Improvement in performance of MgO–CaO refractories by addition of nano-sized ZrO2

MgO–CaO refractories added with different sized ZrO₂ powders were sintered at 1600 °C, the effect of the ZrO₂ powders on performance of MgO–CaO refractories was investigated. The results showed that the densification of the MgO–CaO refractories was appreciably promoted when a small amount of ZrO₂ was added owing to the formation of small size CaZrO₃ facilitated to sintering, and the densification was promoted further with increasing the amount of ZrO₂ due to the volume expansion caused by the reaction of the added ZrO₂ and CaO to form CaZrO₃ in the refractories, and the addition of nano-sized ZrO₂ was more effective. The thermal shock resistance of the MgO–CaO refractories was improved by modification of the microstructure due to the formed CaZrO₃ particles that predominately located on the grain boundaries and triple points in the whole microstructure, and the addition of nano-sized ZrO₂ was more effective attributed to its well dispersion and the critical addition amount was effectively decreased to 6%. The slaking resistance of the MgO–CaO refractories was appreciably improved by addition of ZrO₂ due to its effect on decreasing the amount of free CaO in the refractories, promotion of densification as well as modification of microstructure, the nano-sized ZrO₂ addition was more effective due to its higher activity. The slag corrosion resistance of the MgO–CaO refractories was enhanced by addition of ZrO₂ due to the increase of the viscosity of the liquid phase and thus inhibited further penetration of slag at elevated temperatures.     

Fracture behaviour of microcrack-free alumina–aluminium titanate ceramics with second phase nanoparticles at alumina grain boundaries

Alumina + 10 vol.% aluminium titanate composites were obtained by colloidal filtration and reaction sintering of alumina and titania. The materials were dense with aluminium titanate grains of average sizes 2.2–2.4 μm located mainly at alumina triple points. The reaction sintering schedule promoted the formation of additional nanometric grains, identified as aluminium titanate using STEM–EDX analysis between the alumina grains. This special microstructure led to a change of the toughening mechanism from the typical crack bridging reported for microcrack-free composites fabricated from alumina and aluminium titanate powders to microcracking.

The identification of microcracking as the main toughening mechanism was done from the analysis of stable fracture tests of SENVB samples in three points bending and fractographic observations. Monophase alumina materials with similar grain sizes were used as reference.

Different fracture toughness parameters were derived from the load–displacement curves: the critical stress intensity factor, K_IC, the critical energy release rate, G_IC, the J-Integral and the work of fracture, γ_WOF, and the R curves were also built. The comparison between the linear elastic fracture parameters and the non-linear ones revealed significant toughening and flaw tolerance.     

Fe–Co based metal/spinel to produce light olefins from syngas

New metal/oxide (Co–Fe) catalysts (with no reduction or thermal pre-treatment) are efficient to produce light hydrocarbons with a low selectivity in CO₂ by the Fischer–Tropsch synthesis. The low selectivity in CO₂ is due to the occurrence of the CO₂/H₂ reaction. These materials are stable under reaction conditions, and only few carbides are formed during the Fischer–Tropsch reaction. X-ray analyses indicate that the most degraded phase is the (Co–Fe) alloy phase in CO/H₂ reaction and the spinel phase in the CO₂/H₂ reaction. It was demonstrated that these composites do not behave as the simple sum of a spinel phase and a (Co–Fe) alloy but have their own properties.     

Postsintering Hot Isostatic Pressing of Ceria-Doped Tetragonal Zirconia/Alumina Composites in an Argon–Oxygen Gas Atmosphere

Ceria-doped tetragonal zirconia (Ce-TZP)/alumina (Al₂O₃) composites were fabricated by sintering at 1450° to 1600°C in air, followed by hot isostatic pressing (postsintering hot isostatic pressing) at 1450°C and 100 MPa in an 80 vol% Ar–20 vol% O₂ gas atmosphere. Dispersion of Al₂O₃ particles into Ce-TZP was useful in increasing the relative density and suppressing the grain growth of Ce-TZP before hot isostatic pressing, but improvement of the fracture strength and fracture toughness was limited. Postsintering hot isostatic pressing was useful to densify Ce-TZP/Al₂O₃ composites without grain growth and to improve the fracture strength and thermal shock resistance.     

Properties and Microstructure of Molybdenum Disilicide–β';-SiAlON Particulate Ceramic Composites

Particulate ceramic composites that were composed of a combustion-synthesized β';-SiAlON matrix and dispersed MoSi₂ particles were hot pressed at 1600°C in a nitrogen atmosphere. The physical and mechanical properties of the composites that contained 15, 30, and 45 vol% MoSi₂ were evaluated. The average four-point bend strength, fracture toughness, and Vickers hardness of the composites were in the ranges of 500-600 MPa, 3-4 MP·am^1/2, and 11-13 GPa, respectively. The measured mechanical strength and hardness were very similar to the values that were predicted from the rule of mixtures. The fracture toughness of the combustion-synthesized β';-SiAlON (2.5 MPa·m^1/2) was apparently enhanced by the MoSi₂ particles that were added. The increase in the fracture toughness was predominately attributed to the residual thermal stress that was induced by the thermal expansion mismatch between the MoSi₂ particles and the β';-SiAlON matrix. The composites showed improved electrical conductivity and oxidation resistance over monolithic β';-SiAlON. High-resolution transmission electron microscopy examination of the composites indicated that the MoSi₂ was chemically well compatible with the β';-SiAlON.     

Magnetic properties of Y–Fe–O ultrafine particles containing YFe(3+x)O1.5(4+x) synthesized by rf thermal plasma

Y–Fe–O ultrafine particles containing YFe_(3+x)O_1.5(4+x), -Fe₂O₃, and γ-Fe₂O₃(Fe₃O₄) were fabricated using a thermal plasma evaporation method with rf Ar–O₂. To determine if YFe_(3+x)O_1.5(4+x) in the particles is a ferri-, ferro-, or paramagnetic compound at room temperature (R.T.), the magnetic properties of these particles at R.T. were studied using X-ray diffraction (XRD), vibrating sample magnetometer (VSM), and Mössbauer spectrometry. VSM results showed that the saturation magnetization of particles at R.T. increased after the Curie point (CP) measurement at reduced pressure (4×10⁻³ Pa) from R.T. to an upper limit temperature higher than 460 °C. The saturation magnetization of particles at R.T. after the CP measurement at reduced pressure from R.T. to 700 °C was larger than that from R.T. to 600 °C. In the XRD patterns, the relative quantities of h-YFeO₃ and γ-Fe₂O₃(Fe₃O₄) to that of YFe_(3+x)O_1.5(4+x) increased after the CP measurement at reduced pressure from R.T. to 700 °C, indicating that the saturation magnetization at R.T. increased as the relative quantity of γ-Fe₂O₃(Fe₃O₄) increased. The relative quantities of h-YFeO₃ and γ-Fe₂O₃(Fe₃O₄) to that of YFe_(3+x)O_1.5(4+x) after the CP measurement depended on Fe/Y of the particles, indicating that the increase in saturation magnetization at R.T. after the Curie point measurement depended on the increase in relative quantity of γ-Fe₂O₃(Fe₃O₄). Mössbauer spectrometry before and after the CP measurements showed that YFe_(3+x)O_1.5(4+x) exhibited only a single type of quadrupole splitting and no magnetic splitting, indicating that YFe_(3+x)O_1.5(4+x) is a paramagnetic compound.     

添加浮选镁砂颗粒和Al2O3细粉对方镁石-尖晶石砖性能的影响

以烧结镁砂、浮选镁砂、电熔镁铝尖晶石和煅烧Al₂O₃细粉为主要原料,以木质素磺酸钙为结合剂,经混练、成型、1 650℃烧成制备了方镁石-尖晶石砖,主要研究了以浮选镁砂颗粒替代普通烧结镁砂颗粒以及在此基础上外加不同量煅烧Al₂O₃细粉（质量分数分别为0、3%和5%）对方镁石-尖晶石砖性能的影响。结果表明：1)以浮选镁砂颗粒替代普通烧结镁砂颗粒制备的方镁石-尖晶石砖,显气孔率和常温耐压强度降低,体积密度、抗热震性、高温强度和高温韧性提高。2)在以浮选镁砂颗粒替代普通烧结镁砂颗粒制备的方镁石-尖晶石砖中,随着煅烧Al₂O₃细粉引入量的增多,显气孔率和常温耐压强度降低,体积密度、抗热震性、高温强度和高温韧性提高。     

Effect of quartz particle size on the strength of triaxial porcelain

The effect of the quartz component on strength, σ_f, and fracture toughness, K_Ic, of a triaxial porcelain was studied by varying the size distribution of the quartz particles. Both σ_f and K_Ic were found to increase and then to pass over a maximum as the quartz distribution became finer. Improvements in σ_f and K_Ic of more than a factor of 2 were achieved. The flaw size that controlled failure was shown to equal the maximum quartz particle size in the material containing the coarsest quartz component and to be a constant independent of the quartz size in the other materials. The toughness increase was attributed to microcrack toughening of the glass matrix, the microcracks being formed by thermal mismatch stresses between the quartz particles and the glass matrix on cooling from the sintering temperature.     

BAS, CMAS and CZAS glass coatings deposited by plasma spraying

In this study, three different industrial frits BaO–Al₂O₃–SiO₂ (BAS), CaO–MgO–Al₂O₃–SiO₂ (CMAS), CaO–ZrO₂–Al₂O₃–SiO₂ (CZAS) have been deposited on porcelainized stoneware tiles by plasma spraying. In the as-sprayed conditions, the microstructure of the coatings is defective because of pores, microcracks and low intersplat cohesion. Hot stage microscope and differential thermal analysis measurements made on the glass powders allowed to characterize the frits thermal behaviour. Post process thermal treatments have been arranged, following these indications as well as preliminary tests, in order to achieve the lowest porosity and the highest resistance to abrasion. At the chosen temperatures, a microstructural improvement has been induced, but in the BAS specimens, an optimal sintering has not been accomplished because of the unavoidable full overlapping of the sintering and crystallization processes.     

Brown pigment of the nanopowder spinel ferrite prepared by combustion reaction

The purpose of the present work was to prepare nanometric brown pigments spinel (Zn_1−xNi_x)Fe₂O₄ (with x = 0, 0.5 and 1 mol of the Ni²⁺) structures by combustion reaction and characterize the resulting powders. The compositions were prepared by a single-step solution combustion reaction using nitrates and urea as fuel. Stoichiometric compositions of metal nitrate and urea were calculated using the total oxidizing and reducing valences of the components, which serve as the numerical coefficients for the stoichiometric balance, so that the equivalence ratio Φ_c is equal to unity and the energy released was maximum. The resulting powders were characterized by X-ray diffraction (XRD), nitrogen adsorption by BET, scanning electron microscopy (SEM), helium pycnometry, sedimentation, transmission electron microscopy (TEM), and CIE-L*a*b* color-measurements. The results demonstrated that the synthesis by combustion reaction was very fast and safe resulting in crystalline spinel containing nanoparticles (18–26 nm) for all of the compositions studied. The results demonstrate the viability of using these powders as brown ceramic pigments.     

高热稳定性α-氧化铁的宏量制备及其电化学性能研究

氧化铁（Fe₂O₃）是一种重要的n型半导体材料,被广泛应用于染料、废水处理、光催化和锂离子电池等领域。采用水热法合成了不同直径大小的片状结构的α-氧化铁,其中大尺寸的片状α-氧化铁在1 000 ℃仍能保持原有的表观颜色和形态,证明了其具有高热稳定性,在油漆、染料等领域具有较大的应用潜力。研究了氢氧化钠与三氯化铁溶液浓度及其混合顺序对α-氧化铁材料性能的影响,并且分析了片状α-氧化铁的带隙、锂离子电池性能及粉体表观颜色与颗粒尺寸的依赖关系。结果表明,通过调整氢氧化钠溶液的浓度和氢氧化钠与三氯化铁的滴加顺序可以得到不同尺寸的片状α-氧化铁,α-氧化铁的颜色随着其颗粒尺寸的增大而加深,带隙随着颗粒尺寸的减小呈现上升趋势,并且纳米级颗粒相对于微米级颗粒会提高锂离子电池的实际容量。该研究有助于研发α-氧化铁的宏量制备工艺及发掘其在电化学、陶瓷釉料、颜料等方面的应用,对降低传统能源活动的碳排放、推动中国早日实现“双碳”的国家目标具有重要的意义。     

Thermal Shock Resistance and Fracture Behavior of ZrB2–Based Fibrous Monolith Ceramics

The thermal shock resistance and fracture behavior of zirconium diboride (ZrB₂)-based fibrous monoliths (FM) were studied. FMs containing cells of ZrB₂–30 vol% SiC with cell boundaries composed of graphite–15 vol% ZrB₂ were hot pressed at 1900°C. The average flexure strength of the FMs was 375 MPa, less than half of the strength of hot-pressed ZrB₂–30 vol% SiC. Flexure specimens failed noncatastrophically and retained 50%–85% of their original strength after the first fracture event. A critical thermal shock temperature (Δ T _c) of 1400°C was measured by water quench thermal shock testing, a 250% improvement over the previously reported Δ T _c values for ZrB₂ and ZrB₂–30 vol% SiC of similar dimensions (4 mm × 3 mm × 45 mm). The flexure strength was maintained with Δ T _c values of 1350°C and below. As Δ T _c increased, the stiffness of the flexure specimen decreased linearly. The lower stiffness and improvement in thermal shock resistance is attributed to crack propagation in the cell boundary and crack deflection around the load-bearing cells. The critical thermal shock was attributed to the fracture of the ZrB₂–30% SiC cell material.     

Catalytic flameless combustion of methane over perovskites prepared by flame–hydrolysis

A set of perovskite-type catalysts of general formula LaBO_3±δ (B=Co, Mn, Fe) and a sample of La₂NiO_4±δ were prepared by means of a recently proposed innovative flame–hydrolysis procedure. The catalysts were characterised by nanometer-size particles (20–60 nm), relatively high surface area (ca. 20 m²/g), high thermal stability and high phase purity. Their high activity for the catalytic flameless combustion of methane confirmed the validity and versatility of the preparation method.

Temperature-programmed -desorption and -reaction, coupled with mass spectrometric analysis, allowed to better understand some aspects of the catalytic behaviour shown by the present samples for the cited reaction. In particular, an interesting correlation between the availability of oxygen at various temperatures, as revealed by the so-called and β oxygen desorption peaks, and reaction mechanism was found for the different B metals.     

Influence of a Dispersion of Aluminum Titanate Particles of Controlled Size on the Thermal Shock Resistance of Alumina

The possibility of developing fine-grained (∼0.5–3 μm) and dense (≥0.98ρ_th) alumina (90 vol%)–aluminum titanate (10 vol%) composites with improved thermal shock resistance and maintained strength is investigated. One alumina material and one composite with similar microstructures (porosity and grain-size distribution) were fabricated to investigate the effect of Al₂TiO₅ on thermal shock behavior. The size of the Al₂TiO₅ particles was kept under 2.2 μm to avoid spontaneous microcracking. The mechanical and thermal properties of the materials involved in their response to thermal shock and the results for the evolution of indentation cracks of equal initial crack length with increasing Δ T in samples quenched in glycerine are described. The combination of thermal and mechanical properties—thermal conductivity, thermal expansion coefficient, Young's modulus, and toughness—improve the thermal shock resistance of the alumina–aluminum titanate composite in terms of critical temperature increment (>30%). The suitable structural properties of alumina—hardness and strength—are maintained.     

Transport numbers and oxygen permeability of SrCe(Y)O3-based ceramics under oxidising conditions

Partial conductivities in the SrCe(Y)O_3−δ system have been studied in oxidising conditions in the temperature range 923–1273 K. Compositions with variable Y content (5 and 10 at.%), Sr deficiency (3 at.%), and with the addition of Fe₂O₃ as sintering aid (2 mol%) were analysed. A modified Faradaic efficiency method and oxygen permeation measurements were employed to appraise the oxide-ionic transport. Oxide-ion transference numbers in air lie in the range 0.19–0.80 and decrease with increasing temperature in the range 973–1223 K. Modelling of total conductivity as a function of oxygen partial pressure (p(O₂)) confirmed that protonic transport is minor under the studied conditions. SrCe_0.95Y_0.05O_3−δ exhibits greater oxide-ion conductivity than SrCe_0.9Y_0.1O_3−δ, indicative of dopant–vacancy association at high dopant contents. Conversely, oxygen permeability is slightly higher for SrCe_0.9Y_0.1O_3−δ as a result of faster surface-exchange kinetics. The oxygen flux through Fe-free membranes is dominated by the bulk in low p(O₂) gradients, when the permeate-side p(O₂) is higher than 0.03 atm, but surface exchange plays an increasing role with increasing p(O₂) gradient. Addition of Fe₂O₃ to SrCe(Y)O_3−δ lowers the sintering temperature by 100 K but results in the formation of intergranular second phases which block oxide-ionic and electronic transport, and thus oxygen permeation. The average thermal expansion coefficients (TECs) are (10.8–11.6) × 10⁻⁶ K⁻¹ in the temperature range 373–1373 K for all studied compositions.     

Effect of cooling rate and frequency on the calorimetric measurement of the glass transition

The glass transition of thermoplastics of different polydispersity and thermosets of different network structure has been studied by conventional differential scanning calorimetry (DSC) and temperature modulated DSC (TMDSC). The cooling rate dependence of the thermal glass transition temperature T_g measured by DSC, and the frequency dependence of the dynamic glass transition temperature T measured by TMDSC have been investigated. The relation between the cooling rate and the frequency necessary to achieve the same glass transition temperature has been quantified in terms of a logarithmic difference Δ=log₁₀[|q|]−log₁₀(ω), where |q| is the absolute value of the cooling rate in K s⁻¹ and ω is the angular frequency in rad s⁻¹ necessary to obtain T_g(q)=T(ω). The values of Δ obtained for various polymers at a modulation period of 120 s (frequency of 8.3 mHz) are between 0.14 and 0.81. These values agree reasonably well with the theoretical prediction [Hutchinson JM, Montserrat S. Thermochim Acta 2001;377:63 [6]] based on the model of Tool–Narayanaswamy–Moynihan with a distribution of relaxation times. The results are discussed and compared with those obtained by other authors in polymeric and other glass-forming systems.     

Fractography, Mechanical Properties, and Microstructure of Commercial Silicon Nitride–Titanium Nitride Composites

Commercial-grade Si₃N₄–TiN composites with 0, 10, 20, and 30 wt% TiN content have been characterized. Submicrometer grain-size Si₃N₄ was reinforced with fine TiN grains. Density, Young's modulus, coefficient of thermal expansion, and fracture toughness increased linearly with TiN content. Increased strength was observed in the Si₃N₄+20 wt% TiN, and Si₃N₄+30 wt% TiN composites. Fractography was used to characterize the different types of fracture origins. Improvements in toughness and strength are due to residual stresses in the Si₃N₄ matrix and the TiN particles. A threefold improvement in dry wear resistance of the Si₃N₄+30 wt% TiN composite over the Si₃N₄ matrix was observed.     

Ambient- and High-Temperature Properties of Titanium Carbide–Titanium Boride Composites Fabricated by Transient Plastic Phase Processing

Ambient- and high-temperature properties of a class of titanium carbide-titanium boride composites that have been produced by transient plastic phase processing are presented. The composites produced are comprised of Ti₃B₄, TiB₂, and TiC_0.65 at their equilibrium composition (34.5, 30.5, and 34.9 vol%, respectively), and the Ti₃B₄ phase in these composites occurs either as equiaxed grains or as platelets, depending on the starting mixture composition. Measurements of the ambient- and high-temperature flexure strength and fracture toughness, thermal shock susceptibility, oxidation resistance, and wear resistance of this class of composites are presented. The role of various microstructural parameters—such as the morphology of the Ti₃B₄ phase, the length scale of the microstructure, and the volume fraction of borides—on these properties has been identified.     

A ZrO2 (3Y) Matrix Composite Toughened with Fe3Al Intermetallic

Near fully dense ZrO₂(3Y)/Fe₃Al composites with significantly improved fracture toughness were synthesized by hot-press sintering at 1350°C. High fracture toughness and bending-strength values, 36 MPa·m^1/2 and 1321 MPa, respectively, were achieved in 40 vol% Fe₃Al composite ceramics, whereas those same values for ZrO₂(3Y) alone were 10 MPa·m^1/2 and 988 MPa, respectively. Microscopic observation of the crack path revealed that Fe₃Al particle uniformly dispersed in the matrix have obvious crack-bridging effect. Improved thermal-shock resistance was also obtained, which was attributed to higher toughness, thermal conductivity, and lower Young's modulus by adding of Fe₃Al particles.