Preparation of materials in the system Al2O3 (with additions)-TiB2 and study of their electrical resistance

Materials in the system AI₂O₃ (with additions of ZrO₂, SiO₂, CeO₂, Si₃N₄, AIN)-TiB, containing titanium boride concentrations up to 70 wt. % were prepared by sintering in carbon monoxide (T = 1873 K, t = 3 h). The relative density of these materials did not exceed 69% of theoretical, with a minimum at approximately 38% TiB₂ on the density vs. composition curve. The specific electrical resistance of the materials decreased from 3.410⁹ to 82 Ω cm with increasing titanium boride concentration. An abrupt decrease in electrical resistance occurred in the range 20-30% titanium boride.     

Finely dispersed Si3N4−SiC powders and materials based on them

The objectives of the present research were to investigate the preparation of homogeneous ultrafine composite Si₃N₄−SiC powders by a plasmochemical process and the properties of ceramics produced from them. The chemical and phase compositions of the powders depended on the particle size of the initial powder, silicon input rate, and ratio of ammonium and hydrocarbon flow rates. The particle size and specific surface area of the compounds depended on the concentration of particles in the gas jet, and the cooling rate of the products. Composite powders containing from a few up to 90 mass % SiC, with specific surface areas of 24–80 m²/g and free silicon and carbon content less than 0.5 mass % were obtained. The main phases present were α-Si₃N₄, β-Si₃N₄, β-SiC, and X-ray amorphous Si₃N₄. Dense materials were prepared both by hot pressing at 1800°C under a load of 30 MPa and gas-pressure sintering at 1600–1900°C under a pressure of 0.5 MPa nitrogen. The plasmochemical composites had smaller pore sizes, were finer grained, and densified more rapidly than materials sintered from commercial powders. Institute of Inorganic Chemistry, Latvian Academy of Sciences, Salaspils. Translated from Poroshkovaya Metallurgiya, Nos. 1–2(405), pp. 7–12, January–February, 1999.     

Dielectric characteristics of sintered silicon nitride materials

We have studied and summarized data on structure formation and dielectric characteristics of Si₃N₄-based materials obtained by hot pressing and activated sintering. We have measured the dielectric characteristics in the frequency range 1 kHz to 10 MHz. We have established that the level of the dielectric characteristics of the materials is significantly affected by the content of highly dispersed Si₃N₄ powder obtained by plasmochemical synthesis in the original mix. Translated from Poroshkovaya Metallurgiya, Nos. 3–4(412), pp. 22–26, March–April, 2000.     

Effect of chemistry and particle size of cryochemical powders on the properties of humidity sensors

A comparative analysis of various sintered materials based on cryochemical powders was carried out. The electrical resistance of compacts of pure aluminum oxide and its mixtures with CoO, MgO, ZrO₂, and Y₂O₃ were compared with data on the specific surface area, phase composition, and pore size distribution in the sintered ceramics. Conclusions were drawn about the suitability of the materials as humidity sensors. Highest sensitivity was found in sensors based on aluminum—cobalt spinel, and aluminum—magnesium spinel mixed with zirconium oxide. Deceased. Materials Science Institute, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 3–4(400), pp. 39–43, March–April, 1998.     

Composites with a functional gradient in the systems Si3N4−Al2O3−Y2O3−TiC

Composites with a functional gradient in the system Si₃N₄−Al₂O₃−Y₂O₃−TiC were made by laminating and sintering ceramic films obtained by tape casting. The films had high contents of TiC and Al₂O₃ and were of different thicknesses. Materials with a high density and high fracture toughness (K_1c≈9.3 MPa·m^1/2) were obtained. Warsaw Polytechnic Institute. Translated from Poroshkovaya Metallurgiya, Nos. 7–8(408), pp. 1–7, July–August, 1999.     

In-Situ Reactive Synthesis of Si2N2O Ceramics and Its Properties

Si₂N₂O is considered as a new great potential structural/functional material in place of Si₃N₄ for high-temperature applications. In the present work, Si₂N₂O ceramics were in-situ reactive synthesized by a nitridizing powder mixture of Si and SiO₂ using an optimized two-step sintering process according to thermodynamic analyses. The results showed that the purity of Si₂N₂O in the produced ceramics increased with an increase in final sintering temperature, while the shrinkage and Vickers hardness decreased. After final sintering above 1923?K (1650?°C), pure nanograined Si₂N₂O ceramics can be obtained. Flexural strength and fracture toughness both showed peak values at 1873?K (1600?°C). The reaction mechanism was proposed and then the difference of the produced ceramics was discussed.     

Features of solid solution formation with a fluorite type structure in the system ZrO2-HfO2-Y2O3 with different synthesis methods

X-ray phase, petrographic, and thermal analysis methods are used to study the properties of solid solutions with a fluorite type structure in the ternary system ZrO₂-HfO₂-Y₂O₃ in relationship to preparation method: mixing of the original powders followed by solid-phase sintering and melting in solar furnaces; hydrothermal synthesis of nanocrystalline powders followed by solid-phase sintering. It is shown that in specimens whose composition lies at the isoconcentrate of 10 mole% Y₂O₃ a single phase forms independent of preparation method, i.e. a solid solution with a fluorite type structure. The azeotrope, situated at the liquidus of the limiting binary system ZrO₂-Y₂O₃ in the region of fluorite-like solid solutions, affects the melting temperature of ternary solid solutions and the lattice parameters of specimens after melting in a solar furnace. __________ Translated from Poroshkovaya Metallurgiya, Nos. 1–2(447), pp. 3–11, January–February, 2006.     

Nitrogen Ceramics: Liquid Phase Sintering

Abstract

The role of a minor silicate eutectic liquid phase as a transport medium in sintering hot–pressed silicon nitride (β Si₃N₄) ceramics was identified in the 1970s. A similar mechanism is applicable to hot–pressed Si–Al–O–N ceramic alloys which offer an advantage in control of the final liquid volume and hence in superior high temperature mechanical properties. By increasing the liquid volume it is possible to densify ceramic alloys without application of pressure at the sintering temperature and hence to fabricate components of complex shape. The Lucas Syalon ceramics typify the new range of pressureless–sintered ceramics based on the β Si₃N₄ structure. They are fabricated from the ultrafine compound powders α Si₃N₄, SiO₂, Al₂O₃, Y₂O₃, and a polytypoid phase (a substitute for A1N). The ceramics consist of submicrometre solid solution crystals of general composition Si_3?xAl_xO_xN_4?x(x < 1) within a minor matrix phase which may be either a glassy Y–Si–Al oxynitride or be crystallized to form yttrogarnet. Analysis of matrix glass compositions shows them to be residues of liquids near to a ternary eutectic in the Y₂O₃–SiO₂–Al₂O₃ system which is well below the sintering temperature of ～ 1800°C. Sintering models, based on particle rearrangement due to dissolution of the major α Si₃N₄ component in the eutectic liquid and its reprecipitation as a β Si₃N₄ solid solution, are discussed. Properties and current applications of Syalon ceramics are surveyed briefly. PM/0266     

Activities in CaO-SiO2-Al2O3 slags and deoxidation equilibria of Si and Al

By using the data in previous and present slag-metal equilibrium experiments, the activities of SiO₂ along the liquidus lines in CaO-SiO₂-Al₂O₃ slags were determined at 1823 and 1873 K from the reaction Si+2O=SiO₂ (s), in which the oxygen activities were estimated from the measured oxygen contents or from the combination of nitrogen distribution ratios (L _N) and nitride capacities (C _N). The activities of Al₂O₃ were also determined from the reaction 2Al+3O=Al₂O₃ (s), in which the oxygen activities were estimated from the values for L _N and C _N, or from the reaction 3SiO₂ (s)+4Al=2Al₂O₃ (s)+3Si, in which the activities of SiO₂ and the contents of Al and Si along with the respective interaction coefficients were used. The activities of Al₂O₃ and CaO in the entire liquid region were estimated from the Rein and Chipman’s activities of SiO₂ by using the method of Schuhmann. On the basis of these activities, the deoxidation equilibria of Si and Al in steels were discussed.     

Desilicating Zircon with Plasma Heating and Phase Equilibrium Analyses

Desilicated zirconia has a great variety of applications. The maximum SiO₂ content in desilicated zirconia is <7 mass%. The present techniques to produce zirconia are always accompanied by many drawbacks. So the authors have developed a new plasma process. Desilicated zirconia (88.6 ~ 96.9 mass% ZrO₂) and magnesia‐stabilized zirconia (> 91.54 mass% ZrO₂, < 5.39 mass% MgO) were produced successfully from zircon using a 150 kW plasma rotating furnace. The effects of time and carbon content on the desilication degree were investigated. The mixture treated in the plasma furnace included condensed phases Zr, ZrC, ZrN, ZrO₂, C, Si, SiC, SiO₂ and gaseous phases SiO, O₂, N₂, CO. The established phase equilibrium diagrams of the Si‐C‐N‐O and Zr‐C‐N‐O systems suggest that the formation of Si₃N₄ is thermodynamically impossible, and the formation of ZrC and ZrN is thermodynamically possible in the central high‐temperature region of the plasma furnace. Experimental results supported the analyses.     

High-temperature oxidation of composite AlN-ZrB2-ZrSi2 ceramics

The high-temperature (to 1450°C) oxidation of AlN-ZrB₂-ZrSi₂ powders and compact ceramic materials with different contents of ZrB₂-ZrSi₂ solid solution in air is examined using scanning electron microscopy and differential thermal, thermogravimetric, x-ray phase, and x-ray spectrum microanalyses. It is established that the hot-pressed (practically porousless) ceramic materials have high corrosion resistance up to 1350–1400°C when scale components (individual oxides) interact with each other to form solid solutions based on aluminum oxide, zirconium oxide, and aluminum borate. These phases become sintered in the presence of the liquid B₂O₃ phase, self-reinforced scale being formed. __________ Translated from Poroshkovaya Metallurgiya, Vol. 47, No. 1–2 (459), pp. 196–203, 2008.     

Interaction of ferrosilicon with nitrogen in the presence of zircon and ilmenite concentrates in the process of self-propagating high-temperature synthesis

The equilibria in ferrosilicon-zircon-nitrogen and ferrosilicon-ilmenite-nitrogen systems are calculated. Nitriding of these compositions is investigated experimentally and the experimental and calculated data are compared. It is established that the combustion in the ferrosilicon-additive-nitrogen systems can be implemented with a zircon additive no larger than 60% and that of ilmenite no larger than 40%. The composition of the final product of the mixture with zircon includes Si₄N₄, Fe, Si₂N₂O, and ZrO₂; that with ilmenite includes Si₃N₄, Fe, Si₂N₂O, and TiN. The absence of ZrN in the former case contradicts the results of the calculation and the presence of TiN in the latter case corresponds to them.     

Reaction of a molten mixture of titanium,aluminum, and silicon oxides with hot-pressed silicon nitride

The formation temperature of a liquid phase and the solidification temperature of a molten mixture of Al₂O₃-TiO₂-SiO₂ oxides on a silicon nitride substrate are determined. Data are obtained for the change in kinetics. It is established that the intensity of interaction of molten Al₂O₃-TiO₂-SiO₂ with silicon nitride depends on the oxide mixture composition. With heating there are two possibilities: improvement and worsening of Si₃N₄ crystallite wetting with a liquid phase as well as solidification of the melt. The temperature range where a liquid phase exists for actual materials is about 15°C, which markedly worsens the process of structure formation with Si₃N₄ during sintering.Translated from Poroshkovaya M etallurgiya, No. 5, pp. 39–44, May, 1993.     

Sintered ceramics based on boron nitride and carbide

The formation of BN-B₄C composite materials by sintering in nitrogen is investigated. Structural, mechanical, and chemical characteristics of these materials are examined. Excellent dielectric properties, thermal and chemical stability, and erosion resistance in high-intensity laser beams enable high-temperature application of BN-B₄C composite materials. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 1–2(453), pp. 58–63, 2007.     

A generalized approach to the flood-knapp structure based model for binary liquid silicates: Application and update for the PbO-SiO2 System

The nonideal activity of a metal oxide in a molten binary silicate system is described by treating the liquid as an ideal solution and by considering the formation of a few complexes. Application of this approach to the binary system PbO-SiO₂ shows that the experimentally determined activity of PbO(l) can be modeled by considering the lead silicate melt as an ideal solution of Pb²⁺ and O²⁻,SiO ₄ ⁴⁻ , Si₂O ₇ ⁶⁻ , Si₁₂O ₃₇ ²⁶⁻ , and Si₄O ₁₀ ⁴⁻ . The calculated Gibbs free energy values for the formation of the anionic complexes from O²⁻ and SiO ₄ ⁴⁻ are: ΔGℴ(Si₂O ₇ ⁶⁻ )/J · mol⁻¹ = 38977 − 30.909(T/K); ΔGℴ(Si₁₂O ₃₇ ²⁶⁻ )/J · mol⁻¹ = 200158 − 121.813(T/K); Δℴ(Si₄O ₁₀ ⁴⁻ )/J · mol⁻¹ = 104627 − 28.094(T/K). Values of Gibbs free energy of formation of the solid phases PbO, Pb₄Si0₆, Pb₂SiO₄, PbSiO₃, and SiO₂ which, together with the melt model data, give the best fit to experimental phase relations in the system PbO-SiO₂ were calculated. These values are all in good agreement with literature data.     

Physicochemical Processes of Fabricating Ceramic Materials Based on Nanopowders of Zirconium,Yttrium, Cerium,and Aluminum Oxides

When studying nanoceramics, it is necessary to constantly keep in mind the closest interrelation of their fabrication method, structure, and properties. Nanoceramic materials are used in various branches of technology as structural and functional materials. Nanoceramics are also widely used in medicine. They are harmless, stable, and have great affinity to living organisms. ZrO2-based nanoceramics have a lower elastic modulus than other oxide materials. The specificity of their application lies in their high rupture strength, thermal shock resistance, and chemical stability at high temperatures. However, it is necessary to solve the problem of increasing the fracture toughness of ZrO2-based ceramic materials. The complex alloying of ZrO2 with yttrium and cerium oxides and the use of the Al2O3 additive leads to an increase in the fracture toughness and lowering of the negative effect of materials in the biological medium. In this work, the physicochemical properties of ceramic powders and materials of the ZrO2–2Y2O3–4CeO2–Al2O3 system synthesized by the chemical deposition of inorganic precursors when applying the sol-gel technology are considered based on scientific data and experimental studies. Alloying pure zirconium oxide by stabilizing Y2O3 and CeO2 oxides and thermal hardening of Al2O3 ensure the conservation of the tetragonal structure at room temperature, which makes it possible to retard and control the crack resistance of the material under the load. Investigations into the influence of the sintering temperature and aluminum oxide content on the microstructure and grain size, as well as physicomechanical properties of ceramic materials of compositions ZrO2–2Y2O3–4CeO2 + 1 wt % Al2O3 and ZrO2–2Y2O3–4CeO2 + 3 wt % Al2O3, are carried out.     

Fusion and Dispersion of Oxide Materials in a “Cold” Crucible and in Furnaces with Concentrated Radiant Heating

Melting and dispersion of oxide materials in the system ZrO₂ - HfO₂ - Y₂O₃ is carried out in furnaces with concentrated radiant and induction heating. The polyfractional powders obtained are stabilized solid solutions of the fluorite type. A reduction is established for elementary cell parameters (from 0.5144 to 0.5127 nm) for specimens whose composition relates to an isoconcentrate with 10 mole% Y₂O₃ with an increase in HfO₂ content. Melted and dispersed ZrO₂ - HfO₂ - Y₂O₃ materials are promising for creating dense and highly refractory ceramic materials. __________ Translated from Poroshkovaya Metallurgiya, Nos. 7–8(444), pp. 36–42, July–August, 2005.     

Experimental study of phase equilibria in the PbO-Al2O3-SiO2 system

Equilibrium phase relations in the PbO-Al₂O₃-SiO₂ system have been investigated experimentally by means of high-temperature equilibration, quenching, and electron probe X-ray microanalysis (EPMA). The system has 21 primary phase fields including three monoxides (PbO, Al₂O₃, and SiO₂), seven binary compounds (Al₆Si₂O₁₃, PbAl₂O₄, PbAl₁₂O₁₉, Pb₂Al₂O₅, PbSiO₃, Pb₂SiO₄, and Pb₄SiO₆), and eleven ternary compounds (PbAl₂Si₂O₈, Pb₃Al₁₀SiO₂₀, Pb₄Al₂Si₂O₁₁, Pb₄Al₄SiO₁₂, Pb₄Al₄Si₃O₁₆, Pb₄Al₄Si₅O₂₀, Pb₅Al₂Si₁₀O₂₈, Pb₆Al₂Si₆O₂₁, Pb₈Al₂Si₄O₁₉, Pb₁₂Al₂Si₁₇O₄₉, and Pb₁₂Al₂Si₂₀O₅₅). Three new ternary compounds, Pb₄Al₄SiO₁₂, Pb₄Al₄Si₅O₂₀, and Pb₁₂Al₂Si₁₇O₄₉, were observed and characterized by EPMA. No extensive solid solution in any of the compounds was found in the present study. The liquidus isotherms were experimentally determined in most of the primary phase fields in the temperature range from 923 to 1873 K, and the ternary phase diagram of the PbO-Al₂O₃-SiO₂ system has been constructed.     

Phase Diagrams of the Zr–Si–RE (RE=La and Er) Ternary Systems at 773?K (500?°C)

The isothermal sections of the phase diagram of the Zr–Si–RE (RE=La and Er) systems at 773 K (500 °C) have been investigated using X-ray power diffraction (XRD), scanning electron microscopy (SEM), and optical microscopy (OM) with the aid of metallographic analysis. The existences of 10 binary compounds, namely ZrSi₂, α-ZrSi, α-Zr₅Si₄, Zr₃Si₂, Zr₂Si, RESi₂, RESi_2–x , RESi, RE₅Si₄, and RE₅Si₃ have been confirmed in the Zr–Si–RE (RE=La and Er) systems, respectively. As for the reported binary compound RE₃Si₂, only La₃Si₂ has been observed in the Zr–Si–La system, whereas Er₃Si₂ was not found. No binary compound was found in the Zr–RE binary systems, and no ternary compound was found in the current ternary systems. None of the phases in Zr–Si–La system reveals a remarkable solid solution at 773 K (500 °C). However, the maximum solid solubility of Zr in Er, Er₅Si₃, Er₅Si₄, ErSi, ErSi_1.67, and ErSi₂ is determined to be approximately 12.0 at. pct, 2.4 at. pct, 3.0 at. pct, 3.3 at. pct, 2.2 at. pct, and 1.8 at. pct, respectively. The maximum solid solubility of Er in ErSi₂ is approximately 1.8 at. pct. No remarkable solid solubility of the elements in any of the other phases has been observed.     

Reactive atomization of silicon to formin situ oxide sintering aids

The present investigation demonstrated the feasibility of using reactive atomization to produce Si powder within situ oxide sintering aids. With further process optimization, this powder may be an alternative starting material to the conventional, mechanically blended, Si-plus-oxide powder used to produce commercial sintered reaction bonded silicon nitride (SRBSN). In the reactive atomization approach, yttrium and aluminum additives were introduced into silicon metal during induction melting. Reactive atomization was accomplished using a N₂-5 pct O₂ mixture as the atomization gas. During atomization, oxygen in the atomization gas reacted with Y and Al in the Si melt to produce Y₂O₃ and Al₂O₃, which act asin situ sintering aids. The reactive atomized powder demonstrated a Gaussian distribution with a mean diameter of 36 μm. The powder fines (<38 μm) were used to produce cold isostatically pressed compacts that were subsequently reaction bonded and sintered. The results demonstrate that β-Si₃N₄ formed during reaction bonding and sintering. The density of the SRBSN was 77 pct of theoretical. Transmission electron microscopy (TEM) studies indicated the presence of a glassy phase on the grain boundaries, which is typical in SRBSN and indicative of the presence of thein situ sintering aids. A kinetic model was used to study the influence of processing parameters, such as droplet temperature and oxygen partial pressure, on the kinetics of oxide formation during reactive atomization. The results suggest that the volume fraction of oxides increases with increasing droplet temperature and oxygen partial pressure in the atomization gas mixture. This article is based on a presentation made in the “In Situ Reactions for Synthesis of Composites, Ceramics, and Intermetallics” symposium, held February 12–16, 1995, at the TMS Annual Meeting in Las Vegas, Nevada, under the auspices of SMD and ASM-MSD (the ASM/TMS Composites and TMS Powder Materials Committees).