Thermal stability and hardness of ReB2 type hexagonal OsB2 with the addition of W

This work attempts to understand the effect of W addition on microstructure, thermal stability, and hardness of ReB₂ type hexagonal osmium diboride (h-OsB₂). h-OsB₂ samples with W atomic concentration of (Os+W) from 0% to 30% were synthesized by mechanochemical method combines with pressure-less sintering. The XRD patterns of the as-synthesized powders indicate the formation of Os_1-xW_xB₂ (x?=?0, 0.1, 0.2 and 0.3) solid solution, which has a ReB₂-type hexagonal structure. After being high temperature sintered, part of the h-OsB₂ phase of the pure OsB₂ transformed to orthorhombic (o) phase, while the h-OsB₂ phase was maintained with the addition of W, which suggests that the thermal stability of the sample was remarkably improved. A macroscopically homogeneous structure with some pores can be found from all groups of the as-sintered Os_1-xW_xB₂ (x?=?0, 0.1, 0.2, 0.3) samples, with some B-rich areas distributed in the W doped samples. The lattice parameters of the Os_1-xW_xB₂ (x?=?0, 0.1, 0.2 and 0.3) solid solutions linearly decreased with the increase of the W concentration. The micro-hardness of the OsB₂ sintered samples is 25?±?2?GPa under an applied load of 0.49?N, which increased to 34?±?2?GPa, 38?±?2 and 37?±?2?GPa, respectively when the W concentration increased from 10, 20 and 30?at%. The increased hardness of the h-OsB₂ can be mainly attributed to the improvement of thermal stability with the addition of W.     

Effect of carbon content on the microstructure and mechanical properties of high-entropy (Ti0.2Zr0.2Nb0.2Ta0.2Mo0.2)Cx ceramics

High-entropy (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_x ceramics, with different carbon contents (x=0.55?1), were prepared by spark plasma sintering using powders synthesized via a carbothermal reduction approach. Single-phase, high-entropy (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_x ceramics could be obtained when using a carbon content of x=0.70?0.85. Combined ZrO₂ and Mo-rich carbide phases, or residual graphite, existed in the ceramics due to either a carbon deficiency or excess at x=0.55 and 1, respectively. With the carbon content increased from x=0.70 to x=0.85, the grain size decreased from 4.36 ± 1.55 μm to 2.00 ± 0.91 μm, while the hardness and toughness increased from 23.72 ± 0.26 GPa and 1.69 ± 0.21 MPa·m^1/2 to 25.45 ± 0.59 GPa and 2.37 ± 0.17 MPa·m^1/2, respectively. This study showed that the microstructure and mechanical properties of high-entropy carbide ceramics could be adjusted by the carbon content. High carbon content is conducive to improving hardness and toughness, as well as reducing grain size.     

Influence of Si doping on the microstructure and hardness of an AlTiSiN coating deposited by low pressure chemical vapor deposition

The effect of Si doping on the microstructure and hardness of an AlTiSiN coating deposited by the low pressure chemical vapor deposition (LPCVD) method was studied in detail using grazing incidence X-ray diffraction (GIXRD), field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and a nanoindenter instrument. The results show that Al_0.82Ti_0.18N coating exhibits a typical columnar crystal structure, while Al_0.88Ti_0.09Si_0.03N and Al_0.82Ti_0.08Si_0.10N coatings show a fine equiaxed crystal structure. The number of internal substructures (dislocation, stacking fault, etc.) decreased, while the volume fraction of the amorphous structure increased with the increase of Si content. The results of GIXRD and HRTEM show that all AlTiSiN samples mainly consist of fcc AlN phase with a small amount of hcp AlN phase. Furthermore, a small amount of fcc TiN phase can only be observed in Al_0.82Ti_0.08Si_0.10N coating. The hardness of Al_0.82Ti_0.18N, Al_0.88Ti_0.09Si_0.03N, and Al_0.82Ti_0.08Si_0.10N coatings is 33.0 ± 0.6 GPa, 38.3 ± 1.2 GPa and 27.0 ± 0.8 GPa, respectively. Al_0.88Ti_0.09Si_0.03N coating has obvious potential value for industrial applications.     

Structural,morphological, and electrochemical behavior of titanium-doped SrFe1-xTixO3-δ (x = 0.1–0.5) perovskite as a cobalt-free solid oxide fuel cell cathode

Titanium, Ti-doped SrFe_1-xTi_xO_3-δ (x = 0.1–0.5) perovskite-structured ceramics were synthesized via solution combustion. The structural, morphological, and electrochemical behaviors of the as-synthesized materials were investigated to determine the applicability of SrFe_1-xTi_xO_3-δ as a cobalt-free cathode material for intermediate-temperature solid oxide fuel cells. X-ray diffraction analysis confirmed the formation of a single-phase cubic perovskite structure. The unit volume of this perovskite structure increased as the amount of Ti dopant increased. Morphological analysis revealed that the porosity of the SrFe_1-xTi_xO_3-δ perovskite cathode film was inversely proportional to the amount of Ti dopant. The cathode SrFe_0·9Ti_0·1O_3-δ film exhibited a high porosity of 24.74 ± 0.52%, a low but acceptable hardness value of 0.70 ± 0.01 GPa and an area specific resistance of 0.57 Ω cm². These results suggested that cobalt-free SrFe_1-xTi_xO_3-δ cathode was still not good enough to be compared with the existing cobalt-containing cathode such as lanthanum strontium cobalt ferrite. But, the results obtained from this work can be considered as a major turning point as the literature works on SrFe_1-xTi_xO_3-δ cathode showed excellence electrochemical performance. The contradict result between the present and past studies proved that the use of SrFe_1-xTi_xO_3-δ cathode is worthy of being studied into details to confirm its capability.     

Effect of Mn-doping on the structure and electrical properties of (Pb0.325Sr0.675)TiO3 ceramics

Pb_0.325Sr_0.675Ti_1-xMn_xO₃ ceramics (x?=?0, 0.001, 0.005, 0.01, and 0.05) were successfully prepared by traditional solid-state reaction method. It was found that the lattice constant calculated through Rietveld refinement initially increased and then decreased with increasing Mn content, which was attributed to the variation in valence state of Mn and Ti ions. The microstructure gradually varied from the coexistence of large grains and fine grains for x?=?0 to the uniform grain for x?=?0.05 by increasing the doping Mn ions. With increasing Mn content from x?=?0 to x?=?0.05, the Curie temperature (Tc) dramatically decreased from 25?°C to ??40?°C and dielectric maximum decreased from 27,100 to 13,200. Pb_0.325Sr_0.675Ti_1-xMn_xO₃ ceramics with x?=?0.001 showed the lowest dielectric loss of 0.006 with a relatively high dielectric peak value of ~ 21,000. The grain boundaries resistance obtained from the complex impedance decreased with the increase of Mn content. The decrease in resistance was ascribed to oxygen vacancies and electronics produced by the change of ionic valence state. X-ray photoemission spectroscopy revealed that Ti ions were Ti⁴⁺ and the valences of Mn ions were deduced to be mainly in the form of Mn²⁺ and/or Mn³⁺ for ceramics with low content of Mn, while the Ti ions were in the form of Ti³⁺ and Ti⁴⁺ and Mn ions were diverse valence states with the coexistence of Mn²⁺, Mn³⁺, and Mn⁴⁺ for ceramics with x?=?0.01 and 0.05.     

Structure,mechanical, and thermal properties of Ti1‐xAlxN/CrAlN (x = 0.48, 0.58, and 0.66) multilayered coatings

Nano‐multilayered TiAlN/CrAlN coatings combining advantages of Ti‐Al‐N and Cr‐Al‐N are considered to be promising candidates for advanced machining processes. Here, the structure and thermal properties of Ti_1‐xAl_xN/CrAlN (x = 0.48, 0.58, and 0.66) multilayered coatings as well as referential Ti_1‐xAl_xN and Cr_0.32Al_0.68N monolithic coatings were investigated. Ti_1‐xAl_xN coatings show a structural transformation from cubic structure for x = 0.48 to mixed cubic and wurtzite structure for x = 0.58 and 0.66, and Cr_0.32Al_0.68N coating exhibits a single cubic structure. Through a multilayer arrangement with Cr_0.32Al_0.68N layers, the Ti_0.52Al_0.48N and Ti_0.42Al_0.58N layers can be stabilized in their metastable cubic structure, but the Ti_0.34Al_0.66N layer still tends to crystallize in the mixed cubic and wurtzite structure. The hardness of Ti_0.52Al_0.48N/CrAlN and Ti_0.42Al_0.58N/CrAlN coatings is higher than that of corresponding monolithic coatings regardless of as‐deposited and annealed states. Especially, after annealing at 800°C, the Ti_0.52Al_0.48N/CrAlN and Ti_0.42Al_0.58N/CrAlN coatings reach their peak hardness of ~34.2 and 32.8 GPa due to the spinodal decomposition of Ti_1‐xAl_xN layers. However, the oxidation resistance of Ti_1‐xAl_xN/CrAlN coatings is mainly up to the Al content of Ti_1‐xAl_xN layers, where only the Ti_0.34Al_0.66N/CrAlN coating can survive the 10 h exposure to air at 1000°C.     

Improved non-ohmic and dielectric properties of Cr3+ doped CaCu3Ti4O12 ceramics prepared by a polymer pyrolysis solution route

CaCu_3-xCr_xTi₄O₁₂ (x?=?0.00–0.20) ceramics were prepared via a polymer pyrolysis solution route. Their dielectric properties were improved by Cr³⁺ doping resulting in an optimal dielectric constant value of 7156 and a low tanδ?value of 0.092 in a sample with x?=?0.08. This might have resulted from a decrease in oxygen vacancies at grain boundaries. XANES spectra confirmed the presence of Cu⁺ ions in all ceramic samples with a decreasing Cu⁺/Cu²⁺ ratio due to an increased content of Cr³⁺ ions. All CaCu_3-xCr_xTi₄O₁₂ ceramics showed nonlinear characteristic with improvement in both the breakdown field (E_b) and its nonlinear coefficient (α). Interestingly, the highest values of α, ～ 114.4, and that of E_b, ～8455.0?±?123.6?V?cm^?1, were obtained in a CaCu_3-xCr_xTi₄O₁₂ sample with x?=?0.08. The improvement of dielectric and nonlinear properties suggests that they originate from a reduction of oxygen vacancies at grain boundaries.     

Single-phase formation and mechanical properties of (TiZrNbTaMo)C high-entropy ceramics: First-principles prediction and experimental study

The single-phase formation and related elastic properties of (TiZrNbTaMo)C with one equimolar and twenty non-equimolar systems have been investigated by first-principles calculation. Based on the calculation results, the “composition-structure-elastic properties” correlation heatmapping predicts that Ti element is favorable for increment of hardness and Young’s modulus, while Mo element shows contrary tendency. The (Ti_xZr₂Nb₂Ta₂Mo_4-x)C_10-y (x = 1, 2, 3) have been fabricated by carbothermal reduction assisted hot-pressing sintering. The obtained experimental results validate the prediction trend of first-principles calculation. The optimization hardness and Young’s modulus is achieved at (Ti₃Zr₂Nb₂Ta₂Mo₁)C_10-y, and the corresponding value is 27.1 ± 0.6 GPa at 9.8 N and 490 ± 5 GPa, respectively. Noteworthily, the single-phase formation mainly depends on configuration entropy. The equimolar (Ti₂Zr₂Nb₂Ta₂Mo₂)C_10-y exhibits a single-phase with homogeneous chemical composition, but some element segregation can be found in the other two non-equimolar samples sintered at 2100 ℃.     

Mechanical and thermal expansion studies on Ca0.5Sr0.5Zr4-xTixP6O24 ceramics

Ca_0.5Sr_0.5Zr_4-xTi_xP₆O₂₄ (x?=?0?0.2) ceramics belonging to the NZP family were prepared and dense ceramics with no microcracks were obtained. All of the ceramic samples were still composed of the typical NZP structure with a small amount of Ti⁴⁺ substitution for Zr⁴⁺. The mechanical and thermal expansion properties of the ceramics were characterized and the result showed that the flexural strength monotonically increased to 66.5?MPa. The thermal expansion coefficient varied from 1.8 to 3.4?×?10^?6/°C with Ti⁴⁺ content increasing. Thus, it was clear that the substitution of Ti⁴⁺ for Zr⁴⁺ had obvious effects on the sinterability, mechanical and thermal expansion properties of Ca_0.5Sr_0.5Zr_4-xTi_xP₆O₂₄ ceramics, which were discussed in detail.     

Deposition and Analysis of Al‐Rich c‐AlxTi1−xN Coating with Preferred Orientation

Metastable c‐Al_xT_1?xN is an important and well‐established hard coating in the tool industry. To improve the mechanical and thermal properties, Al‐rich c‐Al_xTi_1?xN coatings with controllable preferred crystal orientations were fabricated via low‐pressure chemical vapor deposition (LP‐CVD) in an industrial plant, using an AlCl₃–TiCl₄–NH₃–Ar–H₂ precursor system. The c‐Al_xTi_1?xN coatings with (100)‐ and (111)‐preferred orientations and average x values of 0.82 and 0.73, respectively, comprised c‐Al(Ti)N/c‐Ti(Al)N nanolamellae with average compositions of c‐Al_0.9Ti_0.1N/c‐Al_0.6Ti_0.4N and c‐Al_0.80Ti_0.20N/c‐Al_0.50Ti_0.50N; the average lamellar periods were 7.7 and 4.5 nm, respectively. High‐resolution transmission electron microscopy indicated that the c‐Al(Ti)N/c‐Ti(Al)N nanolamellae were modulated along the <100> direction, implying coherent spinodal decomposition of c‐Al_xTi_1?xN in the as‐deposited state. The hardness of the c‐Al_xTi_1?xN coatings varied from 33 to 36 GPa, depending on the (100)‐ or (111)‐preferred orientation. Residual stress measurements in the as‐deposited state showed tensile stress values of 1.8 and 4.6 GPa for the (100)‐ and (111)‐oriented c‐Al_xT_1?xN coatings, respectively. This stress may be generated by the difference in the thermal expansion coefficient of the c‐Al_xT_1?xN coating and the carbide substrate and by coherency stress in the c‐Al(Ti)N/c‐Ti(Al)N nanolamellae. In situ high‐temperature X‐Ray diffraction results revealed high thermal stability up to 1000°C.     

High temperature electrical conductivity and electrochemical investigation of La2-xSrxCoO4 nanoparticles for IT-SOFC cathode

Single-phase Ruddlesden popper of La_2-xSr_xCoO₄ nanopowders with x?=?0.7, 0.9, 1.1 and 1.3, were successfully synthesized by a modified sol-gel method. Structural stability and morphology of the prepared samples were examined using HT-XRD analysis, FE-SEM and SEM techniques. HT-XRD analysis of the samples, in the range of room temperature to 850?°C, revealed that the structure of all samples was tetragonal. The electrical conductivity measurements, in the range of room temperature to 850?°C, indicated that by increasing the temperature the electrical conductivity mechanism inverts from variable range hopping to the nearest-neighbor hopping of small polarons. In addition, it was found that by increasing Sr concentration the structure of the sintered samples becomes more stable. The electrochemical characterization was carried out using the impedance spectroscopy (EIS) measurements on symmetrical cells at three different temperatures, 650?°C, 750?°C and 850?°C. The area specific polarization resistance (ASR) of La_2-xSr_xCoO₄-CGO-La_2-xSr_xCoO₄ symmetrical cell, in oxygen flow, was obtained about 1.07, 0.35, 0.33 and 0.43 Ωcm² at 850?°C for the samples with x?=?0.7, 0.9, 1.1 and x?=?1.3, respectively. According to our EIS results, the main rate-limiting step for La_2-xSr_xCoO₄ cathode performance is the dissociation process of oxygen at the surface of cathode at 650?°C and the charge transfer limiting in the cathode/electrolyte at 750?°C and 850?°C. Our results showed that the samples with Sr contents of x?=?0.9 and x?=?1.1 can be the promising cathodes for IT-SOFC applications.     

Thermal stability and oxidation resistance of V-alloyed TiAlN coatings

V-containing nitride coatings recently attract a wide range of research interests owing to their excellent tribological properties. To evaluate their comprehensive properties, a comparative study on the intrinsic thermal stability and oxidation resistance of TiAlN and TiAlVN coatings are conducted here. Ti_0.56Al_0.44N, Ti_0.50Al_0.44V_0.06N, and Ti_0.40Al_0.50V_0.10N coatings, deposited by cathodic arc evaporation, exhibit a single-phase face-centered cubic structure with a hardness of 28.9–29.8 GPa. The V-containing coatings show a pronounced age-hardening upon annealing, which contributes to a hardness increase of 3.7 and 4.8 GPa at 800 °C for Ti_0.50Al_0.44V_0.06N and Ti_0.40Al_0.50V_0.10N, respectively, corresponding to 2.9 GPa for Ti_0.56Al_0.44N. Also, alloying with V retards the formation of wurtzite AlN upon annealing, especially in Ti_0.50Al_0.44V_0.06N, and thus contributes to a higher hardness above 30 GPa even annealing at 1100 °C, while the hardness of Ti_0.56Al_0.44N significantly reduces to 27.8 ± 0.6 GPa. However, alloying with V into TiAlN leads to an earlier formation of rutile TiO₂ and also Ti-rich oxide top-layer on the outside surface instead of dense Al₂O₃, and thus degrades the oxidation resistance. When exposed to air at 700 °C for 10 h, the Ti_0.50Al_0.44V_0.06N and Ti_0.40Al_0.50V_0.10N coatings suffer from a severe oxidation, whereas only a compact oxide scale with a thickness of ~ 80 nm for Ti_0.56Al_0.44N is formed.     

The influence of Ti: Si3N4 ratio on the microstructure evolution of Ti–Si3N4 reaction in Cu melts

In this work, through the study of the obtained Cu–Ti–Si–N intermediate products by controlling the reaction degree of Ti and Si₃N₄ powder in Cu melts at 1250～1300 °C, the effects of different Ti: Si₃N₄ mass ratios on the microstructure evolution of Ti–Si₃N₄ reaction in Cu melts were verified. When the mass ratio of Ti: Si₃N₄ is higher, such as 3.09:1, TiN will cooperate with Ti₅Si₃ and depend on each other to nucleate and grow to form TiN/Ti₅Si₃ composites. The formed TiN are spherical and wetted by Ti₅Si₃ to uniformly disperse in Cu melts. As a result, the TiN–Ti₅Si₃ hybrid reinforced Cu matrix composites will be formed. However, when the mass ratio of Ti: Si₃N₄ is lower, such as 1.37:1, Ti and Si₃N₄ will firstly react to form TiN and Ti–Si liquid. The formed TiN are irregularly polygonal and connect with each other. The Ti–Si liquid will combine with Cu melts to form Cu–Ti–Si liquid and finally form δCu₄Si, ηCu₃Si and τ₁-CuSiTi phases during the colling stage. In this case, TiN are difficult to be wetted, and the Ti–Si₃N₄ compact will keep its original shape and not spread in Cu melts.     

High strengthening effects and excellent wear resistance of Ti3Al(Si)C2 solid solutions

The Ti₃Al_1.2−xSi_xC₂ (x = 0, 0.2, 0.4) powders were synthesized from Ti, Al, Si, and TiC powders, and nearly pure Ti₃Al_1.2−xSi_xC₂ bulks were fabricated by the means of two-time hot-pressing method. Significant strengthening effect in bulks was found after the addition of 0.2 Si and 0.4 Si to form Ti₃Al(Si)C₂ solid solutions. The flexural strengths of Ti₃AlSi_0.2C₂ and Ti₃Al_0.8Si_0.4C₂ were 485 and 554 MPa, 14% and 30% larger than the strength of Ti₃AlC₂, respectively. The Vickers hardness of these compounds were separately, 6.95 and 7.57 GPa, representing the enhancements of 37% and 49% over those of Ti₃AlC₂. The tribological behavior was studied by dry-sliding method with a S45C steel at the sliding speed of 30 m/s and the normal load of 20-80 N. The results showed that after incorporating different contents of Si, the friction coefficient was between 0.22 and 0.30, correspondingly lower wear rate was 3.19-2.61 × 10⁻⁶ mm³/Nm. These excellent tribological performances were attributed to the presence of continuous self-generated oxidized films during tribological examination. Finally, the phase compositions and microhardness of the oxidized films were analyzed and characterized.     

Effect of Si content on the microstructure and properties of Ti–Si–C composite coatings prepared by reactive plasma spraying

In this study, Ti–Si–C composite coatings were synthesized via plasma spraying of agglomerated powders prepared by a spray drying/precursor pyrolysis technology using Ti, Si, and sucrose powders. The influence of Si content, ranging from 0 wt% to 24 wt%, on the microstructure, mechanical properties, and oxidation resistance of the composite coatings was investigated. Results show that the phase composition of the Ti–Si–C composite coatings changes with the increasing Si content. The coatings without Si addition consist of TiC and Ti₃O; the coatings with 6–18 wt% Si are composed of TiC, Ti₅Si₃, and Ti₃O; the coatings with Si content of 24 wt% form only TiC and Ti₅Si₃ phases. As the Si content increases, the hardness of the Ti–Si–C composite coatings increases first and then decreases, depending on the intrinsic hardness of the ceramic phases, the brittleness of Ti₅Si₃, and the defects such as pores and cracks. The Ti–Si–C composite coatings have high wear resistance due to the in-situ synthesized high-hardness TiC and Ti₅Si₃. Owing to the high brittleness of Ti₅Si₃, the increasing Si content leads to higher wear volume loss at room temperature, which can be partially improved in high-temperature wear tests. The oxidation resistance of Ti–Si–C composite coatings increases with the increase of Si content, and the higher the oxidation temperature, the more obvious the influence of the Si addition on oxidation resistance.     

Effect of Ge on microstructure and mechanical properties of Ti3SiC2/Al2O3 composites

Owing to the good physicochemical compatibility and complementary mechanical properties of Ti₃SiC₂ and Al₂O₃, Ti₃SiC₂/Al₂O₃ composites are considered as ideal structural materials. However, TiC and TiSi₂ typically coexist during the synthesis of Ti₃SiC₂/Al₂O₃ composites through an in-situ reaction, which adversely affects the mechanical properties of the resulting composites. In this study, Ti₃SiC₂/Al₂O₃ composites were prepared via in-situ hot pressing sintering at 1450 °C. Ge, which was used as a sintering aid, improved the purity and mechanical properties of the Ti₃SiC₂/Al₂O₃ composites. This is because Ge replaced some of the Si atoms to compensate the evaporation loss of Si to form Ti₃(Si_1-xGe_x)C₂, which showed a crystal structure similar to that of Ti₃SiC₂. Furthermore, the molten Ge accelerated the diffusion reaction of the raw materials, increasing the overall density of the Ti₃SiC₂/Al₂O₃ composites. The optimum Ge amount for improving the mechanical properties of the composites was found to be 0.3 mol. The flexural strength, fracture toughness, and microhardness of the composite with the optimum Ge amount were 640.2 MPa, 6.57 MPa m^1/2, and 16.21 GPa, respectively. The formation of Ti₃(Si_1-xGe_x)C₂ was confirmed by carrying out X-ray diffraction, energy dispersive spectroscopy, and transmission electron microscopy analyses. A model crystal structure of Ti₃(Si_1-xGe_x)C₂ doped with 0.3 mol Ge was established by calculating the solid solubility of Ge.     

Single-source-precursor synthesis and high-temperature evolution of a boron-containing SiC/HfC ceramic nano/micro composite

A boron-containing SiHfC(N,O) amorphous ceramic was synthesized upon pyrolysis of a single-source-precursor at 1000 °C in Ar atmosphere. The high-temperature microstructural evolution of the ceramic at high temperatures was studied using X-ray powder diffraction, Raman spectroscopy, solid-state nuclear magnetic resonance spectroscopy and transmission electron microscopy. The results show that the ceramic consists of an SiHfC(N,O)-based amorphous matrix and finely dispersed sp²-hybridized boron-containing carbon (i.e. B_yC). High temperature annealing of B_yC/SiHfC(N,O) leads to the precipitation of HfC_xN_1-x nanoparticles as well as to β-SiC crystallization. After annealing at temperatures beyond 1900 °C, HfB₂ formation was observed. The incorporation of boron into SiHfC(N,O) leads to an increase of its sintering activity, consequently providing dense materials possessing improved mechanical properties as compared to those of boron-free SiC/HfC. Thus, hardness and elastic modulus values up to 25.7 ± 5.3 and 344.7 ± 43.0 GPa, respectively, were measured for the dense monolithic SiC/HfC_xN_1-x/HfB₂/C ceramic nano/micro composite.     

Defect chemistry and dielectric behavior of Sr0.99Ce0.01Ti1−xO3 ceramics with high permittivity

Sr_0.99Ce_0.01Ti_1-xO₃ (SCT, x?=?0, ±?0.0025, ±?0.0050, 0.0075) ceramics were prepared by solid state reaction methods and sintered in air atmosphere at different temperatures, with a soaking time of 2?h. The dielectric properties of all samples presented excellent temperature independence over a broad temperature range from 25 to 330?℃ and frequency independence between 10?kHz and 1?MHz. Sr_0.99Ce_0.01Ti_0.9925O₃ (SCT0.9925) ceramics sintered in air atmosphere exhibited a high permittivity (~5400) and a low dielectric loss (~0.01) measured at room temperature and 1?kHz. XPS and complex impedance spectroscopy analysis confirmed that the high permittivity and low dielectric loss were attributed to the fully ionized oxygen vacancies and giant defect-dipoles in Ti-deficient samples. However, a higher dielectric loss of Ti-rich samples is owing to the destruction of giant defect dipoles, in which highly localized electrons were transformed into hopping electrons.     

Bond ionicity,lattice energy,bond energy,and microwave dielectric properties of Ca1-xSrxWO4 ceramics

The Ca_1-xSr_xWO₄ (x?=?0, 0.02, 0.04, 0.06, 0.08, 0.10) ceramics were fabricated through solid-state reaction, and the relationships among microwave dielectric properties of Ca_1-xSr_xWO₄, bond ionicity, lattice energy and bond energy were systematically investigated for the first time. The patterns of X-ray diffractions of Ca_1-xSr_xWO₄ presented tetragonal scheelite structure and no second phase appeared throughout the entire compositions. Dielectric properties of Ca_1-xSr_xWO₄ were proved to be related to the microstructures: dielectric constant (ε_r) of Ca_1-xSr_xWO₄ was dependent on the bond ionicity; the quality factor (Q×f₀) of Ca_1-xSr_xWO₄ was affected by W-site lattice energy when intrinsic loss is dominant; the temperature coefficient of resonant frequency (|τ_f|) would increase if B-site bond energy decreased. Ca_1-xSr_xWO₄ ceramic showed excellent microwave dielectric properties, ε_r =?9.42, Q×f₀ =?79876?GHz and τ_f =??18.8?ppm/°C when x?=?0.08 and sintered at 1100?°C for 4?h.     

Synthesis and properties of (Hf1-xTax)C solid solution carbides

Thermodynamically stable (Hf_1–xTa_x)C (x?=?0.1–0.3) compositions were selected by First Principle Calculation and synthesized in nanopowders via high-energy ball milling and carbothermal reduction of commercial oxides at 1450?°C. The formation of a solid solution during powder synthesis was investigated. The solid solution carbide powders were sintered at 1900?°C by spark plasma sintering without a sintering aid. As a result, the (Hf_1–xTa_x)C solid solution carbides exhibited high densities, excellent hardness and fracture toughness (ρ: 98.7–100.0%, HVN: 19.69–19.98?GPa, K_IC: 5.09–5.15?MPa?m^1/2) compared with previously reported HfC and HfC–TaC solid solution carbides.