Self-propagating high-temperature synthesis of advanced ceramics MoSi2–HfB2–MoB

This study focuses on the investigation of the macrokinetic features of SHS (combustion synthesis) of elemental mixtures Mo–Hf–Si–B, in particular the mechanisms of structure and phase formation in the combustion front as well as the structure and properties of consolidated ceramics. Two routes for the fabrication of the composite SHS powder in system MoSi₂–HfB₂–MoB were used: (1) synthesis using Mo–Si–B and Hf–B mixtures followed by mixing of the combustion products and (2) synthesis using the four-component Mo–Hf–Si–B mixture. Dense ceramic samples with a homogeneous structure and low residual porosity (0.8–3.6%) were prepared by hot pressing of SHS powders. Although the particles size distribution and phase composition of SHS powders are similar for both synthesis routes, the structure and properties of both the composite SHS powders and hot-pressed ceramics differ considerably. Synthesis using the four-component Mo–Hf–Si–B mixture allows one to produce hierarchically ordered nanocomposite material with improved mechanical properties: hardness up to 17.6?GPa and fracture toughness up to 7.16?MPa?m^1/2.     

Preparation of Fine Monoclinic Hafnia Powders by Hydrothermal Oxidation

Fine single-phase monoclinic HfO₂ powders were prepared from Hf metal chips and high-temperature high-pressure water by hydrothermal oxidation in closed and open systems. In the closed system, Hf metal was converted to HfO₂ by treatment at ≥600°C under 100 MPa for 3 h. At 500°C half the Hf reacted to produce Hf hydride and a small amount of HfO₂. In the open system, Hf metal scarcely reacted at 500°C, but at 600°C the reaction was more rapid than the corresponding run in the closed system.     

Oxynitride Glass Formation from Gels

Porous, multicomponent silicate gels were heated in flowing and pressurized ammonia. These treatments resulted in dense, homogeneous oxynitride (O-N) glasses which were synthesized at low temperature (∼1000°C) without melting. The O-N glasses exhibited increased microhardness and glass-transition temperature and altered thermal expansion characteristics compared to the respective oxide glasses prepared by densi-fying gels in air or by conventional melting of oxide powders. Spectroscopic evidence indicates that ammonia reacts with all of the network-formers investigated, Si, Al, and B, hut from nitrogen content measurements, it appears that B and/or Al are required for significant nitridation below 1000°C.     

Synthesis and Mechanical Properties of Stoichiometric Aluminum Silicate (Mullite)

Highly dense translucent polycrystalline bodies of stoichiometric aluminum silicate (mullite) were obtained by vacuum-hot-pressing of high-purity submicron mixed oxide powders. The powders were prepared by the hydrolytic decomposition of mixed metal alkoxides. X-ray diffraction and electron microscopy indicated that the initially amorphous needlelike fine particulates transform into highly crystalline orthorhombic mullite at ∼1200°C. Optimum hot-pressing conditions were 5 kpsi and 1500°C for 30 min. Densities within experimental error of the theoretical value of 3.19 g/cm³ were obtained. A typical microstructure consisted of fine interlocking needlelike grains arranged in an overall mosaic or "jigsaw" pattern. Microprobe traverses across the samples indicated homogeneous dispersion of SiO₂ in the AlO₃ matrix. Room-temperature mechanical properties were measured and correlated with the microstructure and crystal structure of the ceramic compact. A slightly higher melting temperature than has been previously reported was observed for the 3Al₂O₃·2SiO₂ studied. All specimens exhibited a smooth surface finish and excellent thermal-shock resistance from 1200°C to room temperature.     

Reaction Kinetics in the Hydrothermal Oxidation of Hf

The hydrothermal oxidation of Hf was studied using metal powders and chips to change the surface-area effect. Fine monoclinic HfO₂ powders (32 to 34 nm) were formed through two parallel reaction paths, one the oxidation of Hf with H₂O and the other the consecutive reaction via formation of Hf hydride. Based on the experimental results at 500°C under 100 MPa, three reaction rate constants were determined for two kinds of starting materials. The relative widths of two paths were proved to be independent of the size of starting materials, and the pulverization and oxidation of both metal powders and chips could be explained by the same mechanism, where the formation of hydride was a driving force in pulverization.     

Preparation of Strontium Barium Niobate by Sol–Gel Method

Crack-free SBN (Sr_xBa_1-xNb₂O₆) thin films have been prepared by a sol–gel method with metal alkoxides. A homogeneous and stable precursor solution was obtained from Sr and Ba metal and Nb(OEt)₅ in ethanol with a key additive of ethoxyethanol. SBN (where x = 0.5) powder crystallized to orthorhombic phase at 700°C, and then transformed completely to tetragonal phase at 1200°C. The formation of tetragonal SBN was observed on sapphire (R) substrates at 700°C, whereas the tetragonal phase began to appear in the powders at 1000°C. SBN films with highly preferred orientation were successfully synthesized on MgO (100) substrates at 670°C.     

On the Theory of Interphase Interaction in a Mixture of Reacting Metal Particles

A mathematical model of interphase interaction in a mixture of powders is constructed on the basis of the principles of mechanics of multiphase, multispecies, and heterogeneous media, as applied to SHS processes. The evolution of structural transformations, chemical reactions, and phase transitions are considered at the level of a mesocell of the mixture. Analytical solutions are presented for conjugate thermodiffusion problems on single particles and in the mesocell, which close the equations of conservation laws. By an example of a single-stage chemical reaction in a mixture of nickel and aluminum powders (Ni + Al = NiAl), the problem of the structure of an SHS heat wave propagating over a finite-thickness semi-infinite rod is solved numerically. It is shown that the wave structure is strictly nonstationary and contains inflections and isothermal sectors at the melting point of the components, which vary periodically in time. To solve the problem of thermal explosion in a mixture of metal powders in a three-dimensional domain of complex geometry, a numerical algorithm with the use of the method of dummy domains was developed. Key words: high-temperature synthesis, intermetallides, thermal explosion, mathematical simulation.     

Synthesis of LiAlO2 Powder by Hydrolysis of Metal Alkoxides

Crystalline fine powders of LiAlO₂ can be prepared by hydrolysis of metal alkoxides for application to the breeder blanket in nuclear fusion. The precipitates were fine crystalline particles of β-LiAlO₂of ∼0.1-μm size under controlled conditions. The transformation from β- to γ-phase was confirmed at temperatures between 700° and 750°C. The microstructure of a sintered body at 1000°C consisted of homogeneous fine particles and pores suitable for the blanket material. A fully dense sintered body could be prepared at 1400°C.     

Preparation of Mullite Cordierite Composite Powders by the Sol-Gel Method: Its Characteristics and Sintering

Mullite/cordierite composite powders containing different proportions of cordierite were prepared by the sol-gel method using boehmite, colloidal silica, and Mg(NO₃)₂·6H₂O. Mullite and cordierite sols were prepared separately and mixed to form the composite sol. Mullitization temperature depends on the cordierite content in the composite. Also, α-cordierite crystallizes at a lower temperature in a mullite-rich (MC20) composite. The XRD patterns of the powders calcined at 1450°C for 12 h showed that mullite and cordierite exist as two different phases, and no additional phases were observed. The IR absorbance spectra of composites showed characteristic peak corresponding to both mullite and cordierite. The sintered density of the powders increases with temperature up to 1450°C and decreases beyound the melting point of cordierite (1455°C). The microstructure of MC30 sintered at 1440°C for 3 h consisted of acicular grains, whereas in MC40 and MC50 equiaxed grain morphology was observed under similar sintering conditions. The flexural strength and Vickers hardness decreases with the increase of cordierite content in the composite. Dielectric constant and thermal expansion showed a similar behavior.     

Preparation of β-BaB2O4 Powders and Thin Films by Sol–Gel Method

β-BaB₂O₄ (β-BBO) powders and films were successfully synthesized by the sol–gel method using metal alkoxides. A homogeneous and stable solution was prepared by the reaction of barium metal with boron triethoxide in ethanol by addition of diethanolamine. The drip-coated precursor films began to crystallize to β-BBO on Pt substrates at 500°C and converted to β-BBO films with preferred orientation to the c -axis at 700°C.     

Investigation of the System Beryllium–Boron

Mixtures of beryllium and boron powders were reacted under argon at elevated temperatures. X-ray single-crystal and powder diffraction analyses, chemical analyses, and Knoop micro-hardness measurements were made. Melting-point ranges were established for compounds of reasonable purity. One compound was identified. Beryllium diboride (BeB₂) had a hexagonal unit cell. Its hardness was 3180 ± 100 (H_k100) and its melting point was greater than 1970°C. Beryllium hexaboride ("BeB₆") had a tetragonal unit cell, a hardness of 2577 ± 75 (H_k100), and a melting point between 2020°) and 2120°C. An anisotropic unknown phase existed whose Be: B ratio was between 6:1 and 2:1 and whose hardness ranged between 623 and 917 (H_k25), depending on orientation. A eutectic between beryllium and this unknown phase was observed at 972°± 15°C.     

Preceramic Polymer Route to Amorphous and Crystalline Potassium Aluminosilicate Powders and Their Electrorheological Properties

The oxide one-pot synthesis (OOPS) process was used to synthesize a polymer precursor to potassium aluminosilicate, KAlSiO₄ (KASp). A KAlSiO₄ gel (KASg) also was produced via a solgel route using the same precursor. The two routes to KAlSiO₄ were explored to compare the effects of the two processing methods on powder properties. The KASp and KASg powders both transformed on heating (.500°C) to amorphous, high-surface-area powders with narrow pore-size distributions (4–24 nm). These anhydrous, amorphous powders were intrinsic electrorheological (ER) materials. Both materials crystallized at } 1070°C, and thermogravimetric analysis, differential thermal analysis, and X-ray diffractometry suggested that they were identical. Diffuse reflectance infrared Fourier transform spectroscopy and scanning electron microscopy proved that the KASp powders were homogeneous, whereas the KASg powders were heterogeneous and segregated. The KASg powders exhibited better ER properties that were associated with the segregated phases.     

Kinetic Analysis of Combustion Synthesis of Lead Magnesium Niobate from Metal Carboxylate Gels

A combustion synthesis route was developed for direct crystallization of perovskite lead magnesium niobate (PMN) from metal carboxylate gels. Direct PMN formation is attributed to a homogeneous cation distribution in the gel and the rapid heating during exothermic gel decomposition in oxidizing atmospheres. The maximum bed temperature and gel decomposition rate are determined by the combined influence of chemical kinetics, heat, and mass transfer. It is shown that there is a temperature window between 700° and 750°C which favors direct PMN formation, whereas pyrochlore formation is favored below 650°C and PbO volatilization occurs above 800°C. The effects of kinetic and transport parameters including oxygen partial pressure, gas flow rate, and bed geometry on PMN formation are discussed on the basis of a thermal ignition model for heterogeneous reactions.     

Electrochemical Synthesis and Sintering of Nanocrystalline Cerium(IV) Oxide Powders

Nanocrystalline CeO₂ powders were prepared electrochemically by the cathodic electrogeneration of base, and their sintering behavior was investigated. X-ray diffraction and transmission electron microscopy revealed that the as-prepared powders were crystalline cerium(IV) oxide with the cubic fluorite structure. The lattice parameter of the electrogenerated material was 0.5419 nm. The powders consisted of nonaggregated, faceted particles. The average crystallite size was a function of the solution temperature. It increased from 10 nm at 29°C to 14 nm at 80°C. Consolidated powders were sintered in air at both a constant heating rate of 10°C/min and under isothermal conditions. The temperature at which sintering started (750°C) for nanocrystalline CeO₂ powders was only about 100°C lower than that of coarser-grained powders (850°C). However, the sintering rate was enhanced. The temperature at which shrinkage stopped was 200°-300°C lower with the nanoscale powder than with micrometer-sized powders. A sintered specimen with 99.8% of theoretical density and a grain size of about 350 nm was obtained by sintering at 1300°C for 2 h.     

Spark plasma sintering of ZrB2- and HfB2-based Ultra High Temperature Ceramics prepared by SHS

The combination of the SHS technique and the Spark Plasma Sintering (SPS) technology was adopted in this work for the fabrication of fully dense MB₂-SiC and MB₂-MC-SiC (M = Zr, Hf) Ultra High Temperature Ceramics (UHTCs). Specifically, Zr or Hf, B₄C, Si, and (for the cases of ternary systems) graphite powders were first reacted by SHS to successfully form the desired composites. The resulting powders were then subjected to consolidation by SPS. In particular, by setting a dwell temperature level of 1800°C, a mechanical pressure of 20 MPa, and a non-isothermal heating time of 10 min, products with relative densities ≥98.5% were obtained for the all systems investigated within 30 min of total processing time. The characteristics of the resulting dense UHTCs, i.e. hardness, fracture toughness, and oxidation resistance, are similar to, and in some cases superior than, those related to analogous products synthesized by alternative, less rapid, methods. The article is published in the original.     

Lead Zirconate Titanate Prepared from Different Zirconium and Titanium Precursors by Sol-Gel

Modified sol-gel processes have been developed for the preparation of lead zirconate titanate (PZT) (52/48) powders. These processes use different starting sources to introduce the titanium and zirconium components, namely tetraethyl orthotitanate, titanium isopropoxide, or titanium diisopropoxide bis(2,4-pentanedionate) for titanium and zirconium propoxide or zirconium acetylacetonate for zirconium. To achieve stable and homogeneous precursor systems, several solvents (acetic acid, 1,2-propanediol, propanol, and distilled water) and chemical modifying additives, such as acetylacetone and nitric acid, were also introduced for the preparation processes. The influence of the different precursors on the crystallization behavior of the sol-gel-derived powders was studied. Well-crystallized single-phase PZT powders were obtained after heat treatment at 600°C for 1 h. The powders obtained sintered well at 1000°C/2 h and a homogeneous microstructure with small grain sizes was obtained.     

Hydrothermal Reaction-Sintering of Monoclinic HfO2

Sintered monoclinic HfO₂ bodies were fabricated below the transformation temperature from Hf metal and water by hydrothermal reaction-sintering. Sintering was observed above 900°C under 100 MPa for 3 h. Generally, both the bulk density and the crystallite size of the sintered bodies increased with increasing temperature. Bodies with the maximum relative density (0.98) were obtained by treatment above 1000°C.     

Synthesis of Red-Emitting, Small Particle Size Luminescent Oxides Using an Optimized Combustion Process

A novel ceramic synthesis technique, combustion synthesis, was explored to produce red-emitting Cr³⁺-doped Y₃A1₅O₁₂ and Eu³⁺-doped Y₂O₃ phosphors with improved physical and luminescent properties. This technique involves the reaction of metal nitrates (oxidizers) and an organic fuel (urea, carbohydrazide, glycine) at 500°C. Resulting powders are well-crystallized, with a large surface area and small particle size. The spectral energy distribution was observed using photoluminescence measurements. The effects of processing parameters such as type of fuel, fuelto-oxidizer ratio, furnace temperature, and batch water content were studied. An increase in phosphor brightness with increasing reaction temperature was observed. Postreaction heat treatments (1000°, 1300°, and 1600°C) increased the luminous intensity of as-synthesized powders. Residual carbon content and chromium site symmetry were investigated as possible explanations for the increase in brightness with increasing heat treatment temperature. By tailoring the reaction chemistry, the optimal conditions for producing the most luminescent phosphors have been identified.     

Sol-Gel Synthesis of a New Oxide-Ion Conductor Sr- and Mg-Doped LaGaO3 Perovskite

Sr- and Mg-doped LaGaO₃ powders were prepared from a salt acetate solution. The stable solution was peptized by reacting ammonium hydroxide with the precursor solution. Thermal analysis (DTA/TGA) was used to characterize first the dehydration and then the thermal decomposition of the organic ligands of the dried gel. The transformation from amorphous powders into a crystallized, homogeneous oxide phase corresponds to two endothermic peaks in the DTA curve; the first one at 150°C is related to the removal of water and is followed by a shoulder at 250°C. The second peak at 300°C corresponds to a superposition of two decomposition reactions: acetate salt into its oxycarbonate and this oxycarbonate into its oxide. Two subsequent exothermic peaks correspond to oxidation of evolved gases such as methane, hydrogen, and carbon monoxide. TEM observations show an average 10 nm particle size of the LaGaO₃powder after annealing at 600°C. X-ray diffraction patterns indicate a pure primitive-cubic phase is formed by 1300°C without formation of any SrLaGaO₃ impurity. The impedance spectroscopy on a 93%-dense sample exhibits no grain-boundary contribution and an ac conductivity σ= 0.11 Ω⁻¹·cm⁻¹ at 800°C.     

Hierarchically Porous Ceramics from Diatomite Powders by Pulsed Current Processing

Hierarchically porous ceramic monoliths have been fabricated by pulsed current processing (PCP) of diatomite powders. The partial sintering behavior of the porous diatomite powders during PCP or spark plasma sintering was evaluated at temperatures between 600° and 850°C. Scanning electron microscopy and mercury porosimetry measurements showed that the PCP method was able to bond the diatomite powder together into relatively strong monoliths without significantly destroying the internal pores of the diatomite powder at a temperature range of 700°–750°C. Little fusion at the particle contact points occurred at temperatures below 650°C while the powder showed partial melting and collapse of both the interparticle pores and the internal structure at temperatures above 800°C.