The simulation for a high-efficiency millimeter wave microstrip antenna by low dielectric loss and wide temperature stable lithium-based microwave dielectric ceramics for LTCC applications

As a common flux agent, B₂O₃–CuO was introduced into Li₂TiO₃ system to reduce the sintering temperature for the requirements of LTCC applications. The optimal mass ratio of CuO to B₂O₃ was innovatively explored. When the mass ratio of CuO to B₂O₃ increased to 1.2:1.0, excellent microwave dielectric properties were obtained in LTMF&LTZN_0.892+CB_1.2 ceramic of ε_r = 13.23, Q × f = 62,749 GHz, τ_f = -2.48 ppm/°C and the sintering temperature was reduced from 1300 to 930 °C. In a wide temperature range, the sample still maintain high temperature stability of |τ_f| < 5 ppm/°C (-40–120 °C). Based on the LTMF&LTZN_0.892+CB_1.2 substrate, a millimeter wave microstrip antenna resonated at 30.12 GHz was designed with a considerably high radiation efficiency of 93.94% and a signal gain of 4.87 dB. Comprehensive microwave dielectric properties make LTMF&LTZN_0.892+CB_1.2 become a candidate material for LTCC applications.     

Binary polymer blend of ArPTU/PI with advanced comprehensive dielectric properties and ultra-high thermally stability

Flexible high-temperature polymeric dielectrics with advanced dielectric properties are urgently demanded in various applications. In this work, series of polymer blend films were prepared from aromatic polythiourea (ArPTU) and polyimide (PI). The experimental results revealed that the blend films were properly engineered to achieve higher breakdown strength, greater dielectric constant, and larger energy density than pure PI film. For instance, the optimum property was obtained from the blend film with 10 wt% ArPTU, exhibiting prominent dielectric properties (K = 4.52, E_b = 443 MV/m), enhanced energy density (4.00 J/cm³) as well as excellent heat resistance (T_g = 419°C). In addition, stable dielectric properties at broad temperature range from −50 to 250°C were also acquired. It is deduced that the good compatibility from ArPTU and PI with similar polarity are responsible for the improved properties. The superior comprehensive properties which combine the advantages of ArPTU and PI suggest the potential applications of ArPTU/PI blend film in high-temperature dielectric areas.     

Accessing effects of aliphatic dicarboxylic acid towards the physical and chemical changes in low temperature hydrothermally reduced graphene hydrogel

Journal of Porous Materials - To date, studies pertaining the usage&nbsp;of aliphatic dicarboxylic acids (DCAs) in&nbsp;the preparation of&nbsp;graphene-based&nbsp;hydrogels are...     

The two-tier tissue reinforcing and toughening mechanism caused by dual-granularity Si powders ingredient,and dielectric properties in reaction bonded and sintered reaction bonded Si3N4 porous ceramics

With the rapid development of hypersonic vehicles and broadband wave-transparent radome, Si₃N₄ porous ceramics (Si₃N₄-PC) have attracted attention due to their excellent intrinsic properties of Si₃N₄ and high porosity. However, its high porosity results in low strength and toughness, which are fundamental properties for radome. Reaction bonded (RB) Si₃N₄-PC has advantages of dielectric properties and cost over general phase transformation sintering (PTS) and sintered reaction bonded (SRB) Si₃N₄-PC while it has been neglected in recent years. In this study, RB and SRB Si₃N₄-PC prepared by non-aqueous gelcasting and the influence of Si powders ingredient on their properties are discussed in order to illuminate the potential of RB Si₃N₄-PC in wave-transparent materials. The results show that RB Si₃N₄-PC with dual-granularity ingredients of 5 μm & 45 μm produces a two-tier tissue of framework of coarse whiskers enhanced by a network of tiny whiskers. SRB Si₃N₄-PC evolves into a two-tier tissue of framework of columnar and rod-like grains joining together to brace each other. The two-tier microscopic tissue strongly reinforces and toughens the structure and results in higher σ_F and γ_wof. As a result, the RB and SRB Si₃N₄-PC of dual-granularity of 5 μm & 45 μm obtain the maximum σ_F of 109.94 MPa and 119.56 MPa as well as maximum γ_wof of 990.74 J m^-2 and 1167.88 J m^-2, respectively. Furthermore, the ε′ and tanδ of RB and SRB Si₃N₄-PC of dual-granularity of 5 μm & 45 μm are about 4.20 and 4.52 as well as 7.01 × 10^-3 and 22.90 × 10^-3, respectively. It is concluded that RB Si₃N₄-PC of dual-granularity has good mechanical and dielectric properties, which are favorable for radome.     

Computational and Theoretical Analysis of the Effect of an Aluminum Borate Oxide Film on the Ignition Conditions of Single Aluminum Diboride Particles

Combustion, Explosion, and Shock Waves - Thermogravimetric analysis data were used to determine the activation energy E&nbsp;=&nbsp;205.9&nbsp;kJ/mol, the pre-exponential factor...     

Lattice vibrational characteristics and structure-property relationships of SrWO4- x wt.% LiF (x = 0.5–3.0) microwave dielectric ceramics

SrWO₄-x wt.% LiF (SWL, x = 0.5–3.0) ceramics were obtained by a conventional solid-state reaction at 850 °C. The effects of the LiF additive on the phase evolution and dielectric response mechanism of the SWL ceramics were comprehensively studied. The densification temperature of the SWL ceramics was successfully reduced to 850 °C by adding LiF as the sintering additive. The X-ray diffraction data indicated that the SWL ceramics were composite ceramics. The lattice vibrational properties were investigated in depth by Raman scattering and Fourier transform infrared reflectance. The intrinsic dielectric property values fitted by the four-parameter semi-quantum model agreed well with the measured dielectric property values and the theoretical values obtained from the Clausius-Mossotti & damping equations. Moreover, structure-property relationships were obtained by studying the lattice vibrational modes of the SWL ceramics. The SWL ceramic achieved the optimum dielectric property at x = 2.0, with ε_r = 9.03 and Q × f = 47,830 GHz.     

Enhanced dielectric constant and breakdown strength in dielectric composites using TiO2@HfO2 nanowires with gradient dielectric constant

To investigate the influence of modification of ceramic fillers on the dielectric properties of polymer-based composites, TiO₂ and core-shell structured TiO₂@HfO₂ nanowires were synthesized, and investigated in this study. TiO₂ nanowires/polyvinylidene fluoride (PVDF) and TiO₂@HfO₂ nanowires/PVDF nanocomposites were prepared using the solution casting method. The experimental results showed that the TiO₂@HfO₂ nanowires/PVDF composites had improved dielectric properties compared with that of the TiO₂ nanowires/PVDF composites. Owing to the enhanced interfacial polarisation by the multilevel interface, the composites with 10 wt % TiO₂@HfO₂ nanowires achieved the highest permittivity of 12.56 at 1 kHz, which was enhanced by ～72% compared to the PVDF matrix. The electric field was evenly distributed by building the fillers with a gradient dielectric constant. The characteristic breakdown strength of the composite with 5 wt % TiO₂@HfO₂ reached 377.76 kV/mm, compared with that of 334.37 kV/mm for the composite with 5 wt % TiO₂ nanowires. This study initiated a novel strategy for preparing dielectrics with high dielectric constant and improved breakdown strength.     

Temperature-dependent dielectric and Raman spectra and microwave dielectric properties of gehlenite-type Ca2Al2SiO7 ceramics

Gehlenite-type Ca₂Al₂SiO₇ ceramics were prepared by the conventional solid-state reaction. Two anomalies were found in the plot of dielectric constant vs temperature, which were associated with space charge polarization. Pure phase crystal structure and no phase transition were observed in the temperature-dependent X-ray diffraction (XRD) patterns and Raman spectra from room temperature (RT) to 900°C. There was relevant relation between Q × f and τ_ƒ with the stretching vibrations of Ca-O bond and O-Ca-O bending in CaO₈ polyhedron. Excellent microwave dielectric properties (ε_r = 8.86, Q × f = 22 457 GHz, and τ_f = −51.06 ppm/°C) were obtained for Ca₂Al₂SiO₇ sintered at 1440°C in air, which had the potential application to use in microwave and millimeter-wave devices such as capacitors and substrates.     

Nonlinear dielectric response and positive dielectric tunability in antipolar CaTiSiO5

Dielectric tunability has been extensively investigated in ferroelectric materials, which exhibit a negative tunability of dielectric permittivity in an external electric field. In contrast, positive tunability is rare and has been reported only in a few antiferroelectric materials. We present positive (and negative) tunability in the titanite, CaTiSiO₅. The dielectric property of CaTiSiO₅ was measured up to an extraordinarily high electric field of 40 MV/m. A nonlinear polarization field loop with no hysteresis was obtained. The dielectric permittivity of ε_r ~ 25 increases up to ε_r ~ 40 at 20 MV/m and room temperature. Although titanite has an antipolar structure and is expected to be “antiferroelectric,” its dielectric response in high electric fields up to ~40 MV/m differs from that of conventional antiferroelectrics. We demonstrate that the phase-transition temperature and dielectric tunability could be modulated through the chemical substitution of Ca_1−xLa_xTiSi_1−xAl_xO₅, in which the destabilization of the long-range antipolar order is revealed by transmission electron microscopy analysis. These results indicate that the observed dielectric response to an electric field may originate from the unique features of the antipolar and domain structures in CaTiSiO₅.     

Influence of the anodization conditions and chemical treatment on the formation of alumina membranes with defined pore diameters

Journal of Porous Materials - Porous anodic aluminum oxide membranes were fabricated via two-step anodization of aluminum in 0.3&nbsp;M H2C2O4, 0.3&nbsp;M H2SO4 and 0.17&nbsp;M H3PO4...     

Enhanced dielectric stability and coercivity of band gap tuned Ba–Al Co-doped bismuth ferrite: An experimental and DFT+U investigation

Employing a modified sol-gel combustion technique, BiFeO₃ and Bi_0.85Ba_0.15Fe_1-xAl_xO₃ (x = 0, 0.025 and 0.050) nanoparticles were synthesized, and the effects of these dopants were investigated analyzing the experimental findings and adopting the DFT + U approach. Rietveld analysis of XRD revealed a deformed perovskite rhombohedral structure for all the Ba–Al co-doped nanoparticles, which was also validated by theoretical study. Suppression of the low-frequency dispersion promoting outstanding dielectric stability was observed due to Al³⁺ doping, and the least dielectric constant along with the highest resistivity was reported for BBFAO-2.5. The band gap of co-doped nanoparticles (x = 2.5 & 5) decreased to 1.95 eV and 1.98 eV, respectively, compared to 2.06 eV, and 2.09 eV for BBFAO-0 and pure BFO. The average particle size of all the co-doped nanoparticles was below the repeat length of the cycloid structure, which is favorable for improved magnetic properties. Remanent magnetization, coercivity, and squareness increased as the motion of domain walls is inhibited by the smaller-sized nanoparticles. DFT + U study further endorses the findings of the experimental study. Analysis of DOS revealed the emergence of shallow impurity states around VBM due to doping, which can operate as active trap centers and impede carrier recombination. Thus, it can be inferred that, Ba–Al co-doped nanoceramics with intriguing dielectric, optical, and magnetic properties are a promising candidate for high-frequency microwave devices, magnetic memory and storage devices, near UV-detector, and solar photocatalysis applications.     

Various Types of Combustion of a Ti + C Granular Mixture with a Different Content of the Gasifying Additive

Combustion, Explosion, and Shock Waves - This paper touches upon the effect of a polyvinyl butyral content (0–2.3%) on&nbsp;the&nbsp;combustion of a Ti&nbsp;+&nbsp;C granular...     

Effect of Si3N4 Ceramic Particulates on Mechanical,Thermal, Thermo-Mechanical and Sliding Wear Performance of AA2024 Alloy Composites

Silicon - In this investigation, hybrid AA2024 – Si3N4 (0–6&nbsp;wt.% @ 2%) – SiC (2&nbsp;wt.%) – graphite (2&nbsp;wt.%) alloy composites have been fabricated as...     

Synthesis of Silver Nanoparticles Using Syzygium malaccense Fruit Extract and Evaluation of Their Catalytic Activity and Antibacterial Properties

Journal of Inorganic and Organometallic Polymers and Materials - In this study, green synthesis of silver nanoparticles, an aqueous&nbsp;Syzygium malaccense&nbsp;fruit extract, was employed...     

CaTiO3 linear dielectric ceramics with greatly enhanced dielectric strength and energy storage density

CaTiO₃ is a typical linear dielectric material with high dielectric constant, low dielectric loss, and high resistivity, which is expected as a promising candidate for the high energy storage density applications. In the previous work, an energy density of 1.5 J/cm³ was obtained in CaTiO₃ ceramics, where the dielectric strength was only 435 kV/cm. In fact, the intrinsic dielectric strength of CaTiO₃ is predicted as high as 4.2 MV/cm. Therefore, it should be a challenge issue to enhance the dielectric strength and energy storage density of CaTiO₃ ceramics by optimizing the microstructures. In the present work, dense CaTiO₃ ceramics with fine and uniform microstructures are prepared by spark plasma sintering, and the greatly enhanced dielectric strength (910 kV/cm) and energy storage density (6.9 J/cm³) are obtained. This can be ascribed to the improved resistivity and thermal conductivity, associated with the fine and uniform microstructures. The different post‐breakdown features of CaTiO₃ ceramics prepared by different process well interpret why the enhanced dielectric strength is achieved in the SPS sample. The energy storage density can be further improved to 11.8 J/cm³ by introducing the amorphous alumina thin films as the charge blocking layer, where the dielectric strength is 1188 kV/cm.     

Shock Compression of Titanium Hydride and Titanium,Tantalum, and Zirconium Deuterides

Combustion, Explosion, and Shock Waves - This paper presents the results of an experimental study of shock compression of&nbsp;titanium hydride&nbsp;(TiH2) and the deuterides of...     

Low dielectric loss LNT/PEEK composites with high mechanical strength and dielectric thermal stability

(3-Aminopropyl)triethoxysilane treated La_(2−x)/3Na_0.06TiO₃ (x = 0.06) (LNT) microparticles filled polyetheretherketone (PEEK) composites were prepared using hot pressing process. The effects of variation of LNT ceramic filling fraction on dielectric properties, water absorption, thermal stability and mechanical strength were investigated. All composites demonstrate low water absorption (less than 0.4%) when the ceramic filling fraction is lower than 0.6V_f. The obtained composites exhibited dielectric permittivities varying from ~4 to ~22 as the ceramic fillers increased from 0.1 to 0.8V_f and low losses (~10⁻⁴ @1 MHz, 3~5 × 10⁻³ at the frequencies of microwave (10 GHz) and millimeter wave (29-50 GHz), respectively). The mechanical strength, dimensional and dielectric thermal stability of the composite are remarkably improved by the addition of LNT ceramic fillers. A composite with near zero temperature coefficients of dielectric permittivity or resonant frequency and flexural strength of ~140 MPa could be obtained. The out-of-plane coefficient of thermal expansion (CTE) could be reduced to ~20 ppm/°C as the ceramic filler loading reached 0.7V_f.     

Thermo-mechanical and high-temperature dielectric properties of cordierite-mullite-alumina ceramics

Heterogeneous ceramics made of cordierite (55–56 wt%), mullite (22–33 wt%) and alumina (23–11 wt%) were prepared by sintering non-standard raw materials containing corundum, talc, α-quartz, K-feldspar, kaolinite and mullite with small amounts of calcite, cristobalite and glass phases. The green specimens prepared by PVA assisted dry-pressing were sintered within the temperature range of 950–1500 °C for different dwelling times (2–8 h). The effects of sintering schedule on crystalline phase assemblage and thermomechanical properties were investigated. The sintered ceramics exhibited low coefficients of thermal expansion (CTE) (3.2–4.2×10⁻⁶ °C⁻¹), high flexural strength (90−120 MPa and high Young modulus (100 GPa). The specimens sintered at 1250 °C exhibited the best thermal shock resistance (∆T~350 °C). The thermal expansion coefficients and thermal shock resistance were studied using Schapery model, the modelling results implying the occurrence of non-negligible mechanical interactions between the phases in bulk. The dielectric properties characterized from room to high temperature (RT– HT, up to 600 °C) revealed: (i) noticeable effects of sintering schedule on dielectric constant (5–10) and dielectric loss factor (~0.02–0.04); (ii) stable dielectric properties until the failure of the electrode material. The thermomechanical properties coupled with desirable dielectric properties make the materials suitable for high density integrated circuitry or high temperature low-dielectric materials engineering.     

Synthesis of a low-firing BaSi2O5 microwave dielectric ceramics with low dielectric constant

In this study, BaSi₂O₅ ceramics with an orthorhombic structure were synthesized by using a traditional solid-state method at a low temperature by doping with Li₂O–B₂O₃–CaO–CuO (LBCC) glass. The phase composition, mechanism of low-temperature sintering, microwave dielectric properties, and changes in the mesophase during the heating of low-temperature sintered BaSi₂O₅ ceramics were examined by performing an X-ray diffraction analysis. A compact matrix of BaSi₂O₅ can be wetted by the liquid phase of the formed LBCC glass. Therefore, LBCC glass with different doping percentages can effectively reduce the sintering temperature of BaSi₂O₅. The microwave dielectric properties of BaSi₂O₅ ceramics sintered at 900 °C at 4 wt% of LBCC glass were determined: ε_r = 7.32, Q × f = 19,002 GHz, and τ_f = ?35.8 ppm/°C. The chemical compatibility of the samples with Ag was studied at 4 wt% doping with LBCC glass, and the samples were fired for 4 h at 900 °C.