Low-temperature reactive hot-pressing of Ta0.2Hf0.8C–SiC ceramics at 1700°C

Temperature above 2000°C and additional pressure is generally required to achieve the full densification of Ta_xHf_1−xC-based ceramics. This work proposed a novel method to fabricate dense Ta_0.2Hf_0.8C ceramics at relatively low temperature. Using a small amount of Si as a sintering aid, Ta_0.2Hf_0.8C was densified at 1700°C by reactive hot-pressing (RHP), with SiC formed in situ. Microstructure evolution mechanisms of the ceramics during RHP were investigated. The effect of silicon content on the densification and mechanical properties of the ceramics was revealed. It is indicated that the apparent porosity of the Ta_0.2Hf_0.8C–SiC ceramics was as low as 0.5%, whereas bending strength and fracture toughness of the ceramics were as high as ∼637 MPa and 6.7 MPa m^1/2, respectively, when the silicon content was 8 wt.%. This work provides a new idea for the low-temperature densification of Ta_xHf_1−xC and other ultrahigh temperature ceramics with high performance.     

Ablation behavior of (Hf–Ta–Zr–Nb–Ti)C high-entropy carbide and (Hf–Ta–Zr–Nb–Ti)C–xSiC composites

The ablation behavior of (Hf–Ta–Zr–Nb–Ti)C high-entropy carbide (HEC-0) was investigated using a plasma flame in air for different times (60, 90, and 120 s) at about 2100°C. The effect of SiC content on the ablation resistance of HEC–xSiC composites (x = 10 and 20 vol%) was also studied. The linear ablation rate of HEC-0 decreases with increasing ablation time, showing the positive role of the oxide layer with a complex composition. The linear ablation rate of HEC–10 vol% SiC (0.3 µm s⁻¹) is only a 10th of that of HEC-0, showing a significant improvement in ablation resistance, probably due to the formation of a protective oxide layer containing melted SiO₂ and refractory Hf–Zr–Si–O oxides.     

Ablation-resistant Ta0.78Hf0.22C solid solution ceramic modified C/C composites for oxidizing environments over 2200 °C

Ta_0.78Hf_0.22C solid solution ceramic was synthesized and introduced into carbon/carbon (C/C) composites by polymer infiltration and pyrolysis (PIP). Effect of the introduction of Ta_0.78Hf_0.22C on the microstructure, ablation resistance, and flexural performance of C/C composites were investigated. Results showed that the flexural strength and modulus of the composites were increased by 108 % and 117 %, respectively, after adding Ta_0.78Hf_0.22C solid solution into C/C composites. In addition, the introduction of Ta_0.78Hf_0.22C improved the ablation resistance of C/C composites under oxyacetylene ablation environment, over 2200 °C. The linear and mass ablation rates were decreased 73 % and 70 %, respectively. The oxide with low melting point, Ta₂O₅, exhibited good sealing and oxygen-barrier capacity, and the formation of new solid solution oxide particles, Hf₆Ta₂O₁₇, could pin cracks during ablation, both of them were contributed to better ablation resistance of modified C/C composites.     

Fabrication and characterization of biomorphic cellular C/SiC–ZrC composite ceramics from wood

Biomorphic cellular C/SiC–ZrC composite ceramics were fabricated from pine and oak wood precursors. Carbonaceous preforms were first prepared by wood pyrolysis and subsequently infiltrated with polyzirconobutanediol (PZC) and polycarbosilane (PCS) to form the composite ceramics. TGA/DTG and dilatometric analysis were used to study the pyrolysis of the wood precursor. XRD and SEM analyses were applied to characterize the microscopic properties of the resulting biomorphic cellular C/SiC–ZrC composite ceramics. Compared with oak, pine was preferable for preparation of cellular C/SiC–ZrC composite ceramics because of its unique microstructure. The SiC–ZrC phase distribution differed within the composite ceramics. In addition, the compression strengths of wood, charcoal, and cellular C/SiC–ZrC composite ceramics were measured in the axis direction. Results showed the improved compression strength of biomorphic cellular C/SiC–ZrC composite ceramics when the impregnation cycles were repeated.     

Physical,mechanical and microstructural characterization of TiC–ZrN ceramics

This study aims to investigate the impact of zirconium nitride (ZrN) additive on the microstructural features and physical-mechanical characteristics of TiC. For this objective, two different samples, namely monolithic TiC and TiC-5 wt% ZrN, were produced by spark plasma sintering method at 1900 °C for 10 min under 40 MPa. X-ray diffraction, field emission scanning electron microscopy, and thermodynamical evaluations confirmed the formation of a single solid solution of (Ti,Zr)(C,N), along with a carbon-rich secondary phase in the doped ceramic. The monolithic TiC provided a higher relative density (95.5%) than the ZrN-doped sample. The fractographical assessment revealed a change in the fracture mode of TiC from transgranular to intergranular with introducing the ZrN additive. Reinforcing TiC with ZrN resulted in a Vickers hardness of 2640 HV_0.1 kg, a flexural strength of 444 MPa, and a thermal conductivity of 14.9 W/mK. Furthermore, the TiC–ZrN sample presented a higher coefficient of friction (0.37 on average) compared to the monolithic TiC (0.34 on average).     

Strength of single-phase high-entropy carbide ceramics up to 2300°C

The mechanical properties of single-phase (Hf,Zr,Ti,Ta,Nb)C high-entropy carbide (HEC) ceramics were investigated. Ceramics with relative density >99% and an average grain size of 0.9 ± 0.3 µm were produced by a two-step process that involved carbothermal reduction at 1600°C and hot pressing at 1900°C. At room temperature, Vickers hardness was 25.0 ± 1.0 GPa at a load of 4.9 N, Young's modulus was 450 GPa, chevron notch fracture toughness was 3.5 ± 0.3 MPa·m^1/2, and four-point flexural strength was 421 ± 27 MPa. With increasing temperature, flexural strength stayed above ~400 MPa up to 1800°C, then decreased nearly linearly to 318 ± 21 MPa at 2000°C and to 93 ± 10 MPa at 2300°C. No significant changes in relative density or average grain size were noted after testing at elevated temperatures. The degradation of flexural strength above 1800°C was attributed to a decrease in dislocation density that was accompanied by an increase in dislocation motion. These are the first reported flexural strengths of HEC ceramics at elevated temperatures.     

Synthesis and characterization of highly durable and reusable superabsorbent core–shell particles

Superabsorbent core–shell particles were synthesized via a two-step process. A silica core was prepared by co-condensation of tetraethyl orthosilicate and vinyl triethoxysilane. The vinyl-functionalized silica particles were then polymerized with acrylamide monomer via free-radical polymerization to yield silica-polyacrylamide (PAM) hybrid particles. The crosslinking density and porosity of PAM on the hybrid particles were controlled by adjusting the concentration of the crosslinker, n,n′-methylenebisacrylamide (MBA). The structure of core–shell particles was confirmed by scanning and transmission electron microscopy techniques. The hybrid particles with 3.0%MBA could absorb water up to 70 g/g. These hybrid particles also removed 80% of methylene blue from solution within 24 h and this efficacy was maintained for seven cycles. The weight remaining of the hybrid particles after nine cycles was higher than that of pure PAM after three cycles indicating the high durability and reusability of the core–shell particles. POLYM. ENG. SCI., 60: 306–313, 2019. © 2019 Society of Plastics Engineers     

Preparation and characterization of organic–inorganic adduct of dinaphtosulfide macrocyclic diamide and silicotungstic acid: study of interaction in solid and solution phase

An adduct compound containing dinaphtosulfide macrocyclic diamide and silicotungstic acid was prepared and characterized by elemental analyses, IR, UV–VIS spectrophotometry and diffuse reflectance spectrometry. The stoichiometry and stability of adduct were studied by conductometry in nitrobenzene (NB) solvent. This is the first report of the interaction of crown ether containing sulfur heteroatom and heteropoly acid. The interaction of the adduct compound with Hg²⁺ was studied by spectrophotometric and pH-metric methods in NB solvent. The formation constant of the Hg²⁺-adduct complex was calculated from the computer fitting of the obtained data. The extraction of Hg²⁺ and Pb²⁺ was studied from aqueous solution using this adduct compound.     

Preparation and dielectric properties of ceramics in the SrBi2(Ta1−xNbx)2O9 solid solution system

Ceramics in the SrBi₂(Ta_1−xNb_x)₂O₉ solid solution system were prepared by a solid state reaction process, and the dielectric characteristics were determined together with the microstructures. An increased dielectric constant and improved temperature coefficient of permittivity were achieved in this solid solution system, while the dielectric loss remained at the same level as that of SrBi₂Ta₂O₉.     

Electrical properties and phase of BaTiO3–SrTiO3 solid solution

It has been known that ABO₃ type perovskite ferroelectrics, such as BaTiO₃ (BTO) and SrTiO₃ (STO), form a complete solid solution. In this study, Ba_1?xSr_xTiO₃ (BST, x=0.0–1.0) solid solution were sintered by a solid-state reaction method using BTO and STO raw powders with appropriate chemical composition. The crystal structure was investigated by a Rietveld refinement method; Fullprof, using X-ray diffraction data. Within the reasonable goodness of fit, tetragonal symmetry was found in BST with x≤0.2, while BST with x≥0.4 were found to be cubic symmetry. However, Ba_0.7Sr_0.3TiO₃ was difficult to decide whether it is cubic or tetragonal because of large uncertainties after final fitting. The composition ratios calculated from the fitted occupancies match well with those measured by EDS within experimental uncertainties. Remnant polarizations of BST with x<0.3 decrease with increasing Sr concentration. Furthermore, measured phase transition temperatures and maximum dielectric constant decrease as increasing Sr concentration. Measured electrical properties of BST were match well with the structural refinement investigations.     

Fabrication and characterization of anorthite–mullite–corundum porous ceramics from construction waste

In this work, anorthite–mullite–corundum porous ceramics were prepared from construction waste and Al₂O₃ powders by adding AlF₃ and MoO₃ as mineralizer and crystallization catalyst, respectively. The effects of the sintering temperature and time on open porosity, mechanical properties, pore size distribution, microstructure, and phase composition were characterized in detail. The results showed that the formation of the mullite whiskers and the properties of the anorthite–mullite–corundum porous ceramics depended more on the sintering temperature than the holding time. By co-adding 12 wt% AlF₃ and 4 wt% MoO₃, mullite whiskers were successfully obtained at sintering temperatures upon 1350 °C for 1 h. Furthermore, the resultant specimens exhibited excellent properties, including open porosity of 66.1±0.7%, biaxial flexural strength of 23.8±0.9 MPa, and average pore size of 1.32 µm (the corresponding cumulative volume percent was 37.29%).     

Preparation and characterization of some ferromagnetic glass–ceramics contains high quantity of magnetite

Ferrimagnetic glass–ceramics are promising candidates for magnetic induction hyperthermia, which is one form of inducing deep-regional hyperthermia, by using a magnetic field. The aim of this work was to study the effect of increasing the amount of crystallized magnetite on the magnetic properties of glass–ceramic samples. Two different ferrimagnetic glass–ceramics with the composition based on wollastonite or hardystonite with high quantity (∼60%) of magnetite were prepared by melting the starting materials at 1450 °C for 2 h. The influences of chemical composition, amount of crystallized magnetite and microstructure of ferrimagnetic glass–ceramics on magnetic properties of ferromagnetic glass–ceramics were investigated using differential thermal analysis (DTA), X-ray diffraction (XRD), transmission electron microscope (TEM) and scanning electron microscope (SEM). The X-ray diffraction patterns show the presence of nanometric magnetite crystals in a glassy matrix after cooling from melting temperature. The amount of crystallized magnetite varies as a function of the chemical composition and heat treatment schedule. The presence of ZnO in the glass–ceramics was found to decrease the viscosity and so cases higher degree of mobility of ions leading to higher degree of crystallinity. The higher heat treatment parameters and so the lower viscosity of the glass containing ZnO are assumed to allow the magnetite to grow to larger crystallite size. Glass transition temperature and thermal stability were found to be functions of chemical composition. Magnetic hysteresis cycles were analyzed using a vibrating sample magnetometer (VSM) with a maximum applied field of 15 kOe at room temperature in quasi-static conditions. From the obtained hysteresis loops, the saturation magnetization (Ms), remanance magnetization (Mr) and coercivity (Hc) were determined. The results showed that these materials are expected to be useful in the localised treatment of cancer.     

Combustion synthesis of AlN–SiC solid solution particles

The feasibility of synthesising AlN–SiC solid solution ceramics by combustion synthesis (CS) reaction is demonstrated through igniting the mixtures of aluminium, silicon and carbon black under different nitrogen pressure values. The effects of the nitrogen pressure and the atomic ratio of (Si+C)/Al on the crystalline phases formed in the reaction product and on the characteristics of combustion behaviour were investigated. Combining thermodynamic analysis and the combustion characteristics, the reaction sequence and the formation of AlN–SiC solid solution by CS were explained.     

Preparation and characterization of Co2+ substituted Li–Dy ferrite ceramics

Stoichiometric compositions of ferrites with the chemical formula Li_0.5?0.5xCo_xFe_2.4?0.5xDy_0.1O₄ with x=0, 0.25, 0.5, 0.75, 1.0 were prepared by the standard double sintering ceramic method. X-ray diffraction analysis confirmed the cubic spinel structure of the prepared samples. The structural, morphological and magnetic properties were studied by X-ray diffraction, infra-red spectroscopy (IR), scanning electron microscopy (SEM), vibrating sample magnetometry (VSM) and ac susceptibility measurements. Lattice constant, grain size and density increase whereas porosity decreases with the increase in Co²⁺ substitution. IR measurements show the characteristic ferrite bands. Spectral absorption bands were observed in IR spectroscopic analysis at ν₁=564?601 cm^?1, ν₂=486?519 cm^?1 and ν₃=551?578 cm^?1. The cation distribution estimated by the X-ray diffraction is supported by magnetization and susceptibility studies. The saturation magnetization decreases from 44.25 to 17.14 emu/g whereas coercivity remarkably increases from 240.69 to 812.14 emu/g with increasing Co²⁺ substitution. The mechanisms involved are discussed.     

Synthesis,processing and properties of TaC–TaB2–C ceramics

Multi-phase ceramics in the TaC–TaB₂–C system were prepared from TaC and B₄C mixtures by reactive pressureless sintering at 1700–1900 °C. The pressureless densification was promoted by the use of nano-TaC and by the presence of active carbon in the reaction products. The presence of TaB₂ inhibited grain growth of TaC and increased the hardness compared to pure TaC. If a coarse TaC powder was used, the compositions did not densify. In contrast, pure nano-TaC was pressureless sintered at 1800 °C by the addition of 2 wt.% carbon introduced as carbon black or graphite. The introduction of carbon black resulted in fully dense TaC ceramics at temperatures as low as 1500 °C. The grain size of nominally pure TaC ceramics was a strong function of carbon stoichiometry. Enhanced grain size in sub-stoichiometric TaC, compared to stoichiometric TaC, was observed. Additional work is necessary to optimize processing parameters and evaluate the properties of ceramics in the TaC–TaB₂–C system.     

Grain size effect on electromechanical properties and non-linear response of dense nano and microstructured PIN–PT ceramics

Nanopowders of 0.63Pb(In_1/2Nb_1/2)O₃–0.37PbTiO₃ were synthesized by solid state reaction using the continuous attrition milling followed by high-energy ball milling techniques in air at room temperature. After milling for 8 h nanopowders of 20–30 nm grain size are obtained. Sintering by hot pressing of PIN–37PT green pellets leads to dense ceramics with average grain size varying from 100 nm to 1 μm. The dielectric and piezoelectric properties of PIN–37PT nanostructured ceramics with grain size bigger than about 160 nm remain roughly unchanged and comparable to those of microstructured ceramics. In addition, the stability of the permittivity and dielectric losses under high ac electric field grows when the grain size decreases. The material becomes less non-linear with decreasing grain size. This result is attractive for acoustic transducer applications.     

Chemical process and solid solution formation in reactive hot pressed ZrB2–SiC ceramics doped with W

ZrB₂–SiC doped with W was prepared from a mixture of Zr, Si, B₄C and W via reactive hot pressing. The fully dense ZrB₂–SiC–WB–ZrC ceramic was obtained at 1900°C for 60 min under 30?MPa in an argon atmosphere. Reaction path and solid solution characteristics of the starting powders were studied through a series of pressureless heat treatment at temperatures between 700 and 1500°C. The solid solution phases of (Zr, W)B₂, (W, Zr)B and (Zr, W)C were formed directly by reactions between the precursors. Homogeneous distribution of solute atoms in solution and the solid solubilities were also studied.     

Highly-efficient preparation of anisotropic ZrB2–SiC powders and dense ceramics with outstanding mechanical properties

Highly-dense ZrB₂–SiC ceramics with excellent mechanical properties including Vickers hardness of 24.5 GPa and fracture toughness of 4.8 MPa/m^1/2 were successfully prepared, by spark plasma sintering of the raw powders synthesized by a novel molten-salt and microwave co-assisted boro/carbothermal reduction (MSM-BCTR) method. Compared with the processing conditions required for synthesizing ZrB₂–SiC by conventional reduction method, the present MSM-BCTR method possessed a variety of significant merits including the smaller material cost, lower processing temperature (1200°C), and remarkably higher efficiency (soaking time as short as 20 minutes). More importantly, the ZrB₂–SiC powders, resultant from MSM-BCTR treatment, were verified to have single-crystalline nature and uniform well-grown anisotropic morphologies (rod-like ZrB₂ and sheet-like SiC) as well as great potential in promoting the mechanical properties of their bulk counterparts. This great achievement was mainly ascribed to the specific MSM-BCTR conditions characterized by microwave heating and molten-salt medium.     

Indentation analysis of elastic and plastic deformation of precursor-derived Si–C–N ceramics

Elastic and plastic deformation behavior of precursor-derived Si–C–N ceramics at room temperature under contact loading was investigated using nano- and microindentation experiments. The observed behavior showed striking similarities to that of anomalous silicate glasses and amorphous carbon based films. The elastic deformation was controlled by the overall rigidity of the material microstructure, that evolved with the increase in the network connectivity due to progressive dehydrogenation in the amorphous materials, and the formation and organization of turbostratic graphite and nanocrystalline SiC in the phase-separated materials. The plastic deformation of the amorphous materials displayed anomalous densification inducing appreciable strain hardening, which reduced with progressive phase separation in the materials. The contrasting evolution of elastic and plastic deformation work quantities in amorphous and phase-separated materials indicate that the plastic deformation in former materials was volume deformation-controlled whereas that in the latter materials was interface deformation-controlled.     

Charge compensation and electrical characteristics of Ta2O5–doped SnO2–CoO ceramics

We investigated the effect of pentavalent donor dopant Ta₂O₅ on microstructure development, electric and dielectric characteristics of SnO₂–CoO based ceramics. Already low additions of Ta₂O₅ (0.05 mol%) effectively reduce the porosity, improve densification and dielectric permittivity and trigger a 3–fold increase in SnO₂ growth rate. Rietveld analysis shows that the amount of Co₂SnO₄ spinel phase drops with the addition of Ta₂O₅ due to incorporation of Co²⁺ and Ta⁵⁺ into SnO₂ structure. With higher additions, however, Ta₂O₅ segregates to the grain boundaries and hinders SnO₂ grain growth, which in turn improves electrical properties. TEM/EDS analysis shows that above 0.5 mol% of Ta₂O₅ the Co:Ta ratio in SnO₂ grains is constant 1:2, which means that a twice lower amount of Ta⁵⁺ is incorporated in the SnO₂ structure compared to the Nb₂O₅-doped SnO₂–CoO system. Accordingly, the following charge compensation mechanism is proposed: 3 Sn(IV)_S^˟_{n (IV)} ⇋ Co(II)_Sn ̎_(IV) + 2 Ta(V)˙_{Sn (IV)}.