On the correlation between solid-particle erosion and fracture parameters in SiC

The fracture toughness,K _Ic, for a hot-pressed SiC and three types of reaction-bonded SiC has been measured using the Hertzian indentation technique with spherical indentors whose radii varied from 2.5 to 10 mm. Excellent agreement between the toughness measured by this technique and the publishedK _Ic value measured using double torsion is obtained for the hot-pressed SiC. The Vickers hardness,H, which was also measured, andK _Ic have been used to predict the solid-particle erosion rate, ΔW, given by ΔW ∞H _a K _c ^b where the values of the exponentsa andb depend on the details of the erosion model. The predicted rates are not in agreement with the measured rates. This discrepancy is probably due to the fact that the models are insensitive to microstructure.     

Erosion mechanisms of aluminium nitride ceramics at different impact angles

In this paper, erosion wear behaviour of aluminium nitride (AlN) ceramics is studied. The influence of particle hardness and shape on erosion of the AlN surface is examined. The effect of varying the impingement angle on the weight loss and the roughness parameters of AlN ceramics testing sample is also determined. Therefore, erosive wear behaviour of AlN ceramics was investigated using SiC and SiO₂ particles as erodents, at following impact angles: 30°, 45°, 60°, 75° and 90°. Scanning electron microscopy (SEM) was used to analyze the eroded surfaces in order to determine erosion mechanisms. The roughness parameters (R_a, R_z and R_max), before and after erosion with SiO₂ and SiC particles at 30° and 90° angles of impingement, respectively, were determined using a profilometer. It was found that the impact angle is influencing the erosion wear of the AlN ceramics and maximum erosion takes place at impact angle of 90°. The results indicate that hard, angular SiC particles cause more damage than softer, more rounded SiO₂ particles.     

Solid particle erosion of SiC-Al2OC ceramics

Erosion rates of SiC-Al₂OC ceramics, with Al₂OC content varying from 5 to 75wt%, were assessed using 240-grit alumina abrasive particles accelerated to a velocity estimated at 120msec^–1 and impacting the target at normal incidence. The target ceramics varied in hardness from 27.1 GPa for SiC-5wt% Al₂OC to 10.8 GPa for SiC-75wt% Al₂OC, but the fracture toughness was essentially independent of composition (K _lc ã 3.5 MPa m^1/2). The erosion weight loss varied linearly with the test duration for all the ceramics and the erosion rate decreased systematically with increasing target hardness; the hardness dependence of the erosion rate was, however, much greater than the predictions of the currently available erosion models.     

Effect of La-doping content on the dielectric and ferroelectric properties of 0.88Pb(Mg1/3Nb2/3)O3–0.12PbTiO3 ceramics

Dense relaxor ferroelectric 0.88Pb(Mg_1/3Nb_2/3)O₃–0.12PbTiO₃ (0.88PMN–0.12PT) ceramics with different La-doping contents (0, 1, 2 and 4 at.%) were sintered by using powders synthesized via a solid-state reaction route. The effects of La doping on the microstructures and electric properties of the 0.88PMN–0.12PT ceramics were investigated. It was found that the average grain size, remanent polarization P _r, coercive field E _c, Curie temperature T _c and leakage-current density J of the ferroelectric ceramics decrease significantly with increasing La doping content. The decrease in P _r, E _c and T _c can be understood in term of the fact that the substitution of Pb²⁺ ions by La³⁺ ions suppresses the long-range coupling of BO₆ octahedrons, while the abatement in J can be explained according to the reduction of oxygen vacancies caused by La doping. By fitting the J–E curves, the conduction mechanism of the 0.88PMN–0.12PT ceramics is confirmed to be Ohmic conduction generated from oxygen vacancies. The dielectric and ferroelectric properties of 0.88PMN–0.12PT ceramics are tunable with manipulating the La doping content.     

Thermal shock resistance of the porous Al2O3/ZrO2 ceramics prepared by gelcasting

Porous Al₂O₃/ZrO₂ ceramics with porosity varying from 6% to 50% were fabricated by gelcasting using polystyrene (PS) as pore-forming agent. The effects of sintering temperature on porosity, strength as well as pore size were investigated. The flexural strength of these porous ceramics at room temperature significantly decreases as the porosity increases. Thermal shock resistance of these ceramics was improved by increasing the porosity. Both the critical difference temperature (ΔT_c) and residual strength of high porosity ceramics were higher than those of low porosity ceramics. These improvements can be attributed to the pores in the specimens which relax the thermal shock stress and arrest the propagation of microcracks effectively, which is confirmed by XRD analysis of specimens which encountered different thermal shock temperature difference.     

Structure and electrical properties of strontium barium niobate ceramics

SBN, x=0.25, 0.35, 0.50, 0.60 and 0.75 series of ceramics prepared by traditional sintering method have been studied systematically. The impact of composition and sintering temperature on structures, microstructures, and electrical properties of SBN ceramics was characterized of X-ray diffraction, scanning electron microscopy, and electrical measurements. It is found that the composition and temperature play an important role on the fabrication of single phase tetragonal TTB SBN ceramics. At x=0.5, TTB SBN ceramics can be obtained at 1200°C. For x<0.5, it consists of a mixture of TTB structure SBN and orthorhombic phase BaNb₂O₆ even at 1300°C; TTB structure SBN and orthorhombic phase SrNb₂O₆ for x>0.5. The complete TTB phase is produced at 1350°C. With Sr content increasing, the electrical performances show a regular change, strongly conforming to the reducing of the Curie temperature. SBN with the Sr composition of x=0.60-0.75 is a promising candidate for electro-optics device applications.     

Penetration scaling in atomistic simulations of hypervelocity impact

We present atomistic molecular dynamics simulations of the impact of copper nano particles at 5 km s⁻¹ on copper films ranging in thickness from from 0.5 to 4 times the projectile diameter. We access both penetration and cratering regimes with final cratering morphologies showing considerable similarity to experimental impacts on both micron and millimetre scales. Both craters and holes are formed from a molten region, with relatively low defect densities remaining after cooling and recrystallisation. Crater diameter and penetration limits are compared to analytical scaling models: in agreement with some models we find the onset of penetration occurs for 1.0 < f/d_p < 1.5, where f is the film thickness and d_p is the projectile diameter. However, our results for the hole size agree well with scaling laws based on macroscopic experiments providing enhanced strength of a nano-film that melts completely at the impact region is taken into account.     

Electrical properties and thermal sensitivity of Ti/Y modified CuO-based ceramic thermistors

The Ti/Y modified CuO-based negative temperature coefficient (NTC) thermistors, Cu_0.988-2yY_0.008Ti _y O (TYCO; y = 0.01, 0.015, 0.03, 0.05 and 0.07), were synthesized through a wet-chemical method followed by a traditional ceramic sintering technology. The related phase component and electrical properties were investigated. XRD results show that the TYCO ceramics have a monoclinic structure as that of CuO crystal. The TYCO ceramics can be obtained at the sintering temperature 970°C-990°C, and display the typical NTC characteristic. The NTC thermal-sensitive constants of TYCO thermistors can be adjusted from 1112 to 3700 K by changing the amount of Ti in the TYCO ceramics. The analysis of complex impedance spectra revealed that both the bulk effect and grain boundary effect contribute to the electrical behavior and the NTC effect. Both the band conduction and electron-hopping models are proposed for the conduction mechanisms in the TYCO thermistors.     

Effect of Zr doping on the microstructure and electric properties of BaHf0.1Ti0.9O3 ceramics

Pure and Zr-doped barium titanate hafnate (BaHf_0.1Ti_0.9O₃, short for BHT) ceramics were prepared by conventional solid state reaction method. The microstructures, dielectric and ferroelectric properties of BaHf_0.1Ti_0.9-x Zr _x O₃ (x = 0, 0.02, 0.04, 0.10) ceramics have been investigated. From the X-ray diffraction patterns it is indicated that Zr⁴⁺ ions have entered the unit cell maintaining the tetragonal perovskite structure of solid solution and the lattice constant of Zr-doped BHT ceramics increases with the increase of Zr content. There is an obvious difference between the grain shape of pure BHT ceramics and that of Zr-doped BHT ceramics. The temperature dependences of dielectric constant indicated that all of the three phase transition temperatures increase after doped zirconium. It is found that well-behaved hysteresis loops can be observed in pure and Zr-doped BHT ceramics. The remanent polarization (2P _r) and the coercive electric field (2E _C) of BaHf_0.1Ti_0.9-x Zr _x O₃ ceramics gradually decrease as the Zr content increases from 2 to 10 mol %.     

Fabrication and characterization of grain-oriented cerium fluoride ceramics from a slip-casting process in a magnetic field

Transparent CeF₃ ceramics may become a promising candidate in high-energy physics application due to their extremely high resistance to hadron-induced damage. To achieve a high transparency, the grain-oriented structure was necessary for CeF₃ ceramics to limit the light scattering caused by the birefringence effects due to its hexagonal phase structure. In the present work, this kind of structure was obtained by a slip-casting process within a magnetic field of either 9 T generated by a superconducting magnet or 0.4 T from a permanent magnet. X-ray diffraction and electron backscattered diffraction analysis proved that the c-axis of grains were well aligned parallel to the magnetic field direction. The Lotgering factor of 93.83 % indicated that a high degree of orientation was achieved by the magnetic field of only 0.4 T. It was due to the large susceptibility and the net magnetic anisotropy Δχ strongly enhanced by the spin–orbit interaction of the 4f electrons of Ce³⁺ ions that highly grain-oriented CeF₃ ceramics can be realized within such a low magnetic field.     

Characterization and dielectric behavior of CuO-doped ZnTa2O6 ceramics at microwave frequency

The effects of CuO addition on the microstructures and microwave dielectric properties of ZnTa₂O₆ ceramics were investigated. CuO was selected as a liquid-phase sintering aid to lower the sintering temperature of ZnTa₂O₆ ceramics. With CuO addition, the sintering temperature of ZnTa₂O₆ can be effectively reduced from 1350 to 1230 °C. The crystalline phase exhibited no phase difference and no second phase was detected at low addition levels (0.25-1 wt.%). The quality factors Q × f were strongly dependent upon the CuO concentration. A Q × f value of 65,500 GHz was obtained for specimen with 0.25 wt.% CuO addition at 1230 °C. For all levels of CuO concentration, the relative dielectric constants were not significantly different and ranged from 34.2 to 35.7. Tunable temperature coefficient of resonant frequency (τ_f) can be adjusted to zero by appropriately turning the CuO content.     

Low temperature sintering of Li2(Mg0.3Zn0.7)Ti3O8-0.12TiO2 microwave dielectric ceramics with controllable grain size

The effects of BaO-B₂O₃-SiO₂ (BBS) frit on sinterability, microstructure and microwave dielectric properties of Li₂(Mg_0.3Zn_0.7)Ti₃O₈-0.12TiO₂ (LMZT) ceramics were systematically investigated. BBS frit can effectively lower the sintering temperature of LMZT ceramics to below 900 °C. Suitable BBS frit addition can accelerate the growth of the LMZT grains while inhibit the abnormal grain growth at the same time. The LMZT ceramics with 2 wt% BBS frit sintered at 900 °C for 3 h show homogeneous microstructure composed of 5–10 μm grains and excellent dielectric properties: ε _r  = 24.1, Q × f = 21,980 GHz, τ _f  = ?4.1 ppm/ °C. It is compatible with Ag electrodes, which makes it a potential candidate material for low temperature co-fired ceramics technology application.     

Electrocaloric properties of (1 − x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 ferroelectric ceramics near room temperature

Electrocaloric effects of (1 − x)Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃ [abbreviated as (1 − x)PMN–xPT] ferroelectric ceramics with x being, 0.08, 0.10, and 0.25, respectively, were measured near room temperature, and the origins of the electrocaloric effects of these ceramics were discussed. It was found that these ceramics possess large electrocaloric effect with ΔT being, more than 1 K under an electric field of 1.5 kV mm⁻¹ in a wide temperature range (more than 10 K) near room temperature, and this effect is due to both of the electrocaloric effect resulting from the electric field induced first-order phase transition and the linear electrocaloric effect. It is expected that these ceramics could be used for multi-stage cascade ferroelectric refrigeration near room temperature.     

等离子喷涂铝硅聚苯酯封严涂层的组织及抗冲蚀性能

抗冲蚀磨损性能是封严涂层最重要的性能指标之一.采用铝硅聚苯酯(Alsi-ployester)粉末和PARXAIR-3710等离子喷涂系统制备了可磨耗封严涂层.采用磨粒冲蚀试验研究了不同冲蚀角度、冲蚀时间、冲蚀颗粒尺寸和冲蚀速度对涂层抗冲蚀性能的影响,并用SEM观察冲蚀表面形貌.结果表明:在相同冲蚀条件下,Alsi-ployester涂层冲蚀率随冲蚀颗粒速度的增加而增加;涂层的冲蚀率受冲蚀角的影响,90°时的冲蚀率大于30°的;30°时涂层损失主要以刮削为主,90°时以凿削为主;涂层失重与冲蚀时间存在良好的线性关系;涂层冲蚀率随着冲蚀颗粒尺寸增加成波动变化,当颗粒尺寸为250μm时,冲蚀率达到最大.在实际工程应用中,Alsi-ployester涂层展示了良好的抗冲蚀性能.     

Defect-dipole-induced two important macroscopic effects: The dielectric relaxation and the ferroelectric aging in hybrid-doped (Ba,Ca)TiO3 ceramics

To observe the dielectric relaxor behavior and the ferroelectric aging effect, the structural and electrical properties of (Ba_0.90Ca_0.10)(Ti_1−xCa_x)O_3−x (BCT-C) ceramics prepared using a conventional dry route were investigated. With increased concentration of Ca on the B-site in BCT-C ceramics, the tetragonal phase is decreased while the multiphases with the cubic pervoskite structure as a major constituent is increased and remained predominant in the BCT-C ceramics with x > 0.03. The increased amount of Ca on the B-site causes the high temperature phase transition to shift to the low temperature and results in an increase in the degree of diffusion and relaxation of the BCT-C ceramics. The observed dielectric relaxation behavior may be understood by a defect-dipole formed random electric field induced domain state. The aging effect controlled by the migration of mobile oxygen vacancies is rarely observed in aged BCT-C ceramics while it can be markedly observed in Bi-doped BCT ceramics. Based on the microscopic mechanism of the aging effect, the possible reasons for the aging effect are given.     

Effect of sintering aid on microwave dielectric behaviors of Ni0.5Ti0.5NbO4 ceramics for LTCC applications

The effects of ZnO glass addition on the microwave dielectric properties of Ni_0.5Ti_0.5NbO₄ (NTN) ceramics prepared by solid-state reaction method have been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The pure NTN ceramics have ε_r of 60.6, Q × f value of 70,100 GHz, and τ_f value of 76.6 ppm ^°C⁻¹ sintered at 1140 °C for 6 h. The results indicate that the addition of ZnO can effectively benefit the densification and further improve the dielectric constant. Moreover, the lower sintering temperature of NTN ceramics from 1140 to 930 °C is obtained by the addition of ZnO glass. However, an excess of ZnO suppresses the grain growth and decreases the Q × f value of NTN ceramics. The NTN ceramics with 2 wt% ZnO sintered at 930 °C for 6 h possess promising microwave dielectric properties: ε_r of 56.3, Q × f value of 67,000 GHz, and τ_f value of 78.6 ppm °C⁻¹, which shows that the materials are suitable for low-temperature co-fired ceramics applications.     

Characterization of negative temperature coefficient of resistivity in (Sn1−x Ti x )0.95Sb0.05O2 (x ≤ 0.1) ceramics

Sb-doped SnO₂-based ceramics substituted by a trace of titanium, (Sn_1?x Ti _x )_0.95Sb_0.05O₂ (x ≤ 0.1), were prepared through a wet chemical process polymerized with polyvinyl alcohol. The phase component and related electrical properties of the ceramics were investigated. The results show that all the ceramics have the tetragonal rutile-type SnO₂ crystalline lattice and show typical effect of negative temperature coefficient (NTC) of resistivity. The room-temperature resistivities and NTC material constants can be adjusted widely by changing the Ti content in the ceramics. The investigations by analyzing the electrochemical impedance spectra at various temperatures show that both grain effect and grain-boundary effect contribute to the NTC feature of the ceramics. The conduction mechanisms combining the electron-hopping model and band conduction are proposed for the NTC effect in the ceramics.     

Study on the plasma erosion resistance of ZrO2p(3Y)/BN–SiO2 composite ceramics

ZrO_2p(3Y)/BN–SiO₂ composite ceramics were prepared by hot pressed sintering. The Kr plasma was incident normally to the sample surface to measure the plasma erosion resistance, and XRD, TEM, SEM were used to characterize the phase composition and morphology of composite ceramics before and after Kr plasma erosion. The composites were composed of h-BN, amorphous SiO₂, m-ZrO₂ and t-ZrO₂, which keep stability in the process of Kr plasma sputtering. Two kinds of zirconia phases (t-ZrO₂, m-ZrO₂) and lamellar h-BN evenly distributed in ZrO_2p(3Y)/BN–SiO₂ composite ceramics, and the lath microstructures were observed in m-ZrO₂ particles. The plasma erosion resistances of ceramic composites increase significantly with the increase of ZrO₂ content. Pits with diameter sizes of several micrometers were observed on the surface of composites after plasma sputtering, which indicate that one or several grains are sputtered out during the plasma sputtering process. The edges of the grains become smoother for the preferential sputtering to the sharp edge of the grains during plasma sputtering.     

NTC characteristic of SnSb0.05O2–BaTi0.8Fe0.2O3−δ composite materials

SnSb _x O₂ (x = 0.003, 0.01, 0.02, 0.05 and 0.07) ceramics and SnSb_0.05O₂–BaTi_0.8Fe_0.2O_3-δ composite ceramics were prepared by using wet-chemical synthesis methods. The phases and related electrical properties of the ceramics were investigated. The results show that all the prepared ceramics have the effect of negative temperature coefficient (NTC) of resistivity over a wide temperature range. The room-temperature resistivities (ρ ₂₅) and material constants (B _25/85) of the SnSb _x O₂ NTC ceramics increase with the Sb concentration increases. The B _25/85 of the ceramics can be enhanced obviously when a certain content of BaTi_0.8Fe_0.2O_3-δ was added in SnSb _x O₂. The analysis of impedance spectra reveals that both grain and grain boundary contribute to the NTC effect of the ceramics. The conduction mechanisms combining the electron-hopping model and band conduction are proposed for the NTC effect in the SnSb_0.05O₂–BaTi_0.8Fe_0.2O_3-δ composite ceramics.     

Microstructure,ferroelectric, piezoelectric and ferromagnetic properties of BiFeO3–BaTiO3–Bi(Zn0.5Ti0.5)O3 lead-free multiferroic ceramics

Multiferroic ceramics of (0.70?x)BiFeO₃–0.30BaTiO₃–xBi(Zn_0.5Ti_0.5)O₃ + 1 mol% MnO₂ with perovskite structure were prepared by a conventional ceramic technique and the effects of Bi(Zn_0.5Ti_0.5)O₃ doping and sintering temperature on the microstructure, multiferroic and piezoelectric properties of the ceramics were studied. All the ceramics possess a pure perovskite structure and no second phases can be detected. After the addition of a small amount of Bi(Zn_0.5Ti_0.5)O₃ (x ≤ 0.05), the ferroelectric and piezoelectric properties of the ceramics are improved and the grain growth is promoted. However, excess Bi(Zn_0.5Ti_0.5)O₃ (x ≥ 0.10) retards the grain growth, degrades the ferroelectricity and piezoelectricity, and induces two dielectric anomalies at high temperature. The ceramics can be well sintered at the very wide range of low sintering temperatures (880–980 °C) and exhibit good densification (relative density: 96.2–98.4 %) and strong electric insulation. The increase in the sintering temperature promotes the grain growth and improves the ferroelectricity of the ceramics. The ceramic with x = 0.05 sintered at 880–980 °C possesses improved ferroelectric and piezoelectric properties with remanent polarizations P _r of 21.9–28.1 μm/cm², piezoelectric constants d ₃₃ of 125–139 pC/N and planar electromechanical coupling factors k _p of 30.1–32.4 %, and high Curie temperatures T _C of 523–565 °C. A weak ferromagnetism with remanent magnetizations M _r of 0.0411–0.0422 emu/g and coercive fields H _c of 1.70–1.99 kOe were observed in the ceramics with x = 0–0.025.