Simultaneous Synthesis and Densification of Titanium Nitride/ Titanium Diboride Composites by High Nitrogen Pressure Combustion

Composites of TiN/TiB₂ were synthesized by a combustion process of BN, Ti in a nitrogen atmosphere. The effect of the BN/Ti ratio and the nitrogen gas pressure on the synthesis of these composites was investigated. Dense TiN/TiB₂ composites with relatively high hardness and toughness were fabricated by combustion synthesis from Ti and BN under a nitrogen pressure of 4.0 MPa. The Vickers microhardness of the products obtained from reactants with a BN/Ti mole ratio of 0.11 increased with an increase in nitrogen pressure and had a maximum value of ∼25 GPa. Fracture toughness, K _IC, of the products increased from 3.1 to 5.9 MPa·m^1/2 as the BN/Ti ratio increased from 0.11 to 0.20. However, products formed under nitrogen pressures higher than 6.0 MPa exhibited circumferential macrocracks due to thermal shock.     

Synthesis and Characterization of Silicon Oxycarbide Glasses

It has been found that X-ray amorphous silica glasses containing up to 18% carbon can be synthesized using a sol/gel process. In this study, the sols were prepared using four different organometallic precursors—methyl-, ethyl-, propyl-, and phenyltrimethoxysilanes. ¹H NMR,¹³C NMR, and TGA revealed that the methoxy groups are hydrolyzed in the solutions and, therefore, are absent in the gels. But the alkyl groups are retained in the dry gels. The ²⁹Si NMR data verified that the Si—C bonds associated with these alkyl groups are intact in the dry gels. Most importantly, however, Si—C bonds are found in the glasses obtained after heat-treating the gels at high temperature in an inert atmosphere; i.e., the synthesis does, in fact, create an oxycarbide network structure. TGA showed that the dense oxycarbide glasses are stable to 1000°C in argon and in air.     

AlON合成及致密化的研究

本文以AlN和Al2O3为原料采用无压烧结的方法制备AlON,研究了不同温度对AlON合成物相的影响,及添加剂MsO、Y2O3和CaO对其致密化的影响.通过X射线衍射仪、扫描电镜和阿基米德原理对其物相、显微结构和体积密度等性能进行表征.研究结果表明:在1700℃时AlON开始生成,随着温度升高AlON相含量越多,最佳合成温度为1800℃,当温度达到1900℃时出现多种AlON相;在1800℃时,添加剂MsO、Y2O3和CaO均能促进AlON的致密化,而添加剂CaO添加量为20wt％时,AlON致密化最佳,气孔率最小为5.43％,密度为3.14 g/cm3.     

Barium Holmium Zirconate, A New Perovskite Oxide: II, Synthesis as Nanoparticles through a Modified Combustion Process

Nanoparticles of barium holmium zirconate, a new complex perovskite ceramic oxide, has been synthesized using a modified self-propagating combustion process. The solid combustion products obtained were characterized by X-ray diffraction (XRD), electron diffraction, differential thermal analysis, thermogravimetric analysis, infrared spectroscopy, particle size analysis, surface area determination, and high-resolution transmission electron microscopy. The XRD and electron diffraction studies have shown that the as-prepared powder is phase pure Ba₂HoZrO_5.5 and has a complex cubic perovskite (A₂BB'O₆) structure with a lattice constant a = 8.428 Å. The transmission electron microscopic investigation has shown that the particle size of the as-prepared powder was in the range 4–16 nm with a mean grain size of 8.2 nm. The nanoparticles of Ba₂HoZrO_5.5 obtained by the present method could be sintered to 98% theoretical density at 1500°C.     

Synthesis of Solid Phosphate Compounds of Ti(III) and Ti(IV)

Several new methods for the synthesis of TiPO₄ are described. Crystalline phosphate compounds containing boron and titanium in a single III or mixed (III, IV) oxidation states have been synthesized and identified by X-ray diffraction. An additional family of phosphate compounds containing only Ti(III, IV) has also been synthesized and identified. All the compounds exhibit attractive colors, e.g., bright green, blue, purple, and black, and are stable at room temperature in air and in boiling water.     

Titanium Diboride–Tungsten Diboride Solid Solutions Formed by Induction-Field-Activated Combustion Synthesis

Solid solutions of titanium diboride–tungsten diboride (TiB₂–WB₂) were synthesized by induction-field-activated combustion synthesis (IFACS) using elemental reactants. In sharp contrast to conventional methods, solid solutions could be formed by the IFACS method within a very short time, ∼2 min. Solutions with compositions ranging from 40–60 mol% WB₂ were synthesized with a stoichiometric ratio (Ti + W)/B =½; however, samples with excess boron were also made to counter the loss of boron by evaporation. The dependence of the lattice constants of the resulting solid solutions on composition was determined. The "a" parameter decreased only slightly with an increase in the WB₂ content, whereas the "c" parameter exhibited a significant decrease over the range 40–60 mol% WB₂. Solid-solution powders formed by the IFACS method were subsequently sintered in a spark plasma sintering (SPS) apparatus. After 10 min at 1800°C, the samples densified to relative density 86%. XRD analysis showed the presence of only the solid-solution phase.     

Surface Damage Resistance of Gel-Derived Oxycarbide Glasses: Hardness, Toughness, and Scratchability

Gel-derived oxycarbide glasses have atomic network structures similar to that of vitreous silica glass but with carbon-rich regions consisting of CSi₄ tetrahedra and C–Si–O bonds finely dispersed in the glass. Therefore, oxycarbide glasses exhibit the so-called anomalous hardness behavior, similar to silica-rich glasses, with a substantial densification–strain component beneath the indenter. However, the role of carbon is twofold: on the one hand, the covalently bonded carbon atoms slightly affect the behavior, similar to the way network modifiers affect the behavior of silicate glasses, and favor a normal indentation behavior; and on the other hand, the free carbon, forming turbostratic graphite domains, provides easy crack initiation sites and low-energy fracture paths. Almost concentric shear steps and microcracks, which follow the turbostratic graphite domains, are observed after indentation. The ultimate coalescence of the microcracks produces Hertzian-type cone cracks.     

Impurities in the Combustion Synthesis of Titanium Carbide

The evolution of solid and gaseous impuritics from powders and their effects during the combustion synthesis (self-propagating high-temperature synthesis) of titanium carbide have been studied. First, the volatiles from the precursor powders, released during bakeout in a vacuum furnace, were measured by residual gas analysis. Second, both condensable and non-condensable materials emitted during the reaction of the Ti + C system were analyzed. The results indicate that the major noncondensing species in both cases were water vapor, hydrogen, carbon monoxide, carbon dioxide, and hydrocarbons. Significant amounts of oxygen and TiO₂ were also enitted during the reaction, but were not observed during bakeout. These impurities are shown to affect not only the reaction process, but also the microstructure of the reaction product.     

Synthesis of Aluminum Nitride–Silicon Carbide Solid Solutions by Combustion Nitridation

AlN–SiC solid solutions were synthesized via a combustion nitridation process. Reactions between powder mixtures of aluminum, silicon, and carbon or aluminum with β-SiC and gaseous nitrogen under pressures of 0.1–8.0 MPa are self-sustaining once they have been initiated. Investigations were made with reactant ratios of Al:Si:C = 7:3:3, 6:4:4, and 5:5:5 and Al:SiC = 7:3, 6:4, and 5:5. For the Al-Si-C system (molar ratio of 6:4:4), the maximum combustion temperature was dependent on the nitrogen pressure, increasing from 2300°C to 2480°C with an increase in pressure, from 0.1 MPa to 6.0 MPa. In all cases, the product contained the solid solution as the primary phase, with minor amounts of silicon. The amount of unreacted silicon decreased as the nitrogen pressure increased; the presence and dependence of unreacted silicon on pressure has been explained in terms of the volatilization of aluminum. The full width at half maximum for the (110) peak of the AlN–SiC solid solution decreased as the nitrogen pressure increased, which indicated the formation of a more homogeneous product.     

Combustion synthesis of B4C/Al2O3/C composite powders and their effects on properties of low carbon MgO-C refractories

To improve the dispersity and oxidation resistance of nano carbon black (CB) in low carbon MgO-C refractories, B₄C/Al₂O₃/C composite powders were prepared by a combustion synthesis method using B₂O₃, CB and Al powders as the raw materials. The phase compositions and microstructures of the synthesized products were characterized by X-ray diffraction (XRD), Raman spectroscopy, and a scanning electron microscopy/energy dispersive spectrometry (SEM/EDS). The results show that an 80 wt% excess of CB is the maximum amount of CB that can be added under the condition of a self-propagating combustion wave, and the phase compositions of the products are B₄C, α-Al₂O₃ and CB. B₄C particles with uniform sizes and cubic polyhedral structures are embedded in the Al₂O₃ matrix. The combustion-synthesized B₄C/Al₂O₃/C powders and mechanically mixed B₄C/Al₂O₃/C powders were added to the low carbon MgO-C refractories, and their corresponding properties were compared. The apparent porosity (AP) of the refractories with the synthesized powders (labelled as M3) is lower than those of the refractories with mechanically mixed powders (labelled as M2) and without composite powders (labelled as M1). The oxidation ratio and slag erosion depth of M3 were lower than those of M2 and M1. The thickness of the decarburized layer of M3 was 10.2% and 22.4% less than that of M2 and M1, respectively. The penetration depth of M3 was 12.0% and 27.9% less than that of M2 and M1, respectively. The thermal shock resistance of M3 was better than that of M2 and M1. The residual strength ratio of M3 was 15.8% and 17.2% more than that of M2 and M1, respectively. These results suggest that the combustion-synthesized B₄C/Al₂O₃/C composite powders can be used as new and promising additives for low carbon MgO-C refractories.     

Effect of Porosity on the Combustion Synthesis of Titanium Nitride

The effect of porosity on the self-propagating reaction between porous titanium and gaseous nitrogen was investigated. The relationship between total nitrogen uptake and porosity showed a maximum at about 44% porosity. High initial sample porosities lead to partial melting of the titanium and subsequently to a lower degree of conversion to the nitride. Low initial porosities limit the conversion through lower reaction interfacial areas and lower gas permeation. Wave velocity measurements, thermogravimetric determinations on the progression of the combustion front, and microstructural analyses demonstrated that the passage of the front is associated primarily with the formation of a surface layer of TiN_1-x with x being 0.10 and 0.06 for lowdensity (49%) and high-density (59%) samples. The product of combustion contained titanium nitride and primary and β-transformed α solid solutions. The relative abundance of the latter two phases was dependent on the initial relative density of the samples.     

Effect of Iron and Boron Carbide on the Densification and Mechanical Properties of Titanium Diboride Ceramics

The effect of Fe and B₄C on the sintering behavior and mechanical properties of TiB₂ ceramics have been studied. Sintering was performed in an Ar atmosphere at 2000° using attrition-milled TiB₂ powder (mean particle size = 0.8 μm). When a small amount of Fe (0.5 wt%) was added, abnormal grain growth occurred and the sintered density was low. In the case of B₄C added along with 0.5 wt% Fe, however, abnormal grain growth was remarkably suppressed, and the sintered density was increased up to 95% of theoretical. But with excess Fe addition (5 wt%), B₄C grains did not act as a grain growth inhibitor, and B₄C grains were frequently trapped in large TiB₂ grains. The best mechanical properties were obtained for the TiB₂–10 wt% B₄C–0.5 wt% Fe ceramics, which exhibited a three-point bending strength of 400 MPa and a fracture toughness of 5.5 MPa · m^1/2.     

Zirconia-Based Metastable Solid Solutions through Self-Propagating High-Temperature Synthesis: Synthesis, Characterization, and Mechanistic Investigations

Cubic Zr_{1− x} Me _x O _y (Me = Fe, Co, Ni, Cu) metastable solid solutions with metal content significantly higher than equilibrium levels have been synthesized by the self-propagating high-temperature synthesis method based on a thermite reaction between metallic zirconium and the transition-metal oxides CoO, Fe₂O₃, CuO, and NiO. Through in situ XRD analysis, it was determined that when heated to 1100°C, the cubic solid solution transformed to the tetragonal phase with the concomitant formation of iron oxide. When cooled to lower temperatures, the tetragonal phase transformed to the monoclinic phase at or below 500°C. Results of auxiliary experiments strongly suggest that the formation of the solid solution takes place behind the combustion front by a reaction between zirconia and the metal.     

Fabrication of Titanium Carbide-Alumina Composites by Combustion Synthesis and Subsequent Dynamic Consolidation

Dispersed-phase composites of TiC-Al₂O₃ were fabricated from low-cost reactants by external ignition of powder compacts followed by dynamic consolidation of the hot products using explosives. Near full densification was achieved using shock pressures as low as 1 GPa. Care must be taken to allow for the escape of evoled gases during exothermic reaction, and rapid cooling of the consolidated composite must be avoided to prevent cracking.     

Effect of Heating Rate on the Sintering of Titanium Dioxide Thin Films: Competition between Densification and Crystallization

Sintering of sol-gel-derived TiO₂ thin films was studied as a function of heating rate by ion-beam analysis, X-ray diffraction, and ellipsometry. The degree of densification increases with increasing heating rate. Densest films containing the lowest amount of hydroxyls are obtained using rapid thermal annealing. The results are rationalized in terms of a competition between densification and crystallization.     

Effect of Nitrogen Pressure and Diluent Content on the Combustion Synthesis of Titanium Nitride

The effects of gas pressure and diluent content on the combustion synthesis of titanium nitride was investigated. Combustion of titanium powder samples containing TiN as a diluent in the concentration range 0 to 60 wt% was carried out in a nitrogen atmosphere at pressures in the range 0.1 to 1.4 MPa. Analysis of the dependence of combustion wave velocity on temperature gave an activation energy of 342 ± 50 kJ. mol⁻¹, which is in good agreement with reported values for the diffusion of nitrogen in titanium nitride.     

Combustion Synthesis of Sialon Powders (Si6-zAlzOzN8-z, Z = 0.3, 0.6)

Fine sialon powders (Si_6-zAl_zO_zN_8-z, Z = 0.3, 0.6) were prepared by nitriding combustion. Silicon powders was reacted with AlN and Si₃N₄ under 10 MPa of nitrogen gas. The reaction temperature reached above 2000°C and single-phase sialon powders were synthesized within minutes. The obtained powders were fully densified without additives by hot isostatic pressing. The sintered body had a flexural strength of 600 MPa by 4-point bending.     

固体Ti(SO4)2催化合成苯甲酸甲酯

以固体Ti(SO4) 2 为催化剂、苯甲酸和甲醇为原料合成了苯甲酸甲酯。考察了催化剂用量、反应时间、醇酸比对酯产率的影响。实验结果表明 ,最佳实验条件为 :在 6 5～ 75℃下 ,催化剂用量为 8% (占苯甲酸质量分数 )、反应时间为 3h、甲醇与苯甲酸用量比为 10∶1,所得到的酯产率达 93 0 %。     

Factors Affecting the Compaction and Densification Behavior of Ti(C,N)‐Based Cermet Powders

In this study, the effects of the major factors on compaction and densification behavior are investigated for Ti(C, N)‐based cermet powders. The relative density equation for green compact of composite powders is modified to predict the green density of Ti(C, N)‐based cermet powders prepared under different degrees of pressing pressures, and the theoretical values are found to be in good agreement with experimental results. It has been found that the composite powders with micron‐sized particles have a better compatibility than those with nano‐sized particles by analyzing the effects of the particle size and purity on the starting mono‐powders. It has also been found that the volume shrinkage and porosity of the former are lower than that of the latter. In addition, it shows that high oxygen content has a negative impact on both the compatibility of composite powders and the uniformity of pore size distribution of sintered cermets. It has also been discussed in this study how the pressing parameters such as pressing pressure, pressing temperature, and dwell time influence the resulting cermets. The results indicate that a better compatibity is reached at a pressing rate of 100 mm/min or a pressing temperature of 100°C.     

Synthesis,densification, microstructure,and mechanical properties of samarium hexaboride ceramic

Samarium hexaboride (SmB₆) powders were synthesized by boro/carbothermal reduction of Sm₂O₃ with B₄C. Nominally pure SmB₆ powder had a mean particle size of about 400 nm and an oxygen content of 0.12 wt%. SmBO₃ formed as an intermediate phase during the synthesis. The synthesized powder was hot pressed at 1950°C to produce SmB₆ ceramics with relative densities >99.6% and a mean grain size of 4.4 μm. Vickers’ hardness was 20.1 ± 0.7 GPa. Young's modulus measured by bending and ultrasonic methods was 271 and 244 GPa, respectively. The flexure strength was 253 ± 79 MPa and fracture toughness was 2.1 ± 0.1 MPa m^1/2. These are the first reported results of the microstructure and bulk mechanical behavior of SmB₆ ceramics.