Rate of formation of C3S in the system CaOSiO2Al2O3Fe2O3MgO with addition of CaF2

The influence of CaF₂ on the kinetics of the reaction C+C₂S → C₃S in five-component diffusion sandwiches (CaOSiO₂Al₂O₃Fe₂O₃MgO) was determined in the temperature interval 1350°C to 1500°C. With the addition of 0.5% CaF₂ to the reacting system the rate of reaction increased by a factor of 2.4 at a reaction temperature of 1350°C, and a factor of 1.2 at 1500°C. Addition of 1% CaF₂ raised the reaction rate 2.8 times at 1350°C and 1.7 times at 1500°C. The effect of CaF₂ on the rate of C₃S formation may be attributed to the fact that the C₃S primary field is much wider in the CSCaF₂ system than in the CSAF system.     

Studies on clinker calcium silicates bearing CaF2 and CaSO4

The system CaOC₂SCaSO₄CaF₂ was studied at temperatures of 1000°C and 1150°C. The solid state equilibrium relations were determined only partially since liquid phase was present in many samples at the aforesaid temperatures. The system contains only a quaternary compound: the flour-ellestadite 3C₂S.3CaSO₄.CaF₂. The tests carried out lead the authors to think that the two silico-fluorides 2C₂S.CaF₂ and 3C₃S.CaF₂ can form only in the cement raw meals with Fwt%/SO₃wt% ratio > 0.158. Therefore the intermediate compounds which can generally interest the firing process of clinker are the fluor-ellestadite and the calcium silico-sulphate (2C₂S.CaSO₄). Analytical determinations carried out also by microprobe showed that the composition of “3C₃S.CaF₂” better corresponds to the formula 19CaO.7SiO₂.2CaF₂.     

Synthesis of YB2C2 by high-energy ball milling and reactive spark plasma sintering

X-ray pure yttrium diborodicarbide (YB₂C₂) was synthesized for the first time via modified spark plasma sintering (SPS) using Y₂O₃, B₄C, and carbon as starting materials. Homogeneous intermixing of the raw powder by high energy ball milling (HEBM) promoted the formation of YB₂C₂ powder with layered structure by boro/carbo-thermal reduction. The average particle size of the synthesized powder at an optimum synthesis temperature (1625℃) was 568 nm. Small amount of YB₂C was formed by consuming YB₄ at and above 1625°C. The metal basis purity of the synthesized YB₂C₂ powder was 99.84%. X-ray pure YB₂C₂ was obtained by densifying the synthesized YB₂C₂ powder at 1900°C for 60 minutes at a pressure of 80 MPa using SPS. This study reports a simple method for the fabrication of X-ray pure rare earth metal diborodicarbides (ReB₂C₂).     

Role of CaF2 on mechanochemically synthesized leucite as dental veneering glass ceramics

Leucite based glass ceramic is widely used in dental ceramics as porcelain fused to metals for veneering applications. Main properties considered here are high coefficient of thermal expansion and good mechanical properties. Owing to these requirements, high expansion phase such as leucite is incorporated in these glass ceramics. The present work was aimed to synthesise leucite using its stoichiometric batch compositions and subsequent high energy ball milling. CaF₂ was also added in another mix to study its role on leucite formation. Further prepared, leucite phase was added in separately prepared low temperature glass frit powders to control amount of glass and leucite content. X-ray diffraction results displayed that high energy ball milling and additive promoted the formation of leucite as a major crystalline phase. Furthermore, CaF₂ also suppressed the subsidiary crystallisation of kalsilite phase. Evaluated average coefficient of thermal expansion in the temperature range of 20–500°C was very close to the theoretical value of pure leucite.     

Synthesis of Alkali Aluminosilicates — Materials for Alkali Contaminated Environments at High Temperatures

Alkali corrosion has become a problem in industrial furnaces especially because of the increasing use of secondary fuels. In the corroded lining material alkali aluminosilicates such as kalsilite, leucite or nepheline could be identified. According to ternary phase diagrams these substances have very high melting points which would make them suitable for high temperature applications in alkali corrosive environments. This study presents systematic synthesis experiments to produce alkali aluminosilicates by thermal (800°C, 1000°C, 1200°C) and hydrothermal (200°C) treatment starting from the nominal compositions of KAlSi₂O₆, KAlSiO₄, and NaAlSiO₄., KOH NaOH, quartz powder, and Al(OH)₃ were used as raw materials. The phase composition was analyzed by using X‐ray diffraction (XRD). The alkali corrosion was tested using alkali salts as corrosive substances. The synthesis experiments resulted in multiphase reaction products. The hydrothermal method yielded only for the initial composition according to the stoichiometry of KAlSiO₄ crystalline phases of the same composition. The thermal method produced for all sets of synthesizing parameters mixtures of stoichiometric and nonstoichiometric alkali aluminosilicates. In the corrosion test, material of the nominal composition of KAlSiO₄ showed the best results. The material was corrosion resistant independently from the initially applied synthesizing parameters.     

Nonisothermal crystallization kinetics and stability of leucite and kalsilite from K2O-Al2O3-SiO2 glasses

The crystallization mechanisms and elemental stability of leucite and kalsilite formed from K₂O-Al₂O₃-SiO₂ glasses were investigated by X-ray powder diffraction (XRD), X-ray fluorescence (XRF), Raman spectroscopy and differential scanning calorimetry (DSC). Glass samples with compositions along the leucite-kalsilite tie-line were produced by melt processing and were then heat-treated at 850, 950, and 1250°C for times ranging from 5 minutes to 1000 hours. Kalsilite is an unstable phase that behaves as an intermediate precursor to leucite. Crystalline materials in which kalsilite is the major phase lose potassium upon prolonged heat treatment (1000 hours at 1250°C), in contrast to those with leucite, in which little or no compositional alteration is detected. The formation of leucite from stoichiometric kalsilite is accompanied by the formation of potassium-doped alumina. The activation energies for leucite and kalsilite crystallization, determined via application of the Kissinger equation to thermal analysis data, were 579 and 548 kJ/mol, respectively. Finally, production of pure leucite can be achieved with more favorable crystallization kinetics when starting with off-stoichiometric compositions.     

Effects of heat-treatment temperature and starting composition on morphology of boron carbide particles synthesized by carbothermal reduction

Carbothermal reduction using B₂O₃ and carbon black was applied for synthesis of B₄C powder and the effects of heat-treatment temperature and starting composition of raw mixture on morphology of B₄C particles were investigated. Morphology of B₄C particles synthesized at 1450 °C was mainly spherical shapes. The B₄C powder synthesized at 1550 °C was large and changed in morphology from polyhedral to skeletal shape, and particle size of B₄C increased with an increase in the amount of B₂O₃ in the starting mixtures. The B₄C powder synthesized beyond 1650 °C consisted from dendrite-like particles aggregated by small primary particles. Morphology of the primary B₄C particles synthesized at 1750 °C changed from polyhedral to rounded shape with increasing the amount of B₂O₃ in the starting mixtures. It is clarified that heat-treatment temperature and the starting compositions of raw mixtures mainly affected B₄C nuclei number along with primary particle size and morphology of primary B₄C particles, respectively.     

AC Impedance Spectroscopy of CaF2‐doped AlN Ceramics

The electrical conductivity of CaF₂‐doped aluminum nitride (AlN) ceramics was characterized at high temperatures, up to 500°C, by AC impedance spectroscopy. High thermal conductive CaF₂‐doped AlN ceramics were sintered with a second additive, Al₂O₃, added to control the electrical conductivity. The effects of calcium fluoride (CaF₂) on microstructure and related electrical conductivity of AlN ceramics were examined. Investigation into the microstructure of specimens by TEM analysis showed that AlN ceramics sintered with only CaF₂ additive have no secondary phases at grain boundaries. Addition of Al₂O₃ caused the formation of amorphous phases at grain boundaries. Addition of Al₂O₃ to CaF₂‐doped AlN ceramics at temperatures 200°C–500°C revealed a variation in electrical resistivity that was four orders of magnitude larger than for the specimen without Al₂O_3. The amorphous phase at the grain boundary greatly increases the electrical resistivity of AlN ceramics without causing a significant deterioration of thermal conductivity.     

Highly transparent AlON fabricated via SPS synthesized by Al2O3/C core–shell starting powder and CRN method

Two types of starting powders were used to synthesis AlON powder via the carbothermal reduction and nitridation (CRN) route, the Al₂O₃ and C mixture, and the Al₂O₃/C core–shell. Al₂O₃/C core–shell resulted in lower carbon content and lower synthesis temperature for CRN method. AlON powder was synthesized using Al₂O₃ and C then was sintered via SPS at 1500, 1600, and 1700°C for 10-30 min in a vacuum atmosphere. A sample with the highest transmittance was annealed at 1250°C for 2, 4, and 8 h in air. The characteristics of AlON powder and bulk samples were investigated by XRD, DLS, FT-IR, elemental analysis of carbon and sulfur, SEM, EDS, and UV–Vis spectroscopy analyses. The annealing process for 2 h improved the transmittance from 31% to 58% at the wavelength of 600 nm. AlON powder synthesized using Al₂O₃/C core–shell, was SPSed at 1700°C for 15 min and annealed at 1250°C for 2 h was used to make a sample of 1.7 mm thickness whose transmittance was found to be 81%.     

Synthesis of CoFe2O4 Nanoparticles by a Low Temperature Microwave‐Assisted Ball‐Milling Technique

Well‐crystallized Cobalt ferrite nanoparticles with mean size of 20 nm and high saturation magnetization (82.9 emu/g) were synthesized at a low temperature (≤100°C) by microwave‐assisted solid–liquid reaction ball‐milling technique without subsequent calcination. CoC₂O₄·4H₂O and Fe powder were used as raw materials and stainless steel or pure iron milling balls with diameter of 1.5 mm were used. As a contrast, solid–liquid reaction ball milling without microwave assistance was also investigated. The results showed that this is a simple, environmentally friendly, and energy‐saving technique for ferrite nanocrystal synthesis.     

The effect of carbon and nickel additions on the precursor synthesis of Cr3C2-Ni nanopowder

Decreasing crystal size to nanoscale is a proven method to enhance material properties. In this study, nanosize Cr₃C₂ and Cr₃C₂-Ni were synthetized and the reaction sequence was studied. Aqueous precursors using only water-soluble raw materials with varying carbon contents and a nickel addition were spray-dried. Glycine was used as a carbon source and chromium acetate hydroxide as a chromium source in the precursor solutions. Nickel nitrate hexahydrate was introduced as a nickel source to yield a metallic binder into the carbide nanopowder.Resulting powders were heat-treating to identify an applicable precursor composition producing the targeted Cr₃C₂ phase with crystal size of tens of nanometers. Thermal synthesis tests of the precursor powders to yield Cr₃C₂ took place at a temperature between 900 and 1300?°C under an Argon atmosphere. The synthesis of nanosize Cr₃C₂-Ni powder was successful at 1000?°C in 30?min, in a case of the best precursor. In order to produce the carbide phase with no residual oxide traces, relative carbon load has to be 48?wt%, while the stoichiometric amount of carbon in Cr₃C₂ is 13?wt%. When also introducing the nickel source into the precursor, an even higher carbon load was required. The carbon surplus needed to enable the Cr₃C₂ synthesis attributes to the non-homogeneity of the precursor composition.The chemical synthesis starting from water-soluble raw materials is a promising way of preparing nanosize Cr₃C₂-Ni with the targeted phase configuration.     

Influence of Yb Concentration on the Optical Properties of CaF2 Transparent Ceramics Codoped with Er and Yb

Er, Yb:CaF₂ nanoparticles with different Yb concentrations were synthesized by a coprecipitation method using nitrates as raw materials. X‐ray powder diffraction and transmission electron microscopy analysis showed that the nanoparticles were single fluorite phase and the nanoparticle size was found to decrease with increasing Yb concentrations. The obtained nanoparticles were hot‐pressed at 800°C under 30 MPa under vacuum environment to fabricate Er, Yb:CaF₂ transparent ceramics. The influence of Yb ion concentrations on the optical transmission, microstructure, and luminescence properties of Er, Yb:CaF₂ transparent ceramics were investigated. The addition of Yb ions was found effectively to reduce grain size and has a positive effect on improving the optical transmission of Er, Yb:CaF₂ transparent ceramics. The highest transmittance in the near‐infrared spectral region of the Er, Yb:CaF₂ transparent ceramic reached about 90%. The green, red, and near‐infrared emission intensities were found to increase with increasing Yb concentration.     

Microstructure of HVOF-sprayed Ag–BaF2⋅CaF2–Cr3C2–NiCr coating and its tribological behavior in a wide temperature range (25 °C to 800 °C)

Ag–BaF₂?CaF₂–Cr₃C₂–NiCr composite powders were prepared by physically blending commercial BaF₂?CaF₂–Cr₃C₂–NiCr and Ag powders. Ag–BaF₂?CaF₂–Cr₃C₂–NiCr composite coatings were deposited on Inconel 718 alloy substrate by high velocity oxy-fuel (HVOF) spraying. The friction and wear behavior of the coatings under dry sliding against Si₃N₄ balls from 25 °C to 800 °C was evaluated with a ball-on-disk high temperature tribometer. The microstructure and composition of the samples were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectrometer. Results showed that the composite coatings were mainly composed of hard phase of Cr₃C₂, binder phase of NiCr, high-temperature lubrication phase of fluorides and low-temperature lubrication phase of Ag. The fluorides existed in the forms of both crystal particles and amorphous state, while the silver featured as typical thermally sprayed splats. Due to the high flame temperature, some fluorides have been oxidized to chromates and around 30 wt% of Ag was lost during spraying. In addition, it was found that Ag content had an important influence on the composite coating, and an appropriate dosage of metallic silver could effectively improve the tribological performance of the coating. The generation of AgCrO₂ at moderate (500 °C and 650 °C) temperature and BaCrO₄ at high temperature (800 °C) could contribute to the decline in friction coefficients and wear rates of Ag–BaF₂?CaF₂–Cr₃C₂–NiCr coatings.     

PREPARATION AND THERMAL EXPANSION OF FE2-xYxW3O12 POWDER BY CITRATE SOL-GEL PROCESS

Fe_2-xY_xW₃O₁₂ powder has been synthesized by the citrate sol-gel process. A model was proposed to calculate the concentration of species in a citric solution. The calculated results could provide valuable information for determining the optimal molar ratio of cation to citric acid and pH value of solution for Fe_2-xY_xW₃O₁₂ preparation. The predicted parameters derived from this model are in good agreement with the experimental results. The prepared gel and the Fe_2-xY_xW₃O₁₂ powder were characterized by X-ray diffraction (XRD) and differential thermal analysis-thermogravimetry (DTA-TG). The results show that it is very difficult to obtain pure Fe₂W₃O₁₂ powder by the citrate sol-gel process in the temperature range 500°–1000°C, however, Y₂W₃O₁₂ can easily be prepared even at 500°C. Y₂W₃O₁₂ annealed at 1000°C for 10 h is favorable for absorbing moisture in air to form Y₂W₃O₁₂·3.3H₂O. The thermal expansion coefficients of Y₂W₃O₁₂·3.3H₂O are: α_a = ? 8.01 × 10^?6°C^?1, α_b = ? 2.51 × 10^?7°C^?1, and α_c = ? 5.55 × 10^?6°C^?1 in 473–1173 K.     

Ethanol‐Water‐Derived Sucrose‐Coated‐Al2O3 for Submicrometer AlON Powder Synthesis

Fluffy and homogenous sucrose‐coated‐γ‐Al₂O₃ structured precursor was prepared by drying ethanol‐water sucrose/Al₂O₃ suspension, in which the ethanol content of 85 vol% was optimized. Using the C/Al₂O₃ mixture pyrolyzed from such precursor with 23.2 wt% sucrose, single‐phase AlON powder was synthesized by two‐step carbothermal reduction and nitridation method at 1550°C for 2 h and 1700°C for another 1.5 h. The particle size of the AlON powder was around 0.6–1.0 μm. Compared with those synthesized by the traditional approaches with mechanical C/Al₂O₃, Al/Al₂O₃, or AlN/Al₂O₃ mixtures, the synthesis temperature was reduced about 50°C, and the AlON powder was fine and exhibited good dispersity. Such superiority of this method was attributed to that the pyrolyzed carbon film on Al₂O₃ particle greatly restrained Al₂O₃ coalescence during the thermal treatment.     

Enhanced nitridation of silicon compacts by Yb2O3 addition

The catalytic effect of ytterbium oxide (Yb₂O₃) on the nitriding reaction of Si compacts was investigated. Si powder mixtures containing Yb₂O₃ were prepared and nitrided in the form of compacts with a multi-step heating schedule over the range of 1200 °C–1450 °C. The nitriding profiles of the powder mixture with increasing temperature indicated that Yb₂O₃ clearly promoted the nitridation of Si compacts at 1200 °C compared with the pure Si compact containing no additives. The critical role of Yb₂O₃ on the nitridation of Si, was elucidated that Yb₂O₃ promotes the loss of initial SiO₂ of the raw Si powder via the measurement of the weight changes at low temperature (1100 °C) and thermogravimetric analysis under N₂ atmosphere. It was also found that the β-ratio of fully nitrided Si was closely related to the intermediate degree of nitridation at 1200 °C and 1300 °C.     

The effect of natural dolomite admixtures on calcium zirconate–periclase materials microstructure evolution

The reaction sintering mechanism of dolomite–zirconia mixtures was investigated using fine grounded dolomite raw material and zirconium powder. The used dolomite raw materials differed by the content of impurities (SiO₂, Al₂O₃ and Fe₂O₃ oxides). The microstructure evolution of MgO–CaZrO₃ and CaZrO₃ sintered materials was presented as a temperature function. One- and two-step firing processes of calcium raw materials powder mixed with chemically pure zirconium oxide were applied. The kinetics of reaction of CaZrO₃ synthesis was estimated by determining the “free” calcium oxide by chemical and XRD analysis. The densification process was evaluated by firing shrinkage, apparent density, pore diameter and pore size distribution measurements. The microstructure of sintered materials was observed by SEM. It was observed that CaZrO₃ synthesis was definitely finished at temperature of 1500 °C in the both applied ways of the synthesis (one- or two-step process). The only phase present in the model material synthesized from chemically pure reagents (CaCO₃ and ZrO₂) after firing at temperature of 1500 °C was calcium zirconate.In the materials synthesized from natural dolomites and ZrO₂ two main phases were present—calcium zirconate and periclase. During firing of CaZrO₃–MgO materials at lower temperatures the presence of transient phases was detected (mainly ferrites and calcium aluminates, 4CaO·Al₂O₃·Fe₂O₃ or 2CaO·Fe₂O₃). These phases disappeared at higher temperatures. This is probably related to the dissolution of impurities in the main phases of CaZrO₃–MgO.The material obtained from the mixture of zirconium oxide and natural dolomite with the high impurities content has the highest densification level (～95% theoretical density of CaZrO₃–MgO) at 1500 and 1600 °C.     

Synthesis,sintering, and grain growth kinetics of Hf6Ta2O17

A systematic study of the solid-state synthesis, pressureless sintering, and grain growth kinetics of Hf₆Ta₂O₁₇ is presented. The ideal conditions for solids-state synthesis of Hf₆Ta₂O₁₇ powder with minimal particle necking was 1250 °C for 2 h in air. The resultant powder has an average particle size of 210 ± 70 nm. The combined synthesis and ball-milling procedure produces highly sinterable Hf₆Ta₂O₁₇ powder, achieving > 97 % of theoretical density after pressureless sintering at 1600 °C for 2 h in air. The grain growth mechanism was sensitive to processing conditions, appearing to be primarily driven by surface diffusion below 1600 °C and grain boundary diffusion above 1650 °C. The respective activation energies for grain growth were found to be Q_S = 659 ± 79 kJ mol⁻¹ and Q_GB = 478 ± 63 kJ mol⁻¹.     

Combustion synthesis of SiC/Al2O3 composite powders with SiC nanowires and their growth mechanism

SiC/Al₂O₃ composite powders with SiC nanowires were synthesized using a one-step combustion synthesis method taking silica fume (SiO₂), aluminum powder (Al) and carbon black (CB) as raw materials, while ferrocene (C₁₀H₁₀Fe) was used as the catalyst. The calculated results for the relationship between the equilibrium phase and temperature of the Al–SiO₂–C system show that SiC and Al₂O₃ are the only equilibrium phases in the system. In addition, the effects of C₁₀H₁₀Fe on the combustion synthesis process and products were studied. It was found that with increasing catalyst content, the amount of residual Si in the products first decreases and then increases, the combustion temperature first increases and then decreases, and the nanowire content continues to increase. For an optimal amount of C₁₀H₁₀Fe of 0.75 wt%, almost no residual Si is observed in the product, while the combustion temperature (T_c) is high (2104 K), the SiC nanowire content is relatively high, and the nanowire aspect ratio is large. In addition, two growth mechanism models for SiC nanowires: VS and VLS were validated.     

The system CaOAl2O3Fe2O3 with added fluoride flux

Effects of fluoride fluxes, especially CaSiF₆·2H₂O upon phase equilibria in the system CaOAl₂O₃Fe₂O₃ are described, presenting liquidus data, new phase field boundaries and invariant points and solid solution areas. The primary phase field for C_12?xA₇·CaF₂ is increased substantially in size, and liquidus or invariant temperatures are depressed by amounts ranging from 20 to 80°C.