1-3 type piezoelectric composites are widely used to fabricate high-sensitivity and broadband ultrasonic transducers due to the advantages like a low dielectric constant, a low acoustic impedance, and a high electromechanical coupling coefficient. However, some conventional manufacturing methods such as the cut-and-fill method, the arrangement-casting process, and the injection molding method have challenges to emanate from geometric complexity and structural integrity. Stereolithography technology based on the mask projection approach is an alternative method for manufacturing piezoelectric ceramics with a rapid prototyping and a high precision. Lead-free barium titanate BaTiO₃ (BTO) ceramics are widely investigated for piezoelectric sensors and dielectric capacitors applications because of their high dielectric constant, good piezoelectric coefficient, and favorable electromechanical coupling coefficient. In this work, digital light processing (DLP) technology was used to manufacture barium titanate based 1-3 type piezoelectric composites. The influence of piezoelectric phase volume fraction (i.e., 20%-45%) on the micrograph, acoustic impedance and electrical properties of BTO 1-3 composites was investigated.

The printed ceramic slurries with 84% (in mass fraction) BTO content, including BTO powders (d₅₀=500 nm), dispersant, and liquid photopolymers, were mixed by a model ZYMC-180V homogenizer (ZYE) at 2 000 r/min for 90 s. The printed models were built by a software named Computer-Aided Design (CAD) and sliced to a thickness of 7 μm. The piezoelectric pillars (150 μm) with a support were printed layer by layer to form the physical models (i.e., Soon solid, TC-Ⅱ). The printed samples were washed in alcohol by aultrasonic cleaner to remove the residual slurry. Afterwards, the green bodies were treated by a two-step heating treatment, i.e., a debonding process and a sintering process, to obtain the piezoelectric phase structures. Two-time vacuum treatments below -100 kPa were introduced to remove air bubbles in epoxy resin (EPO-TEK301) that filled in the kerfs between piezoelectric pillars (width of 120 μm, height of 330 μm). The samples filled with epoxy resin were heat treated at 60 ℃ for 12 h and then coated with gold electrodes. The BTO 1-3 composites were polarized in an electric field of 4 kV/mm in silicone oil. The microstructures of samples were characterized by a model TM4 000 Plus scanning electron microscope (Hitachi Co., Lted., Japan). The dielectric constant ε_r, dielectric loss δ, and frequency impedance spectrum were measured by a model 6500B precision impedance analyzer (Wayne Kerr Co., Ltd., UK). Piezoelectric constant d₃₃ was determined by a model ZJ-3A piezo-d₃₃ meter (Institute of Acoustics, Chinese Academy of Sciences, China). The ferroelectric hysteresis loop was measured by a model 2000E ferroelectric analyzer (aixACCT Co., Germany).

The uniform piezoelectric pillars with a width of (150±10) μm are printed through optimizing the slurry formula, and the minimum distance between pillars is approximately 100 μm. The bending deformation of BTO pillars is negligible, and the lateral shrinkage rate is 20%. The forming mechanism of DLP technology results in a clear lamination of the piezoelectric pillar along the printing direction. The shrinkage of ceramics and the growth of grains fill the gap between interlayers, thus reducing the lamination after sintering. There are no residual bubbles in the epoxy resin after vacuum treatment. The density and acoustic impedance of the piezoelectric composites increase with the growth of the piezoelectric phase volume proportion. Compared with the acoustic impedance of BTO ceramics (i.e., Z=31.64 MRayl, 1Rayl=1 Pa·s/m=10^-6 MRayl), the acoustic impedance of composites (45%, in volume fraction) is decreased by 67%.

The piezoelectric constant d₃₃ and electromechanical coupling coefficient k_t enhance with the piezoelectric phase volume proportion of the composites, meeting their maximum values of 74 pC/N and 0.48, respectively. Compared with the BTO ceramics (k_t=0.37), the electromechanical coupling coefficient is improved by 30%. According to the frequency impedance spectra of piezoelectric composites, 1-3 type composites have a pure thickness vibration mode. The dielectric constant ε_r increases linearly with increasing the piezoelectric phase volume fraction, reaching its maximum of 1 167. The dielectric loss tanδ is lower than 3%, which can enhance the receiving sensitivity of ultrasound transducers. The residual polarization intensity increases with the increase of volume fraction of the piezoelectric phase. The residual polarization intensity of the piezoelectric composite with a volume fraction of 45% reaches a maximum of 1.77 μC/cm² at 4 kV/mm.

1-3 type barium titanate/epoxy resin piezoelectric composites were fabricated by the DLP technology. The effect of volume fraction of piezoelectric phases on the electrical performances of composites was investigated. The electrical properties such as piezoelectric constant d₃₃, electromechanical coupling coefficient k_t, and dielectric constant ε_r were optimized as the volume fraction of the piezoelectric phase (≤45%) was increased. The polymer phase could effectively reduced the acoustic impedance of piezoelectric materials, thus obtaining an acoustic matching between the piezoelectric materials and tissues or water. Compared to pure barium titanate ceramics, 1-3 type composites as a material of ultrasound transducers could effectively suppress the lateral vibration of the material and enhance the thickness vibration mode, which could improve the pulse echo response. The DLP technology with a high resolution provides an effective and low-cost method for fabricating the piezoelectric composites.

Ga₂O₃ is an ultra-wide bandgap semiconductor, which has a direct bandgap of 4.9 eV. Also, it has the superior thermodynamic and chemical stability, making it suitable for the development of deep ultraviolet optoelectronic devices. Ga₂O₃ exhibits the superior photosensitive and resistive properties, which has a promising application in sensor-memory integrated optoelectronic devices. In this paper, the crystal structure, composition, and morphology of the Ga₂O₃ thin film grown via pulsed laser deposition (PLD) was discussed, and then an optoelectronic device with Pt/Ga₂O₃/NSTO/In structure was designed and constructed. The physical mechanism of the Ga₂O₃-based device was investigated. The device process was optimized to achieve both high sensitivity ultraviolet photo-response and stable resistive switching characteristics. This work can provide theoretical guidance and technical support for the development of novel Ga₂O₃-based multifunctional optoelectronic devices.

Ga₂O₃ thin films were grown on (100) Nd∶ SrTiO₃ (NSTO) substrates by the PLD method. The target material used was a pure Ga₂O₃ ceramic target prepared by a solid-state method. Before thin film deposition, the ceramic target was pre-ablated for 3 min to remove surface contaminants. The film deposition was performed at pulse laser energy of 300 mJ, O₂ partial pressure of 3.0 Pa, and substrate temperature of 650 ℃. The target rotation speed was 5 r/min, the substrate rotation speed was 10 r/min, the pulse laser frequency was 5 Hz, and the deposition time was 30 min. Based on the deposited Ga₂O₃ thin films, a device with metal/Ga₂O₃/electrode sandwich structure was further constructed. At room temperature, Pt metal dot electrodes were fabricated on the surface of Ga₂O₃ thin films by a DC magnetron sputtering method as a top electrode of the device, and then weld the metal indium (In) directly onto the back of the NSTO substrate as a bottom contact electrode. The above Pt metal electrode preparation process was run at DC source power of 100 W, working pressure of 1.5 Pa, Ar flow rate of 10 mL/min, substrate rotation of 10 r/min, and sputtering time of 60 s.

The structure of Ga₂O₃ thin films was characterized by a model D8 Discover type high-resolution four circle single crystal X-ray diffractometer. The elemental composition of Ga₂O₃ thin film material was analyzed by a model Escalab 250Xi X-ray photoelectron spectroscope. The surface and cross-sectional morphology of the films was characterized by a model Sigma 500 field emission scanning electron microscope. A semiconductor electrical performance testing system was constructed by a model Keithley 2635 digital source meter, probe station, and electromagnetic shielding dark box, to examine the I-V scanning loop, multi-level resistive state, retention, and fatigue resistance characteristics of the device. A 255 nm LED light source was driven by a digital signal generator to test the time-dependent UV photoresponse characteristics. A Omni-λ 3027 setup consisting of monochromator, 150 W xenon lamp light source, chopper, collimating light converter, and phase-locked amplifier, was utilized to measure the dark current, spectral response, UV-visible suppression ratio, and detectivity of the photodetector.

The deposited β-Ga₂O₃ film with small particles and a flat and dense surface exhibits (004) plane preferential orientation. The Pt/Ga₂O₃/NSTO/In device with both fast self-driven ultraviolet photoresponse and stable resistive switching characteristics was fabricated. Under illumination, the photocurrent of the device rapidly increases to its maximum value and then gradually decreases to a steady state possibly due to the reduction in the Schottky junction built-in electric field caused by the injection of photogenerated carriers into the barrier region. The self-driven photoresponse is attributed to the efficient separation of photo-generated carriers by a built-in electric field formed at the Pt/Ga₂O₃ interface. On the other side, the device exhibits a stable bipolar resistive switching behavior. Alternating -5 V and +3 V pulse voltages with a 10 ms pulse width results in a resistive switching ratio of nearly 10⁴. A rapid switching between multi-level resistance states occurs at different pulse voltages. The intrinsic oxygen vacancy defect in Ga₂O₃ thin film as the trap center dominates the capture/release process of charge carriers, thereby maintaining the resistance state. Changing the pulse voltage can control the amount of electron injection, thus affecting the height of the interface barrier, which can explain the multi-level resistive characteristics of the device. The resistance state shows a good retention, and there is no significant degradation within 10⁴ s, showing the nonvolatile characteristics. In addition, the device also exhibits stable and fast switching characteristics after multiple repeats, indicating potential applications in memory devices. The mechanism of resistive switching effect can be attributed to the interface type resistance mechanism. The variation of the Schottky barrier caused by external electrical injection and the carrier trapping/detrapping process caused by the oxygen vacancy defect trap center jointly determine the resistive switching effect.

The fabricated device had a fast UV photoresponse (τ_r/τ_d=60 ms/120 ms) with a peak responsivity of 13.4 mA/W and 1.49×10¹¹ Jones (λ=250 nm) at 0 V bias. According to the analysis, the Schottky contact formed at the Pt/Ga₂O₃ interface resulted in a fast self-driven photoresponse at 0 V bias. In addition, the fabricated device also exhibited a stable bipolar resistive switching behavior with a high/low resistance ratio of 10⁴. The fast switching between multiple resistance state was achieved with the superior anti-fatigue and retention characteristics at different pulse voltages. The external electric field and oxygen-vacancy trap centers was considered as the main origin of resistive switching effect in Ga₂O₃-based optoelectronic device.

Machinery and equipment are prone to equipment failure and even safety accidents caused by insufficient lubrication under severe working conditions. Adding additives with some superior performance in lubricant is an effective approach to improve the lubricant performance, reduce the friction and wear, and extend the service life of machinery. Lubricants are widely used in machinery and equipment as an important method of anti-wear and friction reduction. Silicate minerals are harmless to the environment and the humanbeing. A low roughness, high hardness friction reaction film can be formed by silicate minerals as lubricant additives during the lubrication process, which can effectively reduce wear. Organic molybdenum as a commonly used lubricant additive has superior antioxidant properties, and can decompose when pressure, temperature and other conditions meet the requirements, and then lead to a chemical reaction and form the protective film containing molybdenum elements, thus preventing the surface oxidation reactions to improve the tribological properties of the lubricant. In this paper, the friction and wear reduction properties of composite lubricant additives of montmorillonite (MMT) and organomolybdenum were investigated to obtain superior lubrication, thereby improving the operational stability and safety of machinery and equipment under severe operating conditions.

The lubrication performance of three molybdenum-based lubricants, i.e., MoDDP (C₂₈H₆₀O₄P₂S₄Mo), MoDTC (C₃₄H₇₂Mo₂N₂O₂S₆) and RS-568, was evaluated by a four-ball friction and wear tester to determine the organic molybdenum type and the optimal mass fraction of organic molybdenum in the composite lubricant additives. Tribological tests were then carried out on lubricant samples at different concentrations of MMT to determine the optimal mass fraction of MMT in the composite additives. After the preparation of the composite additive, the lubricating properties of the MMT/MoDDP composite additives were examined, and the friction reduction and anti-wear mechanism of the composite additive was investigated by energy dispersive spectroscopy, X-ray photoelectron spectroscopyand Raman spectroscopy.

The results show that among the organic molybdenum additives, molybdenum dialkyldithiophosphate (MoDDP) exhibits the optimum lubricant properties. The friction coefficient and wear spot diameter firstly decrease and then increase with the increase in the mass fraction of MMT and MoDDP in the oil, and the optimal mass fractions of MMT and MoDDP are 3.0% and 1.5%, respectively. Compared with the base oil, the friction coefficient is reduced by 41.0% and the wear spot diameteris decreased by 48.9% for oil sample at 1.5% MoDDP, and the friction coefficient is reduced by 40.6% and the wear spot diameter is decreased by 48.2% for the oil sample at 3.0% MMT.

Compared with the oil samples at 3.0% MMT, the friction coefficient and the wear spot diameter was reduced by 26.3% and 7.9%, respectively, for the oil sample with the composite additives. Compared with the oil samples at 1.5% MoDDP, the friction coefficient and the wear spot diameter are reduced by 26.6% and 6.6%, respectively, for the oil sample with the composite additives. The 3.0% MMT/1.5% MoDDP composite lubricant additives can further improve the friction reduction and anti-wear performance of the lubricant. Compared with the base oil, the friction coefficient and wear spot diameter are reduced by 56.3% and 52.3%, respectively, for the oil sample with the composite additives.

During the friction process, a SiO₂ physical adsorption film was deposited on the friction surface due to the adsorption property of MMT additives. Under the action of shear force, the layered MMT particles are subjected to shear, resulting in an interlayer slip, which can reduce the friction between the contact surfaces. The fractured MMT particles are mechanically filled in the surface wear area to improve the carrying capacity of the oil film. Moreover, a layered MoS₂ film with a low shear strength and a high melting point can be generated due to the chemical reaction of the MoDDP additive. Also, the MoO₂, FeS₂, Fe₂O₃ metal compound reaction film can be generated to prevent the direct contact of the friction interfaces. Although the single additive has a great lubrication effect, the MMT and MoDDP composite additives show a better lubrication performance and synergistic effects in the friction process. The composite metal compound layer and the physical adsorption film can be generated during friction to form the high load-bearing capacity of the lubricant film, thus increasing the strength and stability of the lubricant film, and it was able to continuously and effectively reduce friction and wear. The friction tests also verify that MoDDP and MMT have a positive synergistic effect in the friction process. During the lubrication process, chemical reaction films with MoS₂, MoO₂, FeS₂, and Fe₂O₃ and SiO₂ physical adsorption films were generated, thus effectively reducing thefriction and wear.

The 3.0% (in mass fraction) MMT/1.5% (in mass fraction) MoDDP composite additives exhibited the better lubricating properties, compared with a single additive. It was indicated that the MMT/MoDDP composite lubricant underwent a chemical reaction during the friction process, which produced molybdenum sulfides and oxides containing molybdenum (MoS₂, MoO₂) and iron sulfides and oxides containing FeS₂, Fe₂O₃, as well as physically adsorbent films containing SiO₂. The compound lubrication film could effectively reduce the friction of the interface.

With the increasing demand for turbine engine performance, turbine blade structure becomes increasingly complex. The investment of precision casting ceramic core is a serious challenge. The existing solubility is the most important issue restricting the application of alumina-based ceramic cores. Increasing the porosity can enhance the substrate material and the interaction of chemical solutions. The main research direction is to improve the corrosion performance of alumina-based ceramic core. However, the increase of porosity will cause the core strength to plummet, affecting the performance of alumina-based ceramic core. To meet the requirements of investment casting, the organic-inorganic hybrid fiber can improve the porosity and strength of alumna-based ceramic core system. The synergic strengthening mechanism of hybrid fiber modified aluminum ceramic cores pore-system was investigated.

Capacitive corundum powder, fused mullite, and quartz glass powder were used as matrix material, mineralizer, and firing aid, respectively. Meanwhile, organic short-cut fiber as a pore-making agent, inorganic short-cut alumina fiber as reinforcement, coupling agent solution, and hydroxypropyl methylcellulose modified fiber were used to prepare alumina ceramic cores with different hybrid fiber ratios via hot pressing injection. According to aviation industry standard HB 5353.1-3—2004, the key performance parameters of each core sample were obtained. A scanning electron microscope was used to observe the pore characteristics, fracture morphology, and fiber distribution of the core, and analyze the fiber fracture mode. The synergic strengthening mechanism and the pore strength of the hybrid fiber-modified alumina-based ceramic core were investigated.

The introduction of hybrid fibers changes the pore characteristics of the alumina-based ceramic core, and irregularly connected pores and nearly circular closed pores appear in the ceramic core. The fracture mode of the alumina-based ceramic core changes from "transcrystalline fracture" of like-metal to "intergranular fracture" as the proportion of hybrid fibers decreases.

The addition of hybrid fiber reduces the sintering shrinkage rate of the alumina-based ceramic core, the shrinkage rate of the sample NA67 is only 0.2%. For the comparison, the rate is reduced by 77.8% for the sample NA60 without alumina fibers, which may be since an appropriate amount of alumina fiber can effectively increase the center separation of sintered particles, hinder the migration and diffusion of substances, offset the sintering shrinkage stress, and inhibit the sintering shrinkage of the core. The mass-burn loss of alumina-based ceramic core is 19.5% as the proportion of hybrid fibers decreases. As the mass loss mainly comes from the oxidation burn loss of plasticizer and nylon fiber, the alumina fiber has an impact on the migration of crystal water in the sintering process, so the mass-burn loss rate fluctuates slightly.

The number of large pores in the aluminum ceramic core increases, the number of small pores decreases, and the apparent pores in the ceramic core firstly increase and then become stable with the decrease in the proportion of hybrid fibers. Small-aperture pores are mainly composed of closed pores lost by chopped organic nylon fiber (N_sf) fiber, while large-aperture pores are mainly connected pores formed by chopped inorganic alumina fiber (A_sf) crossing closed pores when the hybrid fiber ratio is less than 6∶7. The apparent porosity of the core tends to be stable at 43.31% because the intersected A_sf fibers are clustered among the sintered particles. The effect of A_sf fibers on the increase of the distance between the sintered particles becomes slight, the number and size of connected pores in the ceramic core almost unchange, and the apparent porosity tends to be stable.

The bending strength of the alumina-based ceramic core increases firstly and then decreases as the proportion of hybrid fiber decreases. This is because the appropriate amount of A_sf fiber effectively increases the fiber-matrix interface bonding area. When the core is subjected to external loads, A_sf fiber consumes the more crack propagation energy through the behavior of debonding, fracture, and pull-out, and the bending strength of cores increases significantly. The excessive A_sf fiber is unevenly dispersed in the matrix material, dividing the matrix, reducing the interface bonding area between the agglomerated fiber and the core matrix, weakening the strengthening effect of the fiber on the core, and leading to a slight decline in the bending strength of the core.

The shrinkage rate of the alumina-based ceramic core decreased after firing as the hybrid N_sf/A_sf fiber ratio decreased. The A_sf fibers interspersed in the core matrix effectively hindered the diffusion and migration of sintered particles and reduced the core densification. The shrinkage rate of the sample NA67 after firing was 0.2% when the hybrid fiber ratio was 6∶7, which was 77.8% lower than that of the sample NA60 without hybrid fiber, showing a good dimensional stability.

The increase of apparent porosity of hybrid fiber reinforced alumina-based ceramic core was mainly due to the loss of residual closed porosity by oxidation of N_sf fiber at a high temperature and the formation of connected porosity by A_sf fiber interpenetrating matrix. The number of connected pores, the volume proportion of open pores and the apparent porosity increased, while the volume density decreased as the hybrid fiber proportion decreased.

An appropriate amount of A_sf fiber consumed the crack propagation energy through the mechanism of de-bonding, pulling out, and breaking, and improved the bending strength of alumina-based ceramic core, effectively making up for the strength loss caused by in-situ burning of N_sf. The bending strength of the alumina-based ceramic core reached a maximum value of 20.1 MPa when the hybrid fiber ratio was 6∶7. However, the agglomeration and cross of A_sf fibers could form some penetrating cracks in the core matrix and cut the matrix, affecting the strengthening effect of the fibers when the proportion of hybrid fibers further decreased.

Ordinary concrete rapidly loses its bearing capacity under impact, while the addition of steel fibers effectively enhances the overall integrity and matrix toughness of concrete, thus meeting the concrete performance requirements of high strength, high durability, and high toughness in practical engineering. The existing work on the strengthening and toughening mechanism and failure mechanism of steel fiber reinforced concrete is mainly based on the macroscopic experiments. It is thus difficult to reflect the randomness and non-uniformity of concrete. In this paper, the dynamic response of steel fiber reinforced concrete beam specimens at a low-speed impact was investigated by a drop hammer testing machine to comprehensively reveal the internal damage and failure process of concrete and the toughening mechanism of steel fibers. A three-dimensional microscopic model of steel fiber reinforced concrete beams was proposed, and the mechanical properties and crack development of steel fiber reinforced concrete beams under dynamic impact were analyzed. In addition, the effects of steel fiber volume fraction and impact velocity on the impact resistance and failure morphology of steel fiber reinforced concrete beams were also discussed.

Steel fiber reinforced concrete beams with different steel fiber volume fractions (i.e., 0%, 0.5%, 1.0%, 1.5%, 2.0%, and 2.5%) were prepared based on the experimental mix proportion. The effect of steel fiber volume fraction on the impact performance of steel fiber reinforced concrete beams was analyzed via dynamic three-point bending performance in a model INSTRON-9530 drop hammer impact testing machine.

A steel fiber model was randomly proposed based on the Monte Carlo method, and a spring element was used to simulate the bond slip between steel fibers and concrete matrix. Based on its bond slip relationship, a three-dimensional fine calculation model for steel fiber concrete was established. The dynamic response and failure forms of steel fiber concrete were simulated at different volume fractions (i.e., 0%, 0.5%, 1.0%, 1.5%, 2.0%) and different impact rates (i.e., 1, 2, 3, 4, 5, 10, 20 m/s), respectively, The effects of steel fiber content and impact rate on the impact resistance and failure morphology of steel fiber reinforced concrete beams were analyzed.

The drop hammer impact test is widely used to test the dynamic bending performance of concrete beams, but the hammer impact force measured by a hammer force sensor is not an actual impact force due to its inclusion of structural inertia force. In the low-speed impact test of steel fiber reinforced concrete beams, this study integrated some force sensors with roller bearings to accurately obtain the support reaction force of the beam during the impact process and reduce the influence of inertial force, thereby revealing the true dynamic response of the tested specimens.

The key to improving the mechanical properties of fiber reinforced materials lies in the micro bridging effect of fibers on the concrete matrix. Steel fiber exhibit a large number and random distribution in the concrete matrix, making it difficult to quantitatively evaluate the fiber reinforcement effect at an impact load. This study used the Monte Carlo method to propose a fiber microscopic model and determine the range based on the unique three-dimensional position and direction information of each fiber. The numerical analysis methods were used to more intuitively clarify the deformation and damage situation of steel fibers and concrete matrix during the impact process to obtain the accurate energy quantification evaluation.

The key to improving the mechanical properties of steel fiber reinforced concrete lies in the bond slip between steel fibers and concrete matrix. Simulating the bond slip between concrete matrix and steel fibers accurately becomes a key to simulating steel fiber reinforced concrete. In this paper, spring elements were used to simulate steel fibers. The stiffness of the spring element adopts a "two-stage" model. The simulation data by the model are in reasonable agreement with the experimental results.

The simulation data of three-dimensional steel fiber reinforced concrete beams based on bonded slip spring elements were in reasonable agreement with the experimental results.

The bearing capacity and energy dissipation capacity of steel fiber reinforced concrete beams increased with the increase of steel fiber volume fraction when the fiber volume fraction was small. The peak impact force of the concrete beam was increased by 16.71% at the optimal dosage of steel fiber of 1.5%. However, the energy dissipation capacity and impact resistance of steel fiber reinforced concrete beams decreased as the volume fraction of steel fibers further increased.

The bearing capacity of steel fiber reinforced concrete beams was positively correlated to the rate when the impact rate was in the range of 1 m/s to 20 m/s. The peak impact force of steel fiber reinforced concrete beams increased, but the reinforcement effect of steel fiber on the steel fiber reinforced concrete gradually weakened as the rate continues to increased. The reinforcement effect of steel fiber was slight when the rate increased to 20 m/s. Therefore, for steel fibers with a length of 12.5 mm and an aspect ratio of 20, their enhancement effect on the dynamic mechanical properties of concrete beams was more significant in a low-speed impact range of less than 10 m/s.

The phonon specific heat capacity, phonon group velocity and mean free path of the Bi₂Te₃-based alloy could be effectively controlled by phonon engineering including nano-modification, superlattice structure, nanocomposite, doping and introduction of the dislocation array to achieve a sufficiently low lattice thermal conductivity. The electrical transport properties were optimized by the carrier engineering and energy band engineering techniques, and the synergistic regulation of the electrical and thermal transport properties was realized. As a result, the thermoelectric properties of Bi₂Te₃-based alloys were significantly improved. This provided a scientific and technical support for the application of thermoelectric devices. However, a lot of work remained in improving the thermoelectric properties of Bi₂Te₃ materials by phonon, carrier and energy band engineering. Nevertheless, the large-scale commercialization of Bi₂Te₃-based alloys as thermoelectric materials still attracts much attention. Also, interdisciplinary research in thermoelectricity and other disciplines becomes popular. The future research and large-scale application of Bi₂Te₃ thermoelectric materials are expected.

The functional unit order is an effective strategy to optimize the performance of materials. For Ag₂Se-based TE films, the TE functional units ordering (such as coherent adjacent Ag₂Se grains, good crystalline qualities, continuous grain boundaries, and core-shell nanostructure) and flexible functional units were constructed in Ag₂Se-based films. The effective regulation of electron and phonon transport and flexibility was realized due to the synergistic effect between different functional units, thus increasing the electrical conductivity, reducing the thermal conductivity, and improving the flexibility. The more work are needed to verify whether the relevant strategies are universal.

The dielectric properties were reduced via optimizing film growth conditions. However, some related problems were not solved, i.e., decoupling the physical mechanism of dielectric tunability and dielectric loss, and the optimization of high-frequency film dielectric performance testing and extraction. In terms of microwave devices, more efforts need to be made in microwave device design, simulation, preparation, and testing. In summary, to commercialize ferroelectric thin film-based tunable microwave devices, we need to clarify the physical mechanisms of thin film dielectric properties, optimize the growth conditions preparation processes (i.e., synergistic optimization of dielectric tunability and dielectric loss), and obtain high-quality films, thus laying a foundation for the application prospects of tunable microwave devices.

CSSEs have attracted great attention for the development of SSLBs because they can improve ionic conductivity and enhance the mechanical strength and stability of the SSEs by incorporating inorganic fillers into polymer electrolytes. Recent research progress on the CSSEs were summarized. The main components of the key materials of CSSEs, i.e., inert and active fillers in the polymer matrix, were classified and summarized. The advanced structures of CSSEs to withstand lithium metal reduction and high-voltage positive electrode oxidation were explored. In addition, the possible mechanism of ion conductivity in CSSEs wasalso discussed. Although CSSEs were used in battery applications, they still had some challenges in ion conductivity, lithium-ion transport mechanism, and interface compatibility. Some key issues need to be considered for the future studies, i.e., further increasing in the room-temperature ionic conductivity of CSSEs, understanding of ion conduction mechanism in different types of CSSEs, and optimizing the interphase between the solid electrolytes and the electrodes.

Selection of solar absorbers: Three categories of solar-absorbing materials including carbon materials (such as carbon nanotubes, carbon nanoparticles, and graphene), plasmonic metals (such as Al, Au, Ag, Cu) and ceramic nanomaterials (such as transition metal nitrides and carbides) and semiconducting nanomaterials (such as TiO_2-x, MoS₂) were developed. The present research focuses on the preparation of high-light-absorption photothermal absorbers with a low-cost and a superior chemical stability.

1) Structural design of solar steam generation system: The solar-vapor conversion efficiency (η) can be calculated by

Based on Eq. (1), the solar steam conversion efficiency is directly proportional to water evaporation enthalpy and water evaporation rate. It is proved that the latent heat of water vaporization in a porous nanostructure is lower than the standard value. However, it is still unclear how to obtain a low water evaporation enthalpy by regulating the porous structure of the solar photothermal materials. It is crucial to obtain hierarchical nanostructures and evaluate its water vaporization latent heat. In order to increase the specific surface area of solar photothermal materials, porous foams and gels are designed and prepared, but the preparation of these artificially hierarchical materials is a time-consuming and costly process. It is thus imperative to explore naturally hierarchical plants that are low-cost and easy to obtain.

2) Efficiency and mechanism of VOCs photodegradation in water: Solar-driven steam generation is utilized to produce fresh water by removing salts, heavy metals, micro-organisms, and most organic pollutants in raw water. However, it also accelerates the volatility of the volatile organic compounds (VOCs) and enrich them in the distilled water if the water source is contaminated by VOCs because of their similar boiling point with water, which poses a serious threat to the atmospheric environment and human-being health. VOCs such as alcohols, aldehydes, ketones, olefins, and aromatic compounds tend to migrate and accumulate in different environmental media such as soil, water, and air. The World Health Organization and China's sanitary standards for drinking water quality both have established limit values for VOCs. Consequently, it is crucial to efficiently remove VOC during the solar-driven interfacial evaporation.

3) The relative merits of different electricity generation systems: Solar-driven steam-electricity system is mainly divided into integrating electricity generation system and self-powered electricity generation system. The integrating electricity generation module integrates a solar water evaporation module and an electricity generation module by rationally connecting the solar water evaporation module with photovoltaic device harvesting light energy, thermoelectric module recovering heat energy from water vapor, membrane structure utilizing salt concentration gradients and triboelectric nanogenerator based on steam condensation process. The electricity generation capacity of an integrated electricity generation system typically depends on the electrical output performance of the power generation module itself. Though the integrated electricity generation system can be commercially produced and has a stable performance, it has some problems such as complex structure, high cost and non-adaptability of different modules, which limit its practical application. The self-powered electricity generation system does not connect solar water evaporation module with other auxiliary equipment for electricity generation. It generates electricity directly by a hydrovoltaic effect, and its evaporation-electricity generation performance mainly depends on the microstructure of the photothermal material, such as the size of micro-/nano-pore channels and the zeta potential. The currently reported output currents range from nanoamps to microamps, and exploring current output in the milliamp range is a further research direction.