Enhanced Temperature Stability of Pyroelectric Sensing in Multilayer Potassium Sodium Niobate-Based Ceramics with Graded Polarization Rotation.

Pyroelectric effect which refers to electrical responses induced by time temperature-dependent fluctuations has received extensive attention, showing promising application prospects for infrared (IR) technology. Although enhanced pyroelectric performances are obtained in potassium sodium niobate-based ceramics at room temperature via multi-symmetries coexistence design, the poor pyroelectric temperature stability is still an urging desire that needs to be resolved. Herin, by constructing multilayer composite ceramics and adjusting the proportion of stacked layers, improved pyroelectric coefficient, and figures of merit (FOMs), as well as enhanced temperature stabilities can be achieved. With a remained high pyroelectric coefficient of 5.45 × 10-4 C m-2°C-1 at room temperature, the pyroelectric parameters almost keep unchanged in the temperature range of 30-100 °C, showing great properties advantages compared with previous reports. The excellent properties can be attributed to the graded polarization rotation states among each lamination induced by successive phase transitions. The novel strategy for achieving stable pyroelectric sensing can further promote the application in the IR sensors field.

Characterization of an Impedance-Type Humidity Sensor Based on Porous SnO2/TiO2 Composite Ceramics Modified with Molybdenum and Zinc.

In this study, we report on the room-temperature characteristics of an impedance-type humidity sensor based on porous tin oxide/titanium oxide (SnO2/TiO2) composite ceramics modified with Mo and Zn. The SnO2/TiO2-based composites synthesized in the solid-state processing technique have been structurally characterized using X-ray diffraction, scanning electron microscopy, energy dispersive, and Raman spectroscopy. Structural analysis indicated the desired porous nature of the synthesized ceramics for sensing applications, with an average crystallite size in the nano range and a density of about 80%. The humidity-sensing properties were evaluated within a wide relative humidity range from 15% to 85% at room temperature, and the results showed that a better humidity response had a sample with Mo. This humidity-sensing material exhibits a linear impedance change of about two orders of magnitude at the optimal operating frequency of 10 kHz. Furthermore, fast response (18 s) and recovery (27 s), relatively small hysteresis (2.8%), repeatability, and good long-term stability were also obtained. Finally, the possible humidity-sensing mechanism was discussed in detail using the results of complex impedance analysis.

Effect of Zr2Al4C5 Content on the Mechanical Properties and Oxidation Behavior of ZrB2-SiC-Zr2Al4C5 Ceramics.

ZrB2-SiC-Zr2Al4C5 multi-phase ceramics with uniform structure and high density were successfully prepared through the introduction of in situ synthesized Zr2Al4C5 into ZrB2-SiC ceramic via SPS at 1800 °C. A systematic analysis and discussion of the experimental results and proposed mechanisms were carried out to demonstrate the composition-dependent sintering properties, mechanical properties and oxidation behavior. The results showed that the in situ synthesized Zr2Al4C5 could be evenly distributed in the ZrB2-SiC ceramic matrix and inhibited the growth of ZrB2 grains, which played a positive role in the sintering densification of the composite ceramics. With increasing Zr2Al4C5 content, the Vickers hardness and Young's modulus of composite ceramics gradually decreased. The fracture toughness showed a trend that first increased and then decreased, and was increased by about 30% compared with ZrB2-SiC ceramics. The major phases resulting from the oxidation of samples were ZrO2, ZrSiO4, aluminosilicate and SiO2 glass. With increasing Zr2Al4C5 content, the oxidative weight showed a trend that first increased then decreased; the composite ceramic with 30 vol.% Zr2Al4C5 showed the smallest oxidative weight gain. We believe that the presence of Zr2Al4C5 results in the formation of Al2O3 during the oxidation process, subsequently resulting in a lowering of the viscosity of the glassy silica scale, which in turn intensifies the oxidation of the composite ceramics. This would also increase oxygen permeation through the scale, adversely affecting the oxidation resistance of the composites with high Zr2Al4C5 content.

Improvement of Mechanical Properties of Composites with Surface Modified B4C for Precision Machining.

In order to solve the problem of difficult sintering and high brittleness of B4C-based ceramics, B4C@ZrB2-TiB2 composite powder was synthesized by molten salt method, and B4C-(Zr, Ti)B2 composite ceramics were successfully prepared by spark plasma sintering. The effects of different raw material ratios on the composition, microstructure, and mechanical properties of the prepared composite ceramics were characterized by XRD, XPS, SEM, and TEM. The results show that ZrB2 and TiB2 were grown on the surface of B4C by template mechanism to form a dense nanocrystalline coating, and the original surface of B4C was exposed gradually with the decrease of the ratio of metal powder. When the composite powders were sintered at 1700 °C, ZrB2 and TiB2 formed a solid solution, which can refine grains and improve strength. When the raw material ratio is n(B4C): n(Zr): n(Ti) = 12:1:1, the composite ceramics have excellent comprehensive properties, the Vickers hardness reaches 41.2 GPa.

Fabrication of Large-Area Mullite-Cordierite Composite Substrates for Semiconductor Probe Cards and Enhancement of Their Reliability.

This work aims to fabricate a large-area ceramic substrate for the application of probe cards. Mullite (M) and cordierite (C), which both have a low thermal expansion coefficient, excellent resistance to thermal shock, and high durability, were selected as starting powders. The mullite-cordierite composites were produced through different composition ratios of starting powders (M:C = 100:0, M:C = 90:10, M:C = 70:30, M:C = 50:50, M:C = 30:70, and M:C = 0:100). The effects of composition ratio and sintering temperature on the density, porosity, thermal expansion coefficient, and flexural strength of the mullite-cordierite composite pellets were investigated. The results showed that the mullite-cordierite composite pellet containing 70 wt% mullite and 30 wt% cordierite sintered at 1350 °C performed exceptionally well. Based on these findings, a large-area mullite-cordierite composite substrate with a diameter of 320 mm for use in semiconductor probe cards was successfully fabricated. Additionally, the changes in sheet resistance and flexural strength were measured to determine the effect of the environmental tests on the large-area substrate such as damp heat and thermal shock. The results indicated that the mullite-cordierite composite substrate was extremely reliable and durable.

Synthesis, Phase Transformations and Strength Properties of Nanostructured (1 - x)ZrO2 - xCeO2 Composite Ceramics.

The aim of this work is to study the properties of nanostructured (1 - x)ZrO2 - xCeO2 composite ceramics, depending on the content of oxide components, as well as to establish the relationship between the phase composition of ceramics and strength properties. The choice of (1- x)ZrO2 - xCeO2 composite ceramics as objects of study is due to the great prospects for using them as the basis for inert matrix materials for nuclear dispersed fuel, which can replace traditional uranium fuel in high-temperature nuclear reactors. Using X-ray diffraction, it was found that the variation of the oxide components leads to phase transformations of the Monoclinic-ZrO2 → Monoclinic - Zr0.98Ce0.02O2/Tetragonal - ZrO2 → Tetragonal - Zr0.85Ce0.15O2 → Tetragonal - ZrCeO4/Ce0.1Zr0.9O2 type. As a result of mechanical tests, it was found that the formation of tetragonal phases in the structure of ceramics leads to strengthening of ceramics and an increase in crack resistance, which is due not only to an increase in the crystallinity degree, but also to the effect of dislocation hardening associated with a decrease in grain size. It has been established that a change in the phase composition due to phase transformations and displacement of the ZrO2 phase from the ceramic structure with its transformation into the phase of partial replacement of Zr0.85Ce0.15O2 or Ce0.1Zr0.9O2 leads to the strengthening of ceramics by more than 3.5-4 times. The results of resistance to crack formation under single compression showed that the formation of the ZrCeO4 phase in the structure of ceramics leads to an increase in the resistance of ceramics to cracking by more than 2.5 times.

Electric-Field-Insensitive Temperature Stability of Strain in KNN Multilayer Composite Ceramics.

The prominent advances in both piezoelectricity and temperature stability of potassium sodium niobate-based ceramics make this material system the most potential alternative to toxic lead-based families. However, previous studies have shown that the excellent temperature stability of the electrostrain can be obtained only under a high electric field. This issue can be well solved by our new proposed strategy of constructing multilayer composite ceramics, where an extremely low electric-field-dependent temperature stability of the strain can be achieved, far outperforming the results reported so far. The synergistic contributions from stacking components with different strain responses under different temperatures and electric field strengths realize the dynamic balance of electrostrain of the multilayer composite ceramics, which is also revealed by phase-field simulation. This work provides new ideas for the artificial structural design for the development of stable and reliable high-performance piezo/ferroelectric ceramics.

Study of Radiation-Induced Damage Processes in CeZrO4-YZrO3 Ceramics Caused by Helium Irradiation.

Composite oxide ceramics CeZrO4-YZrO3 obtained by mechanochemical synthesis were chosen as objects of study. The most dangerous type of radiation defect in structural materials is associated with helium accumulation in the structure of the near-surface layer. This can lead to the destruction and swelling of the material, resulting in a decrease in its strength and thermal characteristics. During the studies, it was found that the most significant structural changes (deformation of the crystal lattice, the magnitude of microdistortions of the crystal lattice) are observed with irradiation fluence above 5×1016 ion/cm2, while the nature of the changes is exponential. X-ray diffraction analysis found that the nature of the crystal structure deformation has a pronounced type of stretching due to the accumulation of implanted helium and its subsequent agglomeration. A comparative analysis with data on microdistortions of the crystal lattice and the values of microhardness and softening of ZrO2 and CeO2 showed that two-phase ceramics of the cubic type CeZrO4-YZrO3 are more resistant to radiation-induced degradation than single-phase ZrO2 and CeO2. Results of strength and thermophysical characteristics showed that the presence of two phases increases resistance to destruction and disorder, leading to a decrease in strength and thermal conductivity.

Crystal Growth and Design of Disk/Filament ZnO-Decorated 1D TiO2 Composite Ceramics for Photoexcited Device Applications.

Disk- and filament-like ZnO crystals were decorated on one-dimensional TiO2 nanostructures (TiO2-ZnO) through various integrated physical and chemical synthesis methods. The morphology of the ZnO crystals on TiO2 varied with the chemical synthesis method used. ZnO nanodisks decorated with TiO2 nanorods (TiO2-ZnO-C) were synthesized using the chemical bath deposition method, and ZnO filament-like crystals decorated with TiO2 nanorods (TiO2-ZnO-H) were synthesized through the hydrothermal method. Compared with the pristine TiO2 nanorods, the as-synthesized TiO2-ZnO composites exhibited enhanced photophysiochemical performance. Furthermore, because of their fast electron transportation and abundant surface active sites, the ZnO nanodisks in the TiO2-ZnO-C composite exhibited a higher photoactivity than those in the TiO2-ZnO-H composite. The morphology and crystal quality of the ZnO decoration layer were manipulated using different synthesis methods to realize disk- or filament-like ZnO-decorated TiO2 composites with various photoactive performance levels.

Solid-state NMR study on changes of phosphate and proton species in metal pyrophosphate composite (MP2 O7 -MO2 ) ceramics.

In order to investigate the effects of composite production and the role of the counter cation for metal phosphate conductors, changes in the solid-state nuclear magnetic resonance (NMR) spectra and magnetic relaxation times caused by the removal of volatile products and water washing were examined for various metal pyrophosphate (MP2 O7 -MO2 ; M = Sn, Si, Ti, and Zr). Acidic species could be detected by 1 H MAS NMR spectra for all the composites except ZrP2 O7 -ZrO2 that had the lowest conductivity. The 31 P DD-MAS NMR spectra for MP2 O7 -MO2 composites showed different signal patterns depending on the counter cations participating in the ion exchange as a result of different microstructures. Combinational analysis of 31 P DD-MAS and 31 P CP-MAS NMR spectra of the composites indicated that protonic bulk phosphates were observed at slightly lower fields than non-protonic bulk phosphates in all of the MP2 O7 -MO2 composites. After water washing, the acidic species and the protonic bulk phosphates of MP2 O7 -MO2 composites disappeared or were reduced to trace amounts. The T1 H values of the water-washed composites lengthened because of removal of orthophosphoric acid, although the T1 P values remained almost unchanged. The results of the solid-state NMR studies suggest that the protonic bulk phosphates of MP2 O7 -MO2 composites do not generally distribute in the bulk but exist in the interface between excess H3 PO4 and the bulk. Copyright © 2016 John Wiley & Sons, Ltd.

Lead-Free KNbO3:xZnO Composite Ceramics.

It is a tough issue to develop dense and water resistant KNbO3 ceramics due to high evaporation and hygroscopicity of K2O. Here, KNbO3:xZnO composite ceramics were used to successfully solve this problem, where ZnO particles were randomly distributed into a KNbO3 matrix. The addition of ZnO hardly affects the phase structure of KNbO3, and moreover, the enhancement of electrical properties, thermal stability, and aging characteristics was observed in KNbO3:xZnO composite ceramics. The composites possessed the maximum d33 of 120 ± 5 pC/N, which is superior to that of pure KNbO3 (d33 = 80 pC/N). More importantly, a strong water resistance and an aging-free characteristic were observed in KNbO3:0.4ZnO. This is the first time for KNbO3 ceramics to simultaneously improve electrical properties and resolve the water-absorbing properties. We believe that these composite ceramics are promising for practical applications.