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Two garnet-structured Ca3MgBGe3O12 (B = Zr, Sn) ceramics with tetravalent cations at B-site were prepared by conventional solid state reaction. The crystal structure, microstructure evolution, and microwave dielectric performance were investigated using X-ray powder diffraction, Rietveld refinement, scanning electron microscopy, Raman spectroscopy, and infrared spectroscopy techniques. Dense Ca3MgZrGe3O12 and Ca3MgSnGe3O12 ceramics were obtained at sintering temperatures of 1420 and 1400 °C, respectively. The dielectric constant, unloaded quality factor, and temperature coefficient of resonance frequency of Ca3MgZrGe3O12 were 10.80 ± 0.2; 79,600 ± 1000 GHz (f = 12.61 GHz); and −66.8 ± 1 ppm/°C, respectively, and the corresponding values for Ca3MgSnGe3O12 were 9.68 ± 0.2; 83,400 ± 1000 GHz (f = 14.19 GHz); and −57.9 ± 1 ppm/°C, respectively. The dielectric performances of the two ceramics were compared by analyzing the ionic polarizability, packing fraction, and bond valence. The intrinsic dielectric properties were predicted by fitting the infrared reflectivity spectra.

Sialon/SiC composites were synthesized in situ from SiC, α-Si3N4, AlN, calcined bauxite, quartz and Y2O3 via layered buried sintering at different temperatures (1 540–1 640 °C). The results showed that the O’-sialon/SiC sample with 60 wt% silicon carbide sintered at 1 600 °C exhibited excellent mechanical properties, with apparent porosity of 16.01%, bulk density of 2.06 g·cm−3, bending strength of 52.63 MPa, and thermal expansion coefficient of 5.83×10−6 °C−1. The oxide film formed on the surface was linked closely to O’-sialon, so the oxide film was not easily broken. After 100 h oxidization, the sample surface was smoother and denser, with oxidation weight gain rate 23.6 mg/cm2 and oxidation rate constant 2.0 mg2·cm−4·h−1. Therefore, the sample had the excellent high-temperature oxidation resistance. It was confirmed that the in-situ sialon/SiC composites could be a promising candidate for solar absorber owing to its high-temperature oxidation resistance.

In this paper, sialon/SiC composite ceramics were synthesized in situ from SiC, α-Si3N4, AlN, calcined bauxite, quartz, and Y2O3 via layered buried sintering at different temperatures (1540–1640 °C). The sialon/SiC composite ceramics designed in this study are expected to be used as thermal absorb materials in solar thermal power generation based on the good mechanical character and thermal shock resistance. The results showed that the β-sialon/SiC sample with 60 wt% silicon carbide sintered at 1600 °C exhibited the optimal performances with dense microstructure, high bending strength, and good thermal shock resistance. After 30 thermal shock cycles, the bending strength of β-sialon/SiC sample with 60 wt% silicon carbide was 66.68 MPa with a decrease of 4.79%. In thermal shock tests, SiC and Si3N4 in the sample were oxidized into amorphous SiO2, and small amount of sialon was decomposed into mullite, which resulted in good thermal shock resistance. It was believed that in situ sialon/SiC composite ceramics can be a promising candidate for solar absorber owing to good thermal shock resistance.

Heterogeneous ice nucleation on atmospheric aerosols strongly affects the earth's climate, and at the microscopic level, surface-irregularity-induced ice crystallization behaviors are common but crucial. Because of the lack of visual evidence and effective experimental methods, the mechanism of atomic-structure-dependent ice formation on aerosol surfaces is poorly understood. Here we chose highly oriented pyrolytic graphite (HOPG) to represent soot (a primary aerosol), and environmental scanning electron microscopy (ESEM) was performed for in situ observations of ice formation. We found that hexagonal ice crystals show an aligned growth pattern via a two-stage pathway with one a axis coinciding with the direction of atomic step edges on the HOPG surface. Additionally, the ice crystals grow at a noticeably higher speed along this direction. This study reveals the role of atomic surface defects in heterogeneous ice nucleation and may pave the way to control icing-related processes in practical applications.

Coriolis effect, regarded as one of the universal physical law, describes the phenomenon that an arbitrary object will deflect left relative to the original speed when the object is laid on a reference frame with clockwise rotation, and vice versa. In this Letter, we designed a specialized piezoelectric metamaterial which is consist of a piezoelectric sensing element, an elastic rubber, and a mass element by using the finite element method (FEM). The FEM results suggested that the deflection behavior in this type of piezoelectric metamaterial, could strictly break the Coriolis effect and a negative Coriolis effect was observed. The exploration of negative Coriolis effect in piezoelectric metamaterial would provide theoretical guideline for the further design of ultrasensitive gyroscopes.

Flexoelectricity refers to the mechanical-electro coupling between strain gradient and electric polarization, and conversely, the electro-mechanical coupling between electric field gradient and mechanical stress. This unique effect shows a promising size effect which is usually large as the material dimension is shrunk down. Moreover, it could break the limitation of centrosymmetry, and has been found in numerous kinds of materials which cover insulators, liquid crystals, biological materials, and semiconductors. In this review, we will give a brief report about the recent discoveries in flexoelectricity, focusing on the flexoelectric materials and their applications. The theoretical developments in this field are also addressed. In the end, the perspective of flexoelectricity and some open questions which still remain unsolved are commented upon.

In this paper, we revisit the state-of-the-art amorphous semiconductor InGaZnO4 (a-IGZO), and demonstrate the application of a-IGZO thin film as an electron transporting layer (ETL) for efficient perovskite solar cells (PSCs) by a solution process. The champion device using such a-IGZO ETL has achieved an efficiency of 17.4%. The outstanding performance is attributed to the excellent properties of a-IGZO oxide thin films, including high carrier concentration, high electron mobility and high transparency. Our results definitely revel that amorphous semiconductors (e.g. a-IGZO) offer some unique features and have a great potential for large-area perovskite solar cells.

Complex micro-architectures are of critical importance for mechanical metamaterials. In this paper, we introduce a few novel micro-architectures (e.g. wave-networks and helix-networks) into metamaterials via a finite element method. The observed symmetry and scaling behaviors of the apparent elastic/dielectric/piezoelectric properties are systematically analyzed in order to build a complete structure–property space for these micro-architected metamaterials. The results suggest that the “softness” discovered in curvilinear structures results in ultrahigh piezoelectric responses. The presented scheme for iterative study will provide guidelines for the future design of metamaterials.

Harvesting environmental energy to generate electricity is a key scientific and technological endeavour of our time. Photovoltaic conversion and electromechanical transduction are two common energy-harvesting mechanisms based on, respectively, semiconducting junctions and piezoelectric insulators. However, the different material families on which these transduction phenomena are based complicate their integration into single devices. Here we demonstrate that halide perovskites, a family of highly efficient photovoltaic materials1,2,3, display a photoflexoelectric effect whereby, under a combination of illumination and oscillation driven by a piezoelectric actuator, they generate orders of magnitude higher flexoelectricity than in the dark. We also show that photoflexoelectricity is not exclusive to halides but a general property of semiconductors that potentially enables simultaneous electromechanical and photovoltaic transduction and harvesting in unison from multiple energy inputs., This work was supported by the National Natural Science Foundation of China under grant nos. 51962020, 51972157, 11574126 and 11604135, and partly by the National Key Research and Development Plan of China (2017YFB0406300). L.S. and S.K. thank Nanchang University for support. G.C. acknowledges support from the Generalitat de Catalunya (Grant 2017 SGR 579) and from MINECO (National Plan MAT2016-77100-C2-1-P and Severo Ochoa SEV-2017-0706). M.S. acknowledges the support of MINECO through grants no. MAT2016-77100-C2-2-P and no. SEV-2015-0496, Generalitat de Catalunya (grant no. 2017 SGR1506) and the European Research Council under the European Union’s Horizon 2020 research and innovation programme (grant no. 724529). We thank D Torres for the graphic design of Fig. 2a,b.

A microscopic understanding of the mechanism of direct ice formation from water vapor has a significant benefit for controlling the processes involving ice condensation and evaporation. However, previous studies on this topic have been limited to theoretical simulations or optical observations. Here, by in situ observation via environmental scanning electron microscopy (ESEM), we revealed that hexagonal ice crystals are developed by a step-by-step pathway in a supersaturated water vapor environment. Furthermore, we also discerned that such steps came from two different origins, which are screw dislocations and initial steps. In addition, the relationship between the edge-length (of hexagonal ice crystals) and the growth time was quantitatively studied at controlled temperatures and pressures by experimental data fitting. This study shows that qualitative and quantitative observations of ice formation can be made with simple setups, and it should inspire future investigations toward important physicochemic...

Harvesting environmental energy to generate electricity is a key scientific and technological endeavour of our time. Photovoltaic conversion and electromechanical transduction are two common energy-harvesting mechanisms based on, respectively, semiconducting junctions and piezoelectric insulators. However, the different material families on which these transduction phenomena are based complicate their integration into single devices. Here we demonstrate that halide perovskites, a family of highly efficient photovoltaic materials

In this Letter, we show, both theoretically and experimentally by the molecule dynamics calculation and in-situ Raman scatting, respectively, that the polar molecules in CH3NH3PbI3 perovskite, upon bending, would form a stable molecule alignment, suggesting that hybrid organic–inorganic perovskites are promising candidates for semiconducting ferroelectric materials. Our research provides an alternative paradigm for determining of polarization shifting in ferroelectrics, especially those with large electrical conductivity.

In this work, TiO2-SrRuO3 (SRO) heterojunction was deposited on flexible mica by pulsed laser deposition. TiO2 with anatase phase could be deposited directly on flexible substrate benefit from the high temperature tolerance of mica. Take the advantage of the strong visible light absorption of SRO and the hot electrons injection mechanism, the TiO2/SRO heterojunction shows a very large photocurrent density of 4043.8 μA/cm2 at 0.5 V vs Ag/AgCl. The TiO2/SRO photoanode with high photoelectrochemical activity and good flexibility reveals the application potential in flexible optoelectronics.

Photocatalysts with high flexibility and stability have become hot spots in material science and energy-related research. In this work, a TiO2/Au bilayer was developed on a flexible mica substrate for photoelectrochemical (PEC) water splitting. Due to the high temperature tolerance of mica, anatase TiO2 could be deposited directly on the flexible substrate. By inserting an Au layer, the photocurrent density of the TiO2/Au bilayer was 76 times higher than that of pure TiO2. Moreover, the performance of this bilayer structure has good stability for 500 bending cycles. This study provides a new research direction for the design and manufacture of flexible photoelectrochemical materials.

The in-situ synthesized mullite bonded SiC ceramics for solar thermal tower plant were prepared from Silicon carbide (SiC), manufactured aluminum hydroxide (Al(OH)3) and Suzhou kaolin via semi-dry pressing and pressureless firing. The results indicate that sample B3 (designed mullite content 15 wt%) fired at 1 400 °C exhibited optimal performance with a bending strength of 97.41 MPa. Sample B3 can withstand 30-cycles thermal shock without cracking (wind cooling from 1 100 °C to room temperature), and the bending strength after thermal shock decreased by 17.92%. When the service temperature is 600 °C, the thermal diffusivity, specific heat capacity, thermal conductivity and heat capacity are 6.48×10−2 cm2·s−1, 0.69 kJ·kg−1· K−1, 9.62 W·m−1·K−1 and 977.76 kJ·kg−1, respectively. The XRD and SEM results show that SiC, mullite, α-quartz, and tridymite are connected closely, which gives the material a good bending strength. After 30-time thermal shock cycles, the structure of samples becomes loose. SiC grains are intersectingly arranged with rodshape mullite, exhibiting a favorable thermal shock resistance. The addition of Al(OH)3 and Suzhou kaolin can accelerate the synthesis of mullite, thus to reduce the firing temperature effectively. The volume effect of tridymite is relatively small, improving the thermal shock resistance of materials. A higher designed mullite content yields a lower loss rate of bending strength. The mullite content should not be more than 15 wt% or else the bending strength would be diminished.

For lowering the sintering temperature of silicon carbide ceramics used for solar thermal energy storage technology, O'-Sialon and silicon nitride were employed as composite phases to construct Sialon-Si3N4-SiC composite ceramics. The composite ceramics were synthesized using SiC, Si3N4, quartz, and different alumina sources as starting materials with noncontact graphite-buried sintering method. Influences of alumina sources on the physical properties and thermal shock resistance of the composites were studied. The results revealed that the employment of O'-Sialon and silicon nitride could decrease the sintering temperature greatly to 1540°C. The optimum formula G2 prepared from mullite as alumina source achieved the best performances: 66.7 MPa of bending strength, 10.0 W/(m·K) of thermal conductivity. The composition parameter x = 0.4 of O'-Sialon decreased to 0.04 after 30 cycles thermal shock, and the bending strength increased with a rate of 11.0% due to the increase of O'-Sialon grain size, and the optimization of microstructure caused by the transformation of O'-Sialon grains and densification within the samples. The good thermal shock resistance makes the composites suitable for the use as thermal storage materials of concentrated solar power generation.

Cordierite/silicon carbide (SiC) composite ceramics with 10, 20 and 30 wt% cordierite were in-situ prepared via pressureless firing technique using silicon carbide, alumina, Kaolin and talc as starting materials. The results show that the synthesized composites all exhibited the good physical and mechanical properties. Among those, A2 composite with 20 wt% in-situ cordierite fired at 1280 °C exhibited the better bending strength and thermal shock resistance. The improvement of thermal shock resistance could be attributed to the proper cordierite wrapped around SiC, the homogeneous distribution of pores, the well-grown grains, as well as the formation of some glass phase. During the thermal shock test, a small amount of glass phases generated from the oxidation of SiC could react with Al 2 O 3 and MgO, and then more cordierite was formed due to the incorporation of nucleating agent TiO 2 . With in-situ cordierite content rising, both absorptance and emittance of composite reduced, but A2 composite had the better absorbing ability for the solar energy. It is believed that these in-situ cordierite/SiC composites should be a promising candidate to apply in the field of solar absorber coating owing to the better thermal shock resistance and absorptance of solar energy.

Hybrid perovskite solar cells (PSCs) have been intensively studied in recent years because of their high efficiency and low costs. For PSCs, the electron transport layer (ETL) is a key for its photoelectric conversion efficiency. Here we demonstrate the application of amorphous InGaZnO4 thin films as ETL for efficient PSCs by pulsed laser deposition (PLD). The PSC device using such InGaZnO4 amorphous film as ETL has achieved an efficiency of 15.1%. The outstanding performance is attributed to the excellent properties of amorphous InGaZnO4 oxide thin films, including high electron mobility and high transparency, what is more, the electronic properties of the films can be controlled by changing the partial pressure of oxygen in the deposition chamber and post-deposition annealing process. Our result will be helpful for preparation of large area PSCs and other opto-electric devices at low temperature by physical vapor deposition method.

Flexoelectricity yields electric polarization through graded strains and exists in various kinds of solid dielectrics but usually with a weak coupling response. In this letter, several Bi-doped Pb(Mg2/3Nb1/3)O3-xPbTiO3 (PMN-xPT) ferroelectric ceramics were prepared and the interplay of their piezoelectricity and flexoelectricity was systematically investigated. The largest flexoelectric response (∼300 μC/m) was 30 times larger than that of pure PMN-PT and was observed in the Bi-doped PMN-32PT, where the after poling piezoelectric coefficients were also the largest (990 pC/N) among all the compositions. This suggests that the apparent flexoelectricity in ferroelectrics is highly related to the bulk piezoelectricity. Therefore, introducing a local structural heterogeneity is considered as a feasible approach to achieve an ultrahigh piezoelectric response while also providing an ultrahigh flexoelectricity.

Ultrathin and transparent metal films are considered as desirable materials to substitute indium-tin oxide based transparent electrodes. In this work, we report that an Al-doped ZnO layer effectively induced the formation of epitaxial ultrathin and ultrasmooth Au films on mica. The ultrathin Au film shows optimum electrical conductivity, optical properties, and high tolerance to mechanical bending, including a low sheet resistance of 8 Ω/sq and a high average transmittance of 80.1%, and maintains low electric resistivity even after 1000 continuous bending cycles. Moreover, the Au/AZO anode-based organic light-emitting diode was fabricated with the maximum luminance intensity of 32 540 cd/cm2, which demonstrates that it is promising for application in next generation optoelectronic devices.

In this letter, we report on significant flexoelectric fatigue in lead-free (K,Na,Li)(Nb,Sb)O3 (KNNLS) ceramics. In the ferroelectric phase of KNNLS, the observed effective transverse flexoelectric coefficient, which is initially as high as 1 μC/m, decreases nonlinearly with increasing cycles of strain gradient, and the fatigue regulation is well reproduced by a stretched exponential function. By comparing the time dependence of dielectric permittivity and ferroelectricity in KNNLS ceramics, we conclude that large flexoelectric and flexocoupling coefficients in the ferroelectric phase originate mainly from ferroelectricity and that the related flexoelectric fatigue is likely due to the pinning effect of ferroelectric domains.

Flexoelectricity couples strain gradient to polarization and usually exhibits a large coefficient in the paraelectric phase of the ferroelectric perovskites. In this study, we employed the relaxor 0.3Pb(In1/2Nb1/2)O3-0.35Pb(Mg1/3Nb2/3)O3-0.35PbTiO3 (PIN-PMN-PT) single crystals to study the relationship between flexoelectric coefficients and the crystal structure. The flexoelectric coefficients in PIN-PMN-PT single crystal are found to vary from 57 μC/m at orthorhombic/monoclinic phase to 135 μC/m at tetragonal phase, and decreases to less than 27 μC/m in the temperature above Tm. This result discloses that ferroelectricity can significantly enhance the flexoelectricity in this kind of perovskite.