X-ray phase analyses, we study the influence of hydrogen (hydrogenation, disproportionation, desorption, and recombination) on the phase transformations in Zr_{1 − x} Ti_xCr₂ (x = 0.1 and 0.2) alloys under a pressure of hydrogen of 5 MPa at temperatures varying from the room temperature to 1238°K. For 3–5 MPa and 1163–1223°K, we observe the decomposition of the Laves phase with C14-type structure accompanied by the formation of Zr_{1 − x} Ti_xCr₂H_y and ZrH_x hydrides and Cr. Under a pressure of 5 MPa, on holding at 1223°K for more than 4 h, the compounds completely disproportionate into zirconium and titanium hydrides and chromium. On holding in a vacuum at 1193– 1238°K, the compounds recombine into the phases with C14- or C15-type structure or their mixture.__________Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 6, pp. 67–72, November–December, 2004.     

A melt process to superconductive fibres of Bi2Sr2CaCu2 oxide

A particulate precursor of superconductive Bi₂Sr₂CaCu₂ oxide dispersed in an aqueous poly(vinyl alcohol) solution, was shaped into fibres. The fibres fired at a T _max of 850 °C were very porous. Although they exhibited a Meissner on-set transition at 85 K, electrical resistance at 0.02 Acm^–2 at 77 K was still not zero. Exposing the porous fibres to a temperature slightly above their melting point densified them. The treated fibres had a sharp Meissner transition and could carry critical current (J _c) as high as 331 A cm^–2 at 77 K. They were also more resistant to the effect of a magnetic field than the fibre fired at 850 °C. Eighty percent of the treated fibre was found to be the superconductive 2 2 1 2 phase.     

Effect of annealing treatment on mechanical properties of a ZrB2-SiC-graphite ceramic

The ZrB₂-SiC-graphite (ZSG) ceramic was annealed at 1600, 1700 and 1800 °C for different times, respectively. It was revealed that the annealing treatment is favorable to increase the interface bonding and relative density, and to decrease the thermal residual stress, to result in the crimp of the graphite flake. It was the optimal annealing treatment process conditions of 1700 °C and 90 min according to the best combination of the mechanical properties. To compare the mechanical properties of the specimen before the annealing treatment, the hardness, strength and toughness of the specimen annealed at 1700 °C for 90 min were enhanced by 20.6% and 20.2%, decreased by 8.2%, respectively.     

Coprecipitation-derived mullite and mullite-zirconia composites

Precursor powders of mullite-zirconia (0–40 wt% ZrO₂) were prepared by a hydroxide coprecipitation method and their behaviour during calcination between room temperature and 1500 °C was studied using thermal analysis, X-ray diffraction and electron microscopy. The only crystalline phases present in the precalcined powders were bayerite and gibbsite, and these were stable up to 250 °C. Powders containing ZrO₂ were initially amorphous, but on calcination between 250 and 850 °C produced different crystalline phases at temperatures which depended on the amount of zirconia present. Thus in the case of mullite-40 wt% ZrO₂, zirconia crystallized at about 850 °C and was stable up to 1200 °C, when it reacted with free silica to form zircon (ZrSiO₄). Mullite formed above 1250 °C at the expense of zircon and remained stable at higher temperatures. The oxide powders were very homogeneous, and on sintering produced ceramics with a fine-grained uniform microstructure. The powders were very reactive and could be sintered conventionally to near-theoretical density at 1600–1700 °C without sintering aids. The fracture strength of mullite was about 275 MPa, and this could be improved to 350 MPa by hot isostatic pressing the presintered bodies. Addition of zirconia enhanced the sintering kinetics as well as the fracture strength of mullite.     

Ordering of Fe-Si phases

The microstructure of Fe-Si alloys containing 8 and 15.5 at % Si and heat-treated between 550 and 1200°C is studied by transmission electron microscopy, and the phase composition of alloys containing 19 and 23 at % Si is determined by x-ray diffraction. The Fe-15.5 at % Si alloy heat-treated above 700°C is found to consist of a disordered solid solution and B2 phase. The B2 particles can be thought of as portions of {100} layers consisting entirely of Si atoms and sandwiched between {100} layers of Fe atoms, that is, as a two dimensional phase. At t 675°C, a compositionally modulated microstructure develops in which the Si-enriched zones have the Fe₃ Si stoichiometry and DO₃ structure. At high temperatures, the Fe-19 at % Si alloy consists of the and B2 phases, and the Fe-23 at % Si alloy consists of the and DO₃ phases. These findings are at variance with the generally accepted Fe-Si phase diagram.Translated from Neorganicheskie Materialy, Vol. 41, No. 1, 2005, pp. 28–35.Original Russian Text Copyright © 2005 by Ustinovshchikov, Sapegina.     

In situ synthesis of Mo(Si,Al)2-SiC composites

An in situ reaction was proposed and investigated to produce Mo(Si_{1 – x} Al _x )₂-SiC composites. The starting powders were MoSi₂, Al and C. A direct current hot pressing (DCHP) method was used to prepare these composites. When the mixed powder was hot pressed at temperatures lower than 1500°C, the phase composition was Mo(Si,Al)₂ and -SiC. When the hot pressing temperature was higher than 1600°C, however, Nowotny phase Mo₅Si₃C₁ appeared. The chemical stoichiometry of the proposed in situ reaction becomes difficult because of the formation of solid solution among these phases and the appearance of Mo₅Si₃C phase. The in situ formed SiC phase in the x = 0.3 sample was partly in whisker shape. However, the SiC phase in x = 0.15 sample was in particle shape. These in situ formed SiC particles and whiskers acted as crack deflection and bridging elements and improved the fracture toughness. The Vickers hardness and fracture toughness of the x = 0.3 sample hot pressed at 1700°C for 60 min in vacuum were 15.6 GPa and 5.39 MPa · m^1/2, respectively.     

An internal synthesis method for mullite-chromium carbide composite in a vacuum system

In situ formation of chromium carbide in a mullite matrix through reaction of Cr₂O₃, SiC and A₂O₃ has been studied. Three different chromium compounds, Cr₃Si, Cr₃C₂, Cr₇C₃, and mullite were formed. In a vacuum environment, the Cr₃Si particles formed first and were retained below 1550 °C, while the Cr₇C₃ phase was only dominant above 1600 °C. The Cr₃C₂ phase was the dominant dispersed phase at temperatures of 1450–1500 °C. In an argon environment, the Cr₃C₂ phase was the main product component at temperatures ranging from 1450 to 1550 °C. The mullite phase formed concurrently through the diffusion of the SiO₂ phase into the Al₂O₃. SiO₂ was the product of the reaction between Cr₂O₃ and SiC. The composite hot-pressed at 1450 °C in vacuum gave a flexural strength and fracture toughness of up to 457 MPa and 4.1 MPa m^1/2, respectively.     

Growth of silicon carbide thin films by hot-wire chemical vapor deposition from SiH4/CH4/H2

Silicon carbide (SiC) thin films were prepared by hot-wire chemical vapor deposition from SiH₄/CH₄/H₂ and their structural properties were investigated by X-ray diffraction, Fourier transform infrared absorption and Raman scattering spectroscopies. At 2 Torr, Si-crystallite-embedded amorphous SiC (a-Si_1 − xC_x:H) grew at filament temperatures (T_f) below 1600 °C and nanocrystalline cubic SiC (nc-3C-SiC:H) grew above T_f = 1700 °C. On the other hand, At 4 Torr, a-Si_1 − xC_x:H grew at T_f = 1400 °C and nc-3C-SiC grew above T_f = 1600 °C. When the intakes of Si and C atoms into the film per unit time are almost the same and H radicals with a high density are generated, which takes place at high T_f, nc-3C-SiC grows. On the other hand, at low T_f the intake of Si atoms is larger than that of C atoms and, consequently, Si-rich a-Si_1 − xC_x:H or Si-crystallite-embedded a-Si_1 − xC_x:H grow.