Remarkably enhanced dielectric stability and energy storage properties in BNT—BST relaxor ceramics by A-site defect engineering for pulsed power applications

Lead-free bulk ceramics for advanced pulsed power capacitors show relatively low recoverable energy storage density (Wrec) especially at low electric field condition. To address this challenge, we propose an A-site defect engineering to optimize the electric polarization behavior by disrupting the orderly arrangement of A-site ions, in which $${\rm{B}}{{\rm{a}}_{0.105}}{\rm{N}}{{\rm{a}}_{0.325}}{\rm{S}}{{\rm{r}}_{0.245 - 1.5x}}{_{0.5x}}{\rm{B}}{{\rm{i}}_{0.325 + x}}{\rm{Ti}}{{\rm{O}}_3}$$ Ba 0.105 Na 0.325 Sr 0.245 − 1.5 x □ 0.5 x Bi 0.325 + x TiO 3 ($${\rm{BN}}{{\rm{S}}_{0.245 - 1.5x}}{_{0.5x}}{{\rm{B}}_{0.325 + x}}{\rm{T}}$$ BNS 0.245 − 1.5 x □ 0.5 x B 0.325 + x T , x = 0, 0.02, 0.04, 0.06, and 0.08) lead-free ceramics are selected as the representative. The $${\rm{BN}}{{\rm{S}}_{0.245 - 1.5x}}{_{0.5x}}{{\rm{B}}_{0.325 + x}}{\rm{T}}$$ BNS 0.245 − 1.5 x □ 0.5 x B 0.325 + x T ceramics are prepared by using pressureless solid-state sintering and achieve large Wrec (1.8 J/cm3) at a low electric field (@110 kV/cm) when x = 0.06. The value of 1.8 J/cm3 is super high as compared to all other Wrec in lead-free bulk ceramics under a relatively low electric field (< 160 kV/cm). Furthermore, a high dielectric constant of 2930 within 15% fluctuation in a wide temperature range of 40–350 °C is also obtained in $${\rm{BN}}{{\rm{S}}_{0.245 - 1.5x}}{_{0.5x}}{{\rm{B}}_{0.325 + x}}{\rm{T}}$$ BNS 0.245 − 1.5 x □ 0.5 x B 0.325 + x T (x = 0.06) ceramics. The excellent performances can be attributed to the A-site defect engineering, which can reduce remnant polarization (Pr) and improve the thermal evolution of polar nanoregions (PNRs). This work confirms that the $${\rm{BN}}{{\rm{S}}_{0.245 - 1.5x}}{_{0.5x}}{{\rm{B}}_{0.325 + x}}{\rm{T}}$$ BNS 0.245 − 1.5 x □ 0.5 x B 0.325 + x T (x = 0.06) ceramics are desirable for advanced pulsed power capacitors, and will push the development of a series of Bi0.5Na0.5TiO3 (BNT)-based ceramics with high Wrec and high-temperature stability.

Dynamic oxidation mechanism of carbon fiber reinforced SiC matrix composite in high-enthalpy and high-speed plasmas

This work employed an inductively coupled plasma wind tunnel to study the dynamic oxidation mechanisms of carbon fiber reinforced SiC matrix composite (Cf/SiC) in high-enthalpy and high-speed plasmas. The results highlighted a transition of passive/active oxidations of SiC at 800–1600 °C and 1–5 kPa. Specially, the active oxidation led to the corrosion of the SiC coating and interruption of the SiO2 growth. The transition borders of active/passive oxidations were thus defined with respect to oxidation temperature and partial pressure of atomic O in the high-enthalpy and high-speed plasmas. In the transition and passive domains, the SiC dissipation was negligible. By multiple dynamic oxidations of Cf/SiC in the domains, the SiO2 thickness was not monotonously increased due to the competing mechanisms of passive oxidation of SiC and dissipation of SiO2. In addition, the mechanical properties of the SiC coating/matrix and the Cf/SiC were maintained after long-term dynamic oxidations, which suggested an excellent thermal stability of Cf/SiC serving in thermal protection systems (TPSs) of reusable hypersonic vehicles.

Oxidation behaviors of carbon fiber reinforced multilayer SiC-Si3N4 matrix composites

Oxidation behaviors of carbon fiber reinforced SiC matrix composites (C/SiC) are one of the most noteworthy properties. For C/SiC, the oxidation behavior was controlled by matrix microcracks caused by the mismatch of coefficients of thermal expansion (CTEs) and elastic modulus between carbon fiber and SiC matrix. In order to improve the oxidation resistance, multilayer SiC-Si3N4 matrices were fabricated by chemical vapor infiltration (CVI) to alleviate the above two kinds of mismatch and change the local stress distribution. For the oxidation of C/SiC with multilayer matrices, matrix microcracks would be deflected at the transition layer between different layers of multilayer SiC-Si3N4 matrix to lengthen the oxygen diffusion channels, thereby improving the oxidation resistance of C/SiC, especially at 800 and 1000 °C. The strength retention ratio was increased from 61.9% (C/SiC-SiC/SiC) to 75.7% (C/SiC-Si3N4/SiC/SiC) and 67.8% (C/SiC-SiC/Si3N4/SiC) after oxidation at 800 °C for 10 h.

Si-based polymer-derived ceramics for energy conversion and storage

Since the 1960s, a new class of Si-based advanced ceramics called polymer-derived ceramics (PDCs) has been widely reported because of their unique capabilities to produce various ceramic materials (e.g., ceramic fibers, ceramic matrix composites, foams, films, and coatings) and their versatile applications. Particularly, due to their promising structural and functional properties for energy conversion and storage, the applications of PDCs in these fields have attracted much attention in recent years. This review highlights the recent progress in the PDC field with the focus on energy conversion and storage applications. Firstly, a brief introduction of the Si-based polymer-derived ceramics in terms of synthesis, processing, and microstructure characterization is provided, followed by a summary of PDCs used in energy conversion systems (mainly in gas turbine engines), including fundamentals and material issues, ceramic matrix composites, ceramic fibers, thermal and environmental barrier coatings, as well as high-temperature sensors. Subsequently, applications of PDCs in the field of energy storage are reviewed with a strong focus on anode materials for lithium and sodium ion batteries. The possible applications of the PDCs in Li-S batteries, supercapacitors, and fuel cells are discussed as well. Finally, a summary of the reported applications and perspectives for future research with PDCs are presented.

Influence of binder systems on sintering characteristics, microstructures, and mechanical properties of PcBN composites fabricated by SPS

Cubic boron nitride (cBN) with high hardness, thermal conductivity, wear resistance, and chemical inertness has become the most promising abrasive and machining material. Due to the difficulty of fabricating pure cBN body, generally, some binders are incorporated among cBN particles to prepare polycrystalline cubic boron nitride (PcBN). Hence, the binders play a critical factor to the performances of PcBN composites. In this study, the PcBN composites with three binder systems containing ceramic and metal phases were fabricated by spark plasma sintering (SPS) from 1400 to 1700 °C. The sintering behaviors and mechanical properties of the composites were investigated. Results show that the effect of binder formulas on mechanical properties mainly related to the compactness, mechanical performances, and thermal expansion coefficient of binder phases, which affect the carrying capacity of the composites and the bonding strength between binder phases and cBN particles. The PcBN composite with SiAlON phase as binder presented optimal flexural strength (465±29 MPa) and fracture toughness (5.62±0.37 MPa·m1/2), attributing to the synergistic effect similar to transgranular and intergranular fractures. Meanwhile, the excellent mechanical properties can be maintained a comparable level when the temperature even rises to 800 °C. Due to the weak bonding strength and high porosity, the PcBN composites with Al2O3-ZrO2(3Y) and Al-Ti binder systems exhibited inferior mechanical properties. The possible mechanisms to explain these results were also analyzed.

Thermal conductivity and bending strength of SiC composites reinforced by pitch-based carbon fibers

In this work, pitch-based carbon fibers were utilized to reinforce silicon carbide (SiC) composites via reaction melting infiltration (RMI) method by controlling the reaction temperature and resin carbon content. Thermal conductivities and bending strengths of composites obtained under different preparation conditions were characterized by various analytical methods. Results showed the formation of SiC whiskers (SiCw) during RMI process according to vapor—solid (VS) mechanism. SiCw played an important role in toughening the Cpf/SiC composites due to crack bridging, crack deflection, and SiCw pull-out. Increase in reaction temperature during RMI process led to an initial increase in thermal conductivity along in-plane and thickness directions of composites, followed by a decline. At reaction temperature of 1600 °C, thermal conductivities along the in-plane and thickness directions were estimated to be 203.00 and 39.59 W/(m·K), respectively. Under these conditions, bending strength was recorded as 186.15±3.95 MPa. Increase in resin carbon content before RMI process led to the generation of more SiC matrix. Thermal conductivities along in-plane and thickness directions remained stable with desirable values of 175.79 and 38.86 W/(m·K), respectively. By comparison, optimal bending strength improved to 244.62±3.07 MPa. In sum, these findings look promising for future application of pitch-based carbon fibers for reinforcement of SiC ceramic composites.

Porous, multi-layered piezoelectric composites based on highly oriented PZT/PVDF electrospinning fibers for high-performance piezoelectric nanogenerators

Piezoelectric nanogenerators (PENGs) that can harvest mechanical energy from ambient environment have broad prospects for multi-functional applications. Here, multi-layered piezoelectric composites with a porous structure based on highly oriented Pb(Zr0.52Ti0.48)O3/PVDF (PZT/PVDF) electrospinning fibers are prepared via a laminating method to construct high-performance PENGs. PZT particles as piezoelectric reinforcing phases are embedded in PVDF fibers and facilitate the formation of polar β phase in PVDF. The multi-layered, porous structure effectively promotes the overall polarization and surface bound charge density, resulting in a highly efficient electromechanical conversion. The PENG based on 10 wt% PZT/PVDF composite fibers with a 220 µm film thickness outputs an optimal voltage of 62.0 V and a power of 136.9 µW, which are 3.4 and 6.5 times those of 10 wt% PZT/PVDF casting film-based PENG, respectively. Importantly, the PENG shows a high sensitivity of 12.4 V·N−1, presenting a significant advantage in comparison to PENGs with other porous structures. In addition, the composites show excellent flexibility with a Young’s modulus of 227.2 MPa and an elongation of 262.3%. This study shows a great potential application of piezoelectric fiber composites in flexible energy harvesting devices.

Novel 3D grid porous Li4Ti5O12 thick electrodes fabricated by 3D printing for high performance lithium-ion batteries

Three-dimensional (3D) grid porous electrodes introduce vertically aligned pores as a convenient path for the transport of lithium-ions (Li-ions), thereby reducing the total transport distance of Li-ions and improving the reaction kinetics. Although there have been other studies focusing on 3D electrodes fabricated by 3D printing, there still exists a gap between electrode design and their electrochemical performance. In this study, we try to bridge this gap through a comprehensive investigation on the effects of various electrode parameters including the electrode porosity, active material particle diameter, electrode electronic conductivity, electrode thickness, line width, and pore size on the electrochemical performance. Both numerical simulations and experimental investigations are conducted to systematically examine these effects. 3D grid porous Li4Ti5O12 (LTO) thick electrodes are fabricated by low temperature direct writing technology and the electrodes with the thickness of 1085 µm and areal mass loading of 39.44 mg·cm−2 are obtained. The electrodes display impressive electrochemical performance with the areal capacity of 5.88 mAh·cm−[email protected] C, areal energy density of 28.95 J·cm−[email protected] C, and areal power density of 8.04 mW·cm−[email protected] C. This study can provide design guidelines for obtaining 3D grid porous electrodes with superior electrochemical performance.

A neutron diffraction investigation of high valent doped barium ferrite with wideband tunable microwave absorption

The barium ferrite BaTixFe12−xO19 (x = 0.2, 0.4, 0.6, 0.8) (BFTO-x) ceramics doped by Ti4+ were synthesized by a modified sol—gel method. The crystal structure and magnetic structure of the samples were determined by neutron diffraction, and confirm that the BFTO-x ceramics were high quality single phase with sheet microstructure. With x increasing from 0.2 to 0.8, the saturation magnetization (Ms) decreases gradually but the change trend of coercivity (Hc) is complex under the synergy of the changed grain size and the magnetic crystal anisotropy field. Relying on the high valence of Ti4+, double resonance peaks are obtained in the curves of the imaginary part of magnetic conductivity (μ″) and the resonance peaks could move toward the low frequency with the increase of x, which facilitate the samples perform an excellent wideband modulation microwave absorption property. In the x = 0.2 sample, the maximum reflection loss (RL) can reach −44.9 dB at the thickness of only 1.8 mm, and the bandwidth could reach 5.28 GHz at 2 mm when RL is less than −10 dB. All the BFTO-x ceramics show excellent frequency modulation ability varying from 18 (x = 0.8) to 4 GHz (x = 0.4), which covers 81% of the investigated frequency in microwave absorption field. This work not only implements the tunable of electromagnetic parameters but also broadens the application of high-performance microwave absorption devices.

Density-functional-theory predictions of mechanical behaviour and thermal properties as well as experimental hardness of the Ga-bilayer Mo2Ga2C

Mo2Ga2C is a new MAX phase with a stacking Ga-bilayer as well as possible unusual properties. To understand this unique MAX phase structure and promote possible future applications, the structure, chemical bonding, and mechanical and thermodynamic properties of Mo2Ga2C were investigated by first-principles. Using the “bond stiffness” model, the strongest covalent bonding (1162 GPa) was formed between Mo and C atoms in Mo2Ga2C, while the weakest Ga-Ga (389 GPa) bonding was formed between two Ga-atomic layers, different from other typical MAX phases. The ratio of the bond stiffness of the weakest bond to the strongest bond (0.33) was lower than 1/2, indicating the high damage tolerance and fracture toughness of Mo2Ga2C, which was confirmed by indentation without any cracks. The high-temperature heat capacity and thermal expansion of Mo2Ga2C were calculated in the framework of quasi-harmonic approximation from 0 to 1300 K. Because of the metal-like electronic structure, the electronic excitation contribution became more significant with increasing temperature above 300 K.