Search

An increasing number of new dielectrics are being reported for the development of next-generation ceramic capacitors for power electronics used in clean energy technologies. Here, high discharge energy density (Wdis) ~6.1 J cm−3 with efficiency (η)~72% under a pulsed field (Emax) of 410 kV cm−1 is reported along with temperature stability up to 150 °C (Emax = 200 kV cm−1) for 0.5 K0.5Bi0.5TiO3-0.42BiFeO3-0.08Sm(Mg2/3Nb1/3)O3 (KBT-BF-SMN) bulk ceramics. The optimised composition is chemically heterogeneous but electrically homogenous, similar to several BiFeO3-based dielectrics reported previously and adding to the growing body of evidence that electrical (measured at weak-field) not chemical homogeneity is the best guide to increased Emax and enhanced energy density. KBT-BF-SMN ceramics are therefore considered as promising candidates for pulsed power and power electronics applications.

5G and forthcoming 6G communication systems require dielectric ceramics with low relative permittivity (εr) and near-zero temperature coefficient of resonant frequency (τf) for the lower part of the microwave (MW) band and at sub-Terahertz. Mg2Al4Si5O18 (MAS) ceramics are promising candidates due to their low εr (~ 6) and high-quality factor (Q×f > 40,000 GHz) but have a large τf. In this study, 5.5 wt% TiO2 (MAS-T5.5) was used to adjust τf of MAS to −2.8 ppm/℃ whilst retaining low εr (5.24) and good Q×f (33,400 GHz), properties consistent with those obtained by infrared reflectance. A demonstrator microstrip patch antenna with gain 4.92 dBi and 76.3% efficiency was fabricated from MAS-T5.5.

High temperature power electronics is an advancing area of research, especially given the widespread availability of wide bandgap semiconductors. In this work a suitability analysis of (1−x)BiScO3–xPbTiO3 (BSPT) as a material for high temperature piezoelectric transformers (PTs) is presented and compared to state-of-the-art Pb(Zr1−xTix)O3 (PZT). Initially, both materials will be compared using a variety of metrics to prove the suitability of BSPT as a material for constructing PTs specifically for high temperature applications. PTs based on the two materials are designed and fabricated, and then subsequently characterised using impedance measurements. Finally, half-bridge resonant inverters are constructed using both PTs and the performance of each inverter measured over a range of ambient temperatures. The result of these experiments confirm the suitability of BSPT for PT applications and show the excellent performance of BSPT PTs at temperatures in excess of that possible with PZT based PTs.

Electric field and current applied to an unsintered ceramic body are known to promote low temperature and extremely fast densification, in a process referred to as FLASH sintering. Under the current urgency of the green transition of manufacturing processes, FLASH sintering is a very promising technology for materials industry. Suitable FLASH conditions result in dense ceramics but many issues associated with the effect of electric field and current on local chemistry, structure, and microstructure remain to be understood. We have used FLASH sintering to produce K0.5Na0.5NbO3 (KNN), a lead-free compound suitable for piezoelectric applications. Using a combined X-ray diffraction and Raman spectroscopy study, here we show for the first time that, although the FLASH process may produce homogeneous ceramics with negligible concentration of secondary phase, macroscopic core-localized stresses remain which have significant consequences on the final properties of the sintered material. In addition, the internal stress state and its dependence on the local temperature during FLASH sintering are established by Finite Element Modelling (FEM). The identification of the fine structure of FLASH sintered materials is critical for understanding the unique properties developed under this sintering process and for its development as an alternative low thermal budget sintering technology.

Composite polymer/ceramic filaments for material extrusion-based fused filament fabrication additive manufacturing, using barium strontium titanium oxide (BST) ceramics and acrylonitrile butadiene styrene (ABS) thermoplastics were produced; their dielectric and physical properties have been characterised for the first time. The dielectric properties, relative permittivity (εr), quality factor (Q×f) and dielectric loss (tanδ) were measured as a function of ceramic solid loading (%) at 5 GHz for 3D printed samples. A relative permittivity εr = 6.05, Q×f = 10,433 GHz and dielectric loss tanδ = 0.007 were obtained for a BST/ABS ceramic polymer composite, with 50 wt% (15 vol%) solid loading. The composite materials exhibit reduced dielectric losses compared to standard laminates currently used in the radiofrequency (RF) and telecommunications industry. Based on polymer/ceramic composite filament, a prototype microstrip patch 5 G antenna and a hemispherical dielectric lens were designed and manufactured. Through testing, it shows good antenna performance with a centre frequency of f0 = 3.78 GHz and a (−10 dB) bandwidth of 90.6 MHz. The dielectric lens increased the antenna gain by 3.86 dBi.

Low levels of impurities can have a dramatic influence on the electrical properties of metal oxides. Here, we use a combination of impedance spectroscopy and Seebeck coefficient measurements to show the defect chemistry and p-type conduction mechanism in undoped LaFeO3(LF) ceramics is significantly influenced by impurities in Fe2O3 reagents. A low but significant concentration of impurities associated with 99.9% Fe2O3 reagent is sufficient to compete against the intrinsic Schottky disorder responsible for the p-type behaviour normally observed for undoped LF ceramics. This produces electrically heterogeneous grain (bulk) responses that are strongly dependent on the processing conditions. To achieve electrically homogeneous bulk responses requires adjustment of the oxygen content by annealing in a reducing atmosphere (n-type) or via doping with a significant level of acceptors (mixed ionic/ p-type electronic). These results illustrate the importance of using high purity reagents when studying the electrical properties of ferrite-based perovskites.

Microwave (MW) dielectric ceramics are used in numerous electronic components for modern wireless communication systems, including antennas, resonators, capacitors and filters. However, to date, MW ceramics are manufactured by an energy-intensive, conventional high-temperature (> 1000 °C) sintering technology and thus cannot be co-sintered with low melting point and base electrodes (Ag, Al, etc.

As product development becomes increasingly complex, the demand for the earth's mineral ores increases and with it, the challenge to achieve global “sustainability”. Chemical elements are the building blocks of natural resources which are sourced from across the planet to manufacture globally traded goods. While global technological, social and economic progress accelerates, evaluating the sustainability of these building blocks remains a challenge. Numerous methodologies to evaluate sustainability exist but most rely on high levels of data collection. In this paper, a methodology is presented within a multi-criteria decision analysis and composite indicator framework with the aim of rapidly and comprehensively estimating the sustainability of a chemical element . The framework is based on triple bottom line principles; the environment, economy and society, to measure the sustainability of 59 chemical elements. The output, the chemical element sustainability index (CESI), is a single value supported by the aggregation of the Human Development Index, Global Warming Potential, and National Economic Importance indicators, derived through a rigorous and systematic selection process. Recycling rate is employed within the framework as a control variable given its importance as a sustainability strategy. The results show that the greater the Human Development Index, National Economic Importance and Recycling Rate, and the lower the Global Warming Potential, the more sustainable the chemical element is, and vice-versa. The CESI was validated using three representative piezoelectric materials as a case study. The framework presented is useful for product designers, policy makers and educational bodies, to support decision making towards sustainable production and consumption.

La(Nb0.9V0.1)O4 has been shown by in-situ Raman spectra and X-ray diffraction to undergo a ferroelastic phase transition from monoclinic fergusonite to tetragonal scheelite (TM-T) at 350 oC, accompanied by 3.55% spontaneous strain and an abrupt change in thermal expansion coefficient (αL) from +15.5 ppm/oC to +11.4 ppm/oC. Assuming a linear relation between polarizability and temperature, an anomalous decrease in relative permittivity (εr) at TM-T is predicted from the Clausius–Mosotti relation and Shannon’s additive rule. Such an anomalous decrease in εr at a phase transition has not previously been observed in ferroic and linear dielectrics and may aid in the design of novel microwave dielectric composites.

Ultra-low sintering temperature silver molybdenum oxide (Ag2Mo2O7) ceramics have been printed using direct ink writing (a material extrusion additive manufacturing process) for the first time. An optimum densification conditions of 460 °C / 2 h was determined, resulting in relative permittivity, εr = 13.45, dielectric loss, tanδ = 0.0005, microwave quality factor, Q × f = 17,056 GHz and the temperature coefficient of resonant frequency τf = −121 ppm/°C. The results were comparable to the dielectric properties of conventionally fabricated ceramics. A series of metal/ceramic antenna designs were produced via dual-printing and co-firing, to demonstrate the potential of Ag2Mo2O7, to be used as a co-firable dielectric material for functional integrated circuits and/or microwave RF devices through multi-material direct ink writing.

Dense (1-x)wt%CaSnSiO5-xwt%K2MoO4 (CSSO-KMO) composite ceramics were fabricated by the cold sintering process at 180 °C under 400 MPa for 60 min. X-ray diffraction, Energy dispersive X-ray and Raman spectroscopy confirmed that CSSO and KMO coexisted without intermediate phases. As KMO weight fraction increased, relative permittivity (εr) and temperature coefficient of resonant frequency (τf) decreased and the microwave quality factor (Q×f, where f is resonant frequency) increased. Near-zero τf (-0.5 ppm/°C) was obtained for 65 wt%CSSO-35 wt%KMO with εr ∼ 9.2 and Q×f ∼ 6240 GHz. No chemical reaction between ceramic composites and silver was observed, demonstrating potential for cofiring with Ag-paste. A prototype antenna was fabricated from 65 wt%CSSO-35 wt%KMO composite ceramic with a bandwidth of 144 MHz @ -10 dB, a gain of 5.7 dBi and a total efficiency of 88.4 % at 5.2 GHz, suitable for 5 G mobile communication systems.

Low sintering temperature, ultra-low loss microwave ceramics are envisaged as future dielectrics for fabricating low temperature co-fired ceramic (LTCC) components for 5G applications. Low sintering temperature bismuth molybdate β-Bi2Mo2O9 ceramic powders have been synthesised using a solid-state reaction method. Their additive manufacture and resulting microwave properties are reported for the first time. Optimum densification occurred for 3D printed samples sintered 4 hours at 670°C which resulted in a maximum relative density (ρr) of 92%, relative permittivity (εr) of 34, dielectric loss (tanδ) of 0.0007, giving a microwave quality factor (Qxf) of 10,050 GHz, properties attractive for LTCC applications.

Cold sintered, Li2MoO4-based ceramics have recently been touted as candidates for electronic packaging and low temperature co-fired ceramic (LTCC) technology but MoO3 is an expensive and endangered raw material, not suited for large scale commercialization. Here, we present cold sintered temperature-stable composites based on LiMgPO4 (LMP) in which the Mo (and Li) concentration has been reduced, thereby significantly decreasing raw material costs. Optimum compositions, 0.5LMP-0.1CaTiO3-0.4K2MoO4 (LMP-CTO-KMO), achieved 97% density at

Potassium sodium niobate, K0.5Na0.5NbO3 (KNN) is a lead‐free piezoelectric with the potential to replace lead zirconate titanate (PZT) in electromechanical applications. Due to its cuboid particle morphology and volatile elements, monophasic and dense ceramics are difficult to obtain via conventional sintering. In this work, isothermal FLASH sintering produced uniformly densified KNN ceramics at 900 °C, 200 °C lower than conventional sintering. Specific surface area (SSA) analysis of pre‐FLASH ceramics revealed that a 30 min isothermal hold at 900 °C, before the application of electric field, increased the contact area between particles and was crucial to promote uniform densification. Finite element modelling (FEM) revealed why density is more uniform when using isothermal heating compared with a constant heating rate, commonly used in FLASH sintering. These results extend our understanding of FLASH sintering and illustrate its relevance for the development of lead‐free piezoelectrics.

Ceramic dielectrics are reported with superior energy storage performance for applications, such as power electronics in electrical vehicles. A recoverable energy density (Wrec) of ∼4.55 J cm–3 with η ∼ 90% is achieved in lead-free relaxor BaTiO3-0.06Bi2/3(Mg1/3Nb2/3)O3 ceramics at ∼520 kV cm–1. These ceramics may be co-fired with Ag/Pd, which constitutes a major step forward toward their potential use in the fabrication of commercial multilayer ceramic capacitors. Compared to stoichiometric Bi(Mg2/3Nb1/3)O3-doped BaTiO3 (BT), A-site deficient Bi2/3(Mg1/3Nb2/3)O3 reduces the electrical heterogeneity of BT. Bulk conductivity differs from the grain boundary only by 1 order of magnitude which, coupled with a smaller volume fraction of conducting cores due to enhanced diffusion of the dopant via A-site vacancies in the A-site sublattice, results in higher breakdown strength under an electric field. This strategy can be employed to develop new dielectrics with improved energy storage performance.

Barium strontium titanate (BSTO) thin films were deposited on Pt(111) by high throughput evaporative physical vapor deposition and then annealed at 650 °C for 30 min under N2 atmosphere. Using advanced transmission electron microscopy, energy-dispersive x-ray spectroscopy and electron energy-loss spectroscopy, we directly show that not only does N substitute for O in the BSTO lattice but that it also compensates for Ti3+ ions, suppressing conductivity, thereby reducing dielectric loss and enhancing dielectric tunability. However, this effect is negated near the film edge where we speculate that exposed Pt acts as a reservoir of adsorbed/absorbed O and alters the local N2 concentration during annealing.

Bi2Mo2O9-K2MoO4 (BMO-KMO) composite ceramics with >95% theoretical density were densified by cold sintering at 150 °C. XRD, Raman, back-scattered SEM and EDX spectroscopy indicated that the BMO and KMO phases coexisted in all composites without inter-diffusion and secondary phases. Temperature coefficient of resonant frequency with near-zero value ∼ -1 ppm/°C was acheived for BMO-10%KMO with pemittivity ∼ 31 and quality factor ∼ 3,000 GHz. Cold-sintered composite ceramics were directly pressed/integrated onto a printed circuit board (PCB) using the Cu metallisation as a ground plane for the design and fabrication of a circularly polarized microstrip patch antenna suitable for satellite navigation systems which achieved efficiencies 87% at 1561 MHz (BeiDou) and 88% at 1575 MHz (GPS/Galileo). The low cost, low energy integration of temperature stable, cold sintered ceramics directly onto a PCB represents a step change in substrate fabrication technology for RF devices.

Controlling the sintering and microstructure of lead-free potassium sodium niobate ((K1-xNax)NbO3, KNN) ceramics is of primary importance to optimize its piezoelectric / ferroelectric properties. However, sintering dense and monophasic KNN remains a challenge. Here, we prepare KNN ceramics using spark plasma texturing (SPT), a modified spark plasma sintering (SPS) technique, in which uniaxial pressure is applied in an edge-free configuration, allowing ceramics to deform in the radial direction. Densification at low temperatures (1000 °C) and for short times (20 min) is achieved by SPT accompanied by constrained grain growth (average grain size = 1.4 μm), resulting in enhanced piezoelectric properties (d33 = 108 pC N−1 and g33 = 21.2 × 10−3 Vm N−1). In addition, and of relevance, SPT KNN ceramics reveal a more homogeneous electrical microstructure postulated to be related with a reduced diffusion and local segregation of defects, resulting in grain cores and shells with more similar capacitances and conductivities. Our work brings new practical understanding to sintering of KNN and demonstrates the potential of alternative densification strategies for improved lead-free dielectrics.

A novel Mg-2.0Zn-0.5Ca-0.4Mn alloy has been formulated and processed through melt spinning and hot extrusion to enhance its mechanical and degradation properties. Microstructural characterization of rapidly solidified alloy ribbons consolidated by extrusion revealed a fine and fully recrystallized microstructure with average size of 4 µm. The conventionally extruded alloy consisted of several course second-phase strips as coarse as 100 µm, while the extrusion-consolidated ribbons were devoid of any second phases larger than 100 nm. Rapid solidification followed by extrusion processing resulted in significantly randomized texture where the majority of the basal planes were tilted toward transverse and extrusion directions. Such a weak texture resulted in higher activity of basal planes and thereby considerably improved the fracture elongation from 4% to 19%, while retaining relatively high tensile strength of 294 MPa. In addition to high strength and ductility due to the reduced activity of deformation twining during compression, the extrusion-consolidated alloy ribbons showed lower yielding asymmetric ratio than that measured for the conventionally extruded alloy (1.25 versus 1.61). Electrochemical measurements and immersion tests indicated that application of rapid solidification followed by extrusion remarkably reduced the corrosion rate from 2.49 mm/year to 0.37 mm/year due to recrystallization completion and suppression of coarse second-phase formation.

FLASH sintering is a field-assisted technique that allows the densification of ceramics in a few seconds at temperatures significantly lower than those of conventional cycles. There is still discussion among the scientific community about the mechanism behind this sintering process, that has been typically attributed to Joule heating, defect creation and movement or liquid phase assisted sintering. Computational modelling can be a powerful tool in helping to explain and predict this process. Using potassium sodium niobate (KNN) as a case study, a lead-free piezoelectric, this work explores Finite Element Modelling to evaluate the dependence of Joule heating generation and temperature distribution as a function of the cubic particle orientation.

Existing equivalent circuit extraction techniques are inaccurate for piezoelectric transformers (PTs) with low-input capacitance or high damping. A new method is presented, offering improved accuracy in both damping resistance and resonant frequency extraction compared with state-of-the-art methods. Effectiveness is evaluated on two sample PTs, with the proposed method achieving up to 84% decrease in error compared with previous methods.

CaTiO3-K2MoO4 (CTO-KMO) dielectric composites were successfully cold-sintered at 150 °C for 30 min with a uniaxial pressure of 200 MPa. X-ray diffraction, Raman spectroscopy, back-scattered SEM and energy dispersive x-ray mapping confirmed the coexistence of CTO and KMO with no evidence of interdiffusion and parasitic phases either between the two ceramic end-members or with Ag internal electrodes. As KMO concentration increased, the temperature coefficient of resonant frequency (TCF) and relative pemittivity (εr) decreased but the microwave quality factor (Q × f) increased. A near-zero TCF composition was obtained for CTO-0.92KMO composites which exhibited εr ∼ 8.5 and Q × f ∼ 11,000 GHz. A microstrip patch antenna was designed and fabricated using cold sintered CTO-0.92KMO as a substrate (40×40×1.4 mm), which gave a s11 of −14.2 dB and a radiation efficiency of 62.0 % at 2.51 GHz.

Gehlenite‐typed Ca2Al2SiO7 ceramics were prepared by the conventional solid‐state reaction. Two anomalies were found in the plot of dielectric constant vs. temperature, which was associated to space charge polarization. Pure phase crystal structure and no phase transition were observed in the temperature dependent XRD patterns and Raman spectra from room temperature to 900°C. There was relevant relation between Q× f and τƒ with the stretching vibrations of Ca‐O bond and O‐Ca‐O bending in CaO8 polyhedron. Excellent microwave dielectric properties (εr = 8.86, Q × f = 22,457GHz, and τf = −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.

Additive manufacturing (AM) of co-fired low temperature ceramics offers a unique route for fabrication of novel 3D radio frequency (RF) and microwave communication components, embedded electronics and sensors. This paper describes the first-ever direct 3D printing of low temperature co-fired ceramics/floating electrode 3D structures. Slurry-based AM and selective laser burnout (SLB) were used to fabricate bulk dielectric, Bi2Mo2O9 (BMO, sintering temperature = 620–650°C, εr = 38) with silver (Ag) internal floating electrodes. A printable BMO slurry was developed and the SLB optimised to improve edge definition and burn out the binder without damaging the ceramic. The SLB increased the green strength needed for shape retention, produced crack-free parts and prevented Ag leaching into the ceramic during co-firing. The green parts were sintered after SLB in a conventional furnace at 645°C for 4 h and achieved 94.5% density, compressive strength of 4097 MPa, a relative permittivity (εr) of 33.8 and a loss tangent (tan δ) of 0.0004 (8 GHz) for BMO. The feasibility of using SLB followed by a post-printing sintering step to create BMO/Ag 3D structures was thus demonstrated.

AY2Si3O10 (A = Sr, Ba) trisilicate ceramics were synthesized by traditional high temperature solid state reaction method. X-ray diffraction patterns and Rietveld refinement revealed that AY2Si3O10 (A = Sr, Ba) ceramics belonged to triclinic and monoclinic crystal systems with Pī and P21/m space groups, respectively. The vibrational modes of [SiO4] tetrahedra, [YO6] octahedra and [(Sr/Ba)O8] polyhedra were analyzed by Raman spectroscopy. The infrared spectroscopy fitting analysis was used to determine intrinsic dielectric properties. Excellent microwave dielectric properties were measured for SrY2Si3O10 and BaY2Si3O10 with ɛr = 9.3, Qf = 64100 GHz, τf = −31 ppm/°C and ɛr = 9.5, Qf = 65600 GHz, τf = −28 ppm/°C, respectively. Both trisilicate ceramics are considered potential candidates for 5G and mm wave technology, provided τf can be further tuned.