Microwave dielectric ceramics in (Mg1/3Sb2/3)O2-ZrO2-TiO2 system with near zero τf and low loss in a very wide range

A series of (ZrTi)_1-x(Mg_1/3Sb_2/3)_2xO₄ (0.04?≤?x?≤?0.36) ceramics were successfully synthesized through the conventional solid-state processing. Appropriate content of CuO was added as sintering aids to promote the density of ceramics. The XRD analysis revealed that the main crystalline phase of ceramics sintered at optimal temperature belonged to α-PbO₂-type structure. Raman spectroscopy and far infrared reflectivity (FIR) spectra were employed to study the phonon modes of ceramics, which explained the relationship between microwave dielectric properties and structure. It is interesting that the τ_f are near-zero (+6.6 ~ ?4.6?ppm/°C) and meanwhile the Q×f are relatively high (29,000–41,800?GHz) for samples with x in a very wide range of 0.10–0.36. In this range, their dielectric constants (ε_r) can be adjustable from 35.4 to 24.4. The results demonstrated this ceramic system is a potential candidate for microwave dielectric applications requiring an adjustable dielectric constants and near zero τ_f.     

Preparation of nanocrystalline MoSi2 with enhanced lithium storage by sol–gel and carbonthermal reduction method

Herein nanocrystalline MoSi₂ with enhanced lithium storage was successfully synthesized via a sol-gel and carbonthermal reduction method. Reduction of the gel mixture of Mo precursor and Si precursor by carbon at a desired temperature resulted in the formation of MoSi₂ nanoparticles. The gel mixture was obtained through the hydrolysis of TEOS and ammonium molybdate and the polymerization of hydrolysis products of TEOS. The reducing agent carbon was produced via decarburition of sucrose's hydrolysis products, which have been wrapped in the gel during its formation process. Addition of HCl to the mixed solution controlled the hydrolysis and polymerization rate, and enabled the formation of a gel mixture with homogeneously distributed hydrolysis products of ammonium molybdate, TEOS and sucrose. This achievement likely generates a novel route to synthesize non-oxide compounds such as silicide, carbide through the sol–gel and carbonthermal reduction process. In addition, the as-received MoSi₂ nanoparticles showed considerable activities in the reversible lithiation and delithiation process. When using as an anode for Li-ion batteries, MoSi₂ nanoparticles delivered a specific capacity of 325 mAh g^?1 at C/12 and showed an increasing capacity with cycling.     

Structure,electrical and dielectric properties of Ca substituted BaTiO3 ceramics

BaTi_1-xCa_xO_3-x [BTC100x] ceramics were synthesized via solid-state reaction method. Effect of Ca substitution on the structure, electrical and dielectric properties of BTC100x ceramics was systematically investigated. Calcined BTC100x powders were in tetragonal phase when x?≤?0.01, whereas transformed to cubic at x?>?0.01. Additionally, the diffraction peak (200) shifted to lower angles with increasing x, indicating increased unit cell volume. Meanwhile, Ba_0.97Ca_0.03TiO₃ [BC3T] ceramic was prepared and studied, to compare with BaTi_0.97Ca_0.03O_2.97 (BTC3). It was found that pure BaTiO₃ [BT] and BC3T ceramics had the similar structural and dielectric properties, whereas BTC3 ceramic showed much difference，XRD patterns, Raman spectrum, impedance spectra and dielectric-temperature spectra provided strong evidence of Ca²⁺ substitution at Ti site in BT lattice. Finally, BTC100x ceramics were produced and dielectric properties were investigated. With increasing x, the Curie temperature decreased from 128?°C (BT) to 42?°C (BTC5).     

Enhanced high-temperature dielectric properties and microwave absorption of SiC nanofibers modified Si3N4 ceramics within the gigahertz range

Si₃N₄ ceramics modified with SiC nanofibers were prepared by gel casting aiming to enhance the dielectric and microwave absorption properties at temperatures ranging from 25?°C to 800?°C within X-band (8.2–12.4?GHz). The results indicate that the complex permittivity and dielectric loss are significantly increased with increased weight fraction of SiC nanofibers in the Si₃N₄ ceramics. Meanwhile, both complex permittivity and dielectric loss of SiC nanofibers modified Si₃N₄ ceramics are obviously temperature-dependent, and increase with the higher test temperatures. Increased charges mobility along conducting paths made of self-interconnected SiC nanofibers together with multi-scale net-shaped structure composed of SiC nanofibers, Si₃N₄ grains and micro-pores are the main reason for these enhancements in dielectric properties. Moreover, the calculated microwave absorption demonstrates that much enhanced microwave attenuation abilities can be achieved in the SiC nanofibers modified Si₃N₄ ceramics, and temperature has positive effects on the microwave absorption performance. The SiC nanofibers modified Si₃N₄ ceramics will be promising candidates as microwave absorbing materials for high-temperature applications.     

Effects of structural characteristics on microwave dielectric properties of low-loss (Zn1-xNix)ZrNbTaO8 ceramics

Low-loss (Zn_1-xNi_x)ZrNbTaO₈ (0.02?≤?x?≤?0.10) ceramics possessing single wolframite structure are initiatively synthesized by solid-state route. Based on the results of Rietveld refinement, complex chemical bond theory is used to establish the correlation between structural characteristics and microwave performance in this ceramic system. A small amount of Ni²⁺ (x?=?0.06) in A-site with the fixed substitution of Ta⁵⁺ in B-site can effectually raise the Q?×?f value of ZnZrNb₂O₈ ceramic, embodying a dense microstructure and high lattice energy. The dielectric constant and τ_f are mainly affected by bond ionicity and the average octahedral distortion. The (Zn_0.94Ni_0.06)ZrNbTaO₈ ceramic sample sintered at 1150?°C for 3?h exhibits an outstanding combination of microwave dielectric properties: ε_r =?27.88, Q?×?f?=?128,951?GHz, τ_f =?–39.9?ppm/°C. Thus, it is considered to be a candidate material for the communication device applications at high frequency.     

Quantitative analysis of toughening effect of crack deflection on Si3N4/Si2N2O composites through improved indentation method

In this study, Si₃N₄/Si₂N₂O composite ceramics prepared by hot pressing were used as an example, and the material fracture morphology and fracture mechanism were analyzed. Based on the formula of fracture toughness measured by an indentation method, a quantitative computation method was proposed to determine the toughened effect of ceramic materials resulting from the crack deflection by the second phase. The grain size and sintering density are increased with the increase of sintering temperature. The toughening effects resulting from the crack deflection is increased, and the main mode of fracture is transformed into the transgranular fracture. The Si₂N₂O grains can play a role in the toughening process because these grains can hinder the cracks extending along the radial direction. However, when the cracks extend in the axial direction, the toughening effect of Si₂N₂O grains is not obvious because of the internal stacking faults in the axial direction. The improved indentation method can quantitatively analyze the toughening effect of the second phase of composite ceramics, and the validity of this method are verified by comparing the fracture toughness of Si₃N4/Si₂N₂O and fine grained β- Si₃N₄ ceramics.     

Synergistic reinforcement of surface modification on improving the bonding of veneering ceramics to zirconia

Veneering ceramics should be strongly bonded to zirconia core in order to achieve successfully long-term clinical practice. Indeed, to pursue the high zirconia core–veneering ceramic bonding is still a concerned issue. In this regard, this study was to treat zirconia surface using a 3?wt% Si₃N₄ solution in 4?M NaOH and to investigate the effect of soaking time (5, 10, and 20 days) on the surface properties of zirconia and shear bond strength between zirconia and veneering ceramics. The residual veneering ceramics on zirconia surfaces and failure modes were also examined after fracture. The results showed that the phase composition of zirconia before and after surface modification was not changed. The elemental mapping and depth profiling consistently revealed the soaking-time-dependent Si content on the zirconia surface. The surface roughness of zirconia was significantly (P?<?0.05) increased with the increasing soaking time. When zirconia was treated for 10 days, the shear bond strength value of 27.4?MPa was significantly (P?<?0.05) higher than the control (18.6?MPa), associated with greater remaining amounts of veneering ceramics on the zirconia surface. The failure mode of the treated zirconia was almost the mixed failure. On the basis of the data, surface modification using Si₃N₄ in NaOH solution for zirconia core could be a simple and effective method for enhancing the veneering ceramic–zirconia bonding.     

AHP-based procedure for optimization of microwave-assisted blackberry sugar osmotic process

Due to the limitations of single index analysis, the merits and demerits of microwave-assisted permeation process cannot be comprehensively evaluated. However, the analytical hierarchy process (AHP) was applied to optimize the conditions of microwave-assisted diffusion of sugar in blackberry. In this paper, the results of a single-factor experiment were compared with blackberry preserved after the treatment of microwave. An orthogonal rotation test procedure was designed to calculate the composite score of preserved blackberry fruit using AHP, and to determine the optimum processing conditions based on sugar osmosis characteristics. The results showed that the sugar content, vitamin C (VC) content, and color difference of treated blackberry increased with the increase of microwave power, the residence time, and the resulting sugar concentration. The composite grade points are listed to show that the optimum results were as follows: power level, the residence time, and the sugar concentration were, respectively, 210?W, 12?min, and 35%.     

Power Generation from Coke Oven Gas Using Chemical Looping Combustion: Thermodynamic Simulation

A chemical looping combustion (CLC) combined cycle with coke oven gas as fuel and NiO/NiAl₂O₄ as an oxygen carrier is proposed. The system was simulated by Aspen Plus® and the oxygen carrier circulation ratio was calculated. The effects of key operational temperatures and different gas turbines on the system performance were investigated. Under optimized conditions, a high CO₂ capture efficiency could be achieved. To capture CO₂ thoroughly, the PG6561B gas turbine can be employed, allowing for nearly 100 % CO₂ capture efficiency.     

Effects of spark plasma sintering on ferroelectricity of 0.8Bi3.15Nd0.85Ti3O12-0.2CoFe2O4 composite ceramic

The 0.8Bi_3.15Nd_0.85Ti₃O₁₂ (BNdT)-0.2CoFe₂O₄ (CFO) composite multiferroic ceramics have been fabricated by spark plasma sintering (SPS) at 850?°C. The relative density of as-sintered SPS ceramic reaches 97.4 (±0.3)%. The composites are composed of pure BNdT and CFO phases without any preferred c-orientation. The a-orientation preference is more obvious perpendicular to the pressure direction. The average grain-sizes of BNdT and CFO are 163 and 146?nm, respectively. The BNdT phase has more grains below 100?nm (～20%). The super energy-dispersive X-ray analyses suggest no serious reaction between BNdT and CFO. The Raman spectrum verifies the nano-structure of the SPS ceramic via the broadening bands and peak shifts. The Curie temperature of the SPS ceramic declines to 560?°C with stabilized dielectric loss. The grain boundary resistance plays a dominant role on impedance above 700?°C. The remanent polarization approaches to 15.2?μC/cm² (300?kV/cm) with lower coercive fields (?89/+95?kV/cm).