Ultra-high performance concrete (UHPC), as an advanced cementitious fiber-reinforced composite material, is often widely used as a load-bearing member in the critical parts of various types of large-span and high-rise structures. However, the increasingly frequent fire disasters have posed a great challenge to the safe serviceability of UHPC. Steel fiber, as an important factor for UHPC to achieve ultra-high toughness, the cracking mechanism of steel fibers exposed to high-temperature will directly affect the high-temperature residual properties of UHPC. Although some scholars have carried out studies on the residual properties of UHPC after high temperature, there are less systematic studies on the influence of steel fibers on the mechanical and thermal properties of UHPC, especially on the deterioration mechanism of steel fibers exposed to high temperature. In view of this, this work systematically investigates the influence of steel fiber on the mechanical and thermal properties of UHPC exposed to high temperature. The mathematical model between steel fiber dosage-temperature-mechanical properties/thermal properties was established. In addition, the changes in the pull-out behavior of steel fibers from the UHPC matrix at different temperatures were investigated for the first time, and the deterioration mechanism of steel fibers exposed to high temperature was elucidated based on SEM tests.

The Onoda P.II 52.5 cement and the blending material produced by Jiangsu Sobute New Material Co. were used, respectively. At the same time, an appropriate amount of polypropylene fiber was mixed to reduce the risk of explosive spalling of UHPC. UHPC with 0%, 1.0%, 2.0% and 2.5% steel fiber volume mixing were prepared. The concrete molding size was 100 mm×100 mm×100 mm, and 28 d standard curing was carried out after the casting was completed. Then the mechanical and thermal properties were tested after high temperature. The changes in the pull-out mechanism of steel fibers after high temperature were also investigated.

The compressive strength of UHPC after high temperature with different steel fiber dosage increases and then decreases with temperature. The addition of steel fibers can improve its overall compressive strength to different degrees. The tensile strength of UHPC shows almost monotonically decreasing with the increase of temperature, and the degree of loss rate is greater. Among them, the enhancement of steel fiber addition can obviously improve the tensile strength of UHPC at room temperature as well as at 200 ℃, but the degree of its enhancement with the increase in temperature gradually weakened. The elasticity modulus of UHPC with the increase in temperature first increased and then decreased, and the overall rule of change is similar to the compressive strength. The thermal conductivity of UHPC shows a monotonically decreasing law with temperature, which is mainly because the high temperature makes the organic fiber in UHPC melt, free water continues to evaporate, and the hydration products continue to decompose and so on to reduce the apparent density of the system. The addition of the steel fibers effectively improves the thermal conductivity of UHPC. The specific heat capacity showed the opposite law to the thermal conductivity. In addition, the maximum pull-out load, pull-out work and bond strength of steel fibers at 200 ℃ increased to varying degrees, compared to room temperature. At 400 ℃, relevant parameters began to have a substantial decline. When the temperature reached 600 ℃, the failure mode of steel fibers changed from pull-out to pull-off mode.

The compressive strength of UHPC increases at the beginning and then decreases with the increase of temperature, and reaches the peak value at 200 ℃. The addition of steel fiber does not affect the trend of UHPC residual compressive strength exposed to high temperature, but improve absolute value of its residual strength. Temperature changes pull-out mode of steel fiber. At 25-200 ℃, the surface of the steel fibers was smooth when they were pulled out, and the tensile strength of UHPC did not change much. At 400 ℃, steel fibers were attached to part of the matrix when they were pulled-out, UHPC tensile strength began to drop significantly. At 600 ℃, the failure mode of steel fibers changed from pull-out to pull-off mode. The tensile strength of UHPC with different steel fibers continues to decline, and the difference gradually narrowed. The elasticity modulus of UHPC reached its peak at 200 ℃ and began to decline rapidly, and its sensitivity to high temperature is much greater than the compressive and tensile strength changes, the value of the proposal can be used as an important indicator for the assessment of the safety of the relevant building after the fire, and the incorporation of steel fibers does not affect the overall development of its law. Steel fiber can significantly increase the thermal conductivity of UHPC, but will not change the overall trend of thermal conductivity with the increase in temperature. The thermal conductivity of UHPC decrease with the increase of temperature and the gradual decrease in the overall trend of change; specific heat capacity shows the opposite development law with the thermal conductivity. Copper-plated steel fiber as UHPC commonly used metal fiber, can maintain good mechanical properties and play a toughening crack resistance before 400 ℃. When the temperature reached at 600 ℃ and later, the degree of deterioration of copper-plated steel fiber itself is greater than the fiber-matrix interface transition zone, which leads to UHPC tensile and compressive drop in coordination, it is recommended that the steel fiber's own high-temperature performance enhancement should be focused on under this condition.

De-icing salt has been a common way to melt snow accumulated on concrete structures. But this often leads to the deterioration of concrete structures. Therefore, frost resistance durability of concrete subjected to de-icing salt is crucial. Conventionally, concrete is allowed to freely deform during single-side freezing and thawing (SSFT) tests. However, a concrete specimen is only equivalent to a unit of the concrete component in actual, and its deformation subjected to SSFT is restrained by the surrounding concrete, leading to alterations in salt frost damage. The differences in the evolution of concrete's frost damage between the laboratory and actual engineering are highlighted. The different internal damage of concrete subjected to freezing and thawing with pure water and de-icing salt is usually related to the water migration and phase transformation inside the concrete. However, the only difference between restrained concrete and unrestrained concrete is the restraint effect. The significant difference in internal damage of the two concretes subjected to SSFT cannot be explained by classical theory. Additionally, the influence of restraint effect on water migration in concrete also needs to be clarified. Thus, it is crucial to study the effect of restraint on the internal damage of concrete subjected to SSFT, which will provide guidance for the mix design of high frost resistant concrete in cold regions. In this paper, restraint ring and anchors were designed to confine the deformation of concrete to simulate the serving condition during SSFT tests. The relative dynamic elastic modulus and water absorption in volume of restrained and unrestrained concrete with a water cement ratio of 0.60 were tested after every 4 SSFT cycles. The temperature and humidity sensors were used to monitor the evolution of the relative humidity (U_IRH) inside the concrete during the SSFT test. The influence and mechanism of restraint effect on the internal damage of concrete subjected to SSFT were clarified from the perspective of moisture migration.

P·I 42.5 cement complying GB 8076—2024, natural river sand with a fineness modulus of 2.68, and graded stone by uniformly mixing the crushed stone with 5-10 mm and 10-20 mm in a mass ratio of 4∶6 were used to prepare fresh concrete with water to cement ratios of 0.60. The slump and air content of fresh concrete are 120 mm and 1.4%, respectively, tested according to GB/T 50080—2016. The 28 d cube compressive strength of the concrete specimen is 28.3 MPa, tested according to GB/T 50081—2019.

Two groups of moulds were used to cast concrete specimens. For unrestrained specimens, cylindrical plastic moulds with a diameter of 100 mm and a height of 70 mm were used. For restraint specimen, a restraint device was designed, which is used as the mould to cast a restrained concrete specimen meanwhile used as the restraining device to limit concrete deformation in SSFT tests. The device consists of a restraint ring, 24 restraint anchors, a counter-force ring, and a flange. All parts of the device are made of 304 stainless steel.

The curing and pre saturation of concrete specimens were carried out according to the salt freezing method in CIF test and GB/T 50082—2009. The CDF/CIF TESTER produced by Schleibinger Geräte was used to provide SSFT cycles. After every 4 SSFT cycles, the relative dynamic elastic modulus (r_n) and water absorption in volume (V_n) of the concrete specimens were tested. The evolution of internal relative humidity (U_IRH) of 28 SSFT cycles was monitored, and the pore size ratios of restrained and unrestrained concrete during the first cooling cycle were calculated.

The results show that after every one freeze-thaw cycle, the U_IRH of both restrained and unrestrained concrete increased, resulting in hysteresis of the U_IRH-temperature curves, which reflects the process of water migration inside concrete subjected to SSFT cycles. The hysteresis of the U_IRH-temperature curves of unrestrained concrete is more significant than that of restrained concrete. After 28 SSFT cycles, the U_IRH increment, r_n loss and V_n of restrained concrete are 30%, 32% and 28% less than those of unrestrained concrete, respectively.

The calculated results based on the U_IRH-temperature curves show that the radius of pores (r_pr) in restrained and unrestrained concrete increases with SSFT cycles. After 28 SSFT cycles, the r_pr of restrained concrete is 58% less than that of unrestrained concrete. A model for the internal water migration of concrete subjected to SSFT is established to analyze the influence of restraint on the water migration. The ratio of pore radius of the restrained and unrestrained concrete before freezing point during the cooling process in first SSFT cycle was calculated. The results indicate that under the influence of restraint, the water migration inside the concrete driven by SSFT is effectively weakened.

The main conclusions of this paper are summarized as following. The conventional SSFT test of unrestrained concrete overestimates the internal damage. The restraint effect weakens the water migration before the freezing point of concrete during the cooling process, which is the basis for reducing internal damage induced by SSFT cycles. Moreover, the restraint effect also reduces the pore size expansion of concrete subjected to SSFT cycles, which is manifested macroscopically as less loss of relative dynamic elastic modulus and less increase of water absorption in volume.

White light emitting diode (LED) is widely used as a high-quality lighting source, and its performance depends critically on the selected light-emitting materials. Early white LEDs were mainly composed of a combination of blue light chips and yellow light emitting phosphors, but suffered from low color rendering index, high color temperature, and blue light hazards. To solve these problems, researchers have turned to improving the color rendering performance of white LEDs by using red phosphors that can be excited by blue light.

Professor Zhiguo Xia's group at the State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, has proposed a new material design principle for Eu²⁺-activated oxide phosphors that are difficult to obtain red light emission: Eu²⁺ occupying a low coordination number of polyhedra can cause a large crystal field cleavage, which can in turn realize red light or even near-infrared emission. A novel Sr₃Ga₄O₉ phosphor matrix with multiple low-coordination cation lattice sites was designed and synthesized, and broadband red light emission under 450 nm blue light excitation was successfully achieved by doping Eu²⁺, with the emission peak located at 618 nm.

Sr₃Ga₄O₉∶Eu²⁺ and Sr₃Ga₄O₉∶Eu²⁺, 0.05Zn²⁺ phosphors were prepared by high-temperature solid-phase method. According to the proportional content of the different raw materials in the chemical formula, SrCO₃ (A.R.: 99.5% analytically pure), Ga₂O₃ (A.R.), H₃BO₃ (A.R.) and Eu₂O₃ (99.99%) were weighed and ground in an agate mortar for half an hour. The mixture was transferred to an alumina crucible and fired at 900 ℃ in an air atmosphere for 6 h, then cooled naturally to room temperature. The mixture was ground again to powder form and refilled into the crucible, which was placed in a tube furnace with a continuous flow of reducing gas ([V(N₂)∶V(H₂)] = 80%∶20%). The samples were burned at 1 200 ℃ for 10 h, cooled to room temperature naturally, and then ground carefully to obtain the desired phosphor samples.

A white light illumination device was fabricated using a blue LED chip stacked with green and red phosphors. The green light was partly provided by Lu₃Al₅O₁₂∶Ce³⁺ (LuAG∶Ce³⁺) commercial green phosphor, while the red light was contributed by the experimentally prepared Sr₃Ga₄O₉∶Eu²⁺,0.05Zn²⁺ phosphor.

The Sr₃Ga₄O₉ cell structure belongs to the P-1 space group of the triclinic crystal system. The results of XRD characterization tests indicate that the prepared samples are in pure phase. A heterovalent substitution strategy was used to introduce Zn²⁺ into Sr₃Ga₄O₉, and the XRD results showed that this co-doping did not change the structure of the matrix material. Further analysis revealed that Zn²⁺ ions replaced Ga³⁺ sites, while Eu²⁺ may occupy two 6-coordinated Sr²⁺ lattice sites.

Under the excitation of 450 nm, the Sr₃Ga₄O₉∶xEu²⁺ phosphor emits red light at 618 nm, and the emission intensity reaches the maximum at x = 0.04. The low-temperature emission spectra at 78 K can be decomposed into two Gaussian peaks, which indicates that the Eu²⁺ occupy two different luminescent centers and emit the red light, respectively. The fluorescence lifetime decay curves of the Sr₃Ga₄O₉∶0.04Eu²⁺ sample at low temperature (78 K) can be well fitted by the double-exponential function, which again demonstrates that the Eu²⁺ ions occupy two different lattice sites.

After Zn doping, the luminescence intensity of Sr₃Ga₄O₉∶Eu²⁺,0.05Zn²⁺ phosphor was enhanced and the thermal stability was also improved significantly. The enhancement of thermal stability is from the presence of defects in the crystal, which compensates for the heat loss. The average decay lifetime of the co-doped Zn²⁺ is significantly higher than that of the sample without Zn²⁺, and a clear bulge of new pyroelectric peaks at 210 ℃ and 325 ℃ is clearly seen. Thus the addition of Zn²⁺ can increase the concentration of traps and contribute to the formation of deeper defect energy levels.

For Sr₃Ga₄O₉∶0.04Eu²⁺, the thermal stability is poor due to its shallow trap depth, which causes the electrons to escape from the shallow traps immediately at room temperature. Whereas, Sr₃Ga₄O₉∶Eu²⁺,0.05Zn²⁺ has deeper trap 2 and trap 3 in it, and thus maintains a good thermal stability even at high ambient temperatures.

The white LED device was fabricated to cover the entire visible region of the emission spectrum with a color rendering index of Ra = 87, a color temperature of CCT = 3 500 K, and CIE color coordinates of (0.410, 0.380).

Novel Sr₃Ga₄O₉∶Eu²⁺ red phosphors that can be excited by blue light were designed and prepared, and their crystal structures and spectral properties were investigated. It is found that the Sr₃Ga₄O₉∶Eu²⁺ red light emission comes from two low-coordination polyhedra Sr1O₆ and Sr3O₆ occupied by Eu²⁺ with 5d-4f leaps. Zn²⁺/Ga³⁺ heterovalent substitution occurs by the introduction of Zn²⁺ into Sr₃Ga₄O₉, which is proved to introduce a defective energy level through the fluorescence decay lifetime and thermoluminescence to improve the thermal stability of the phosphor. A white LED device prepared using a commercial blue LED chip, Sr₃Ga₄O₉∶Eu²⁺,0.05Zn²⁺ red phosphor, and a commercial green phosphor, LuAG∶Ce³⁺, has a high color-rendering index, Ra = 87, and a low correlated color temperature, CCT = 3 500 K, which suggests that it is suitable for warm-white-light lighting.

Bi₂Te₃-based alloys are the most well-known thermoelectric materials for room-temperature applications. The existing Bi₂Te₃-based thermoelectric materials produced commercially are prepared by a zone melting method, resulting in highly oriented materials with high thermoelectric properties. However, the ingots produced by the zone melting method are prone to disintegration due to the weak van der Waals forces between the layers of Te atoms, leading to the poor mechanical properties and the proclivity for fracture during processing and practical applications. It is thus important to improve the mechanical properties and machinability of Bi₂Te₃-based materials with the excellent thermoelectric properties. This work was to prepare a n-type Bi₂Te_2.7Se_0.3 thermoelectric material via spark plasma sintering combined with hot extrusion (SPS+HE). The parameters affacting hot extrusion process and preparation of n-type Bi₂Te_2.7Se_0.3 materials under different process conditions were optimized, and the thermoelectric performance and mechanical properties were investigated. In addition, the machinability of the extruded Bi₂Te_2.7Se_0.3 material was also analyzed.

The commercial zone-melting crystal rods were pulverized and sintered via spark plasma sintering (SPS) at 350 ℃ and 20 MPa for 5 min. The ingot was then put into a hot extrusion mold, and treated at 390-550 ℃, extrusion speed of 0.05-0.50 mm/s, extrusion ratio of 9∶1, and extrusion angle of 30°. Under the determined hot extrusion parameters, n-type Bi₂Te_2.7Se_0.3 materials were prepared by different processes, i.e., the raw materials were ground for 1 h and then sintered by SPS and subsequent hot extrusion (HE); the raw materials were melted at 700 ℃ for 5 h and then cold-pressed (MT), and sintered by subsequent hot extrusion (HE); the raw materials were melted and cold-pressed, followed by annealing and hot extrusion (MT+AN+HE); and the raw materials were ground for 1.5 h and cold-pressed, followed by hot extrusion (BM+HE). The phase structure was analyzed by X-ray diffraction (XRD, D8 Advance, Bruker Co. Ltd., USA) at room temperature. The microstructure and fracture morphology were determined by scanning electron microscopy (SEM, Supra 55, ZEISS Co., Germany). The electrical conductivity and the Seebeck coefficient were measured by ZEM-3 (ULVAC Co. Ltd., Japan). The thermal diffusivity (λ) was measured by a laser flash method (LFA457, Netzsch Co., Germany). The thermal conductivity was calculated according to κ=λρC_p, where C_p was the heat capacity according to the Neumann-Kopp rule, and the density ρ was measured based on the Archimedes prinicple.

The results show that n-type Bi₂Te_2.7Se_0.3 thermoelectric materials can be prepared under optimum process conditions (i.e., extrusion temperature of 400 ℃ and extrusion speed of 0.05 mm/s). The XRD patterns of all the hot-extruded samples agree well with a rhombohedral structure (JCPDS 50-0954) of Bi₂Te_2.7Se_0.3. No impurity peaks appear. The SEM images reveal that Bi, Te, and Se are homogeneously distributed in the materials without any impurity phases. The electrical conductivity σ of all the samples decreases with increasing temperature, having a metallic conduction behavior. The σ of the MT+AN+HE sample is the maximum value of 1.4×10⁵ S/m. All the samples have negative Seebeck coefficients S, indicating that electrons are the major charge carriers. The S of the SPS+HE sample is the maximum value of 191.0 μV/K. At 300 K, the SPS+HE sample has the maximum power factor PF of 37.0 μW/(cm·K²). However, the PF of the material gradually decreases with the increase of temperature, which is consistent with the change of electrical conductivity and Seebeck coefficient due to the intrinsic excitation. The lattice thermal conductivity of MT+HE sample is the minimum value (i.e., 0.9 W·m^-1·K^-1). The maximum zT value reaches 0.83 at 400 K for SPS+HE sample and its Vickers hardness is 602.5 N/mm², which is twice greater than that of the zone-melting sample. This is mainly due to the dynamic recrystallization process of grain refinement during the hot extrusion process. The machinability of the extruded Bi₂Te_2.7Se_0.3 material are improved. The thermoelectric micron-sized particles with a minimum size of 147 μm can be achieved, providing a promising material for the development of Bi₂Te₃-based thermoelectric micro-devices.

The optimal hot extrusion process parameters for n-type Bi₂Te_2.7Se_0.3 thermoelectric materials were obtained (i.e., extrusion ratio 9∶1, extrusion angle 30°, extrusion temperature 400 K and extrusion speed 0.05 mm/s). A series of n-type Bi₂Te_2.7Se_0.3 thermoelectric material samples were prepared via hot extrusion under the optimum process conditions. The Bi₂Te_2.7Se_0.3 sample prepared by SPS and sebsequent hot extrusion (SPS+HE) had the maximum zT value 0.83 at 400 K, and its mechanical properties were improved. The Vickers hardness of the SPS+HE sample could reach 602.5 N/mm², which was twice greater than that of the zone-melting Bi₂Te_2.7Se_0.3 material. In addition, the machining performance was also improved, and the thermoelectric micron-sized particles with the minimum size of 147 μm could be machined for the extruded Bi₂Te_2.7Se_0.3 sample, providing a promising material for the Bi₂Te₃-based thermoelectric micro-devices.

β-Ga₂O₃ as a ultra-wide bandgap semiconductor material with a great development potential has a bandgap width of 4.8 eV, and a theoretical breakdown electric field of 8 MV/cm. Its Baliga advantage value of β-Ga₂O₃ is also several times greater than that of silicon carbide (SiC) or gallium nitride (GaN). However, the low thermal conductivity of β-Ga₂O₃ seriously affected the device's heat dissipation and the application in the high-power field. Wafer bonding is an indispensable key technology in semiconductor devices, which enables the formation of new structures, efficient fabrication, device integration, and cost reduction. The silicon substrate with the advantages of low cost and high thermal conductivity is an important substrate material for heterogeneous integrations. SiO₂ layer is easily formed on the surface of silicon, and the surface structure of SiO₂ consists of a Si-O-Si three-dimensional network structure together with an unsaturated Si-O structure, and the existence of this structure is conducive to reducing the probability of bubble generation, thus improving the quality of the bonding bonding interface, so it is easier to realize the bonding of silicon and β-Ga₂O₃ by using SiO₂ as an intermediate layer. This paper was to analyze the effect of O₂ plasma activation on the wafer surface, including changes in contact angle, roughness, and surface chemical composition. In addition, the effect of annealing temperature on the strength and thermal stability of the bond structure and the structure and composition of the β-Ga₂O₃/SiO₂ bond interface was also discussed.

SiO₂/Si substrate with the thickness of 550 μm and β-Ga₂O₃ single crystals sunstrate with the thickness of 650 μm were prepared. The two wafers were cleaned with alcohol and acetone via ultrasonic cleaning , and isopropanol for 10 min, then rinsed with deionized water and blown dry with N₂. Also, β-Ga₂O₃ and SiO₂/Si substrates were processed by oxygen plasma in a reactive ion etching (RIE) equipment (PlasmaPro 100 Cobra, Oxford Instrument Co., UK) at 50 mtorr and 300 W. The two activated wafers to be bonded were initially aligned and bonded, and pressure was applied to the wafer surface with tweezers to fully expel the gas from the bonding interface and make the bonding closer. Finally, the bonded wafers were placed on a pressurized platform for pressurized resting to complete the pre-bonding process. The pre-bonded wafers were placed in the annealing furnace for low-temperature annealing to enhance the bonding strength. A testing machine was used to determine the bond strength of the bonded pairs at different annealing temperatures. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to determine the bonded interface images and elemental distribution.

The contact angle and hydroxyl density with treatment time were investigated via adjusting the treatment time of O₂ plasma. The hydrophilicity of SiO₂ and β-Ga₂O₃ wafers greatly enhances, and the hydroxyl density on the wafer surface improves as the O₂ plasma activation time increases. O-H in β-Ga₂O₃ is produced by the breakage of oxygen atoms (O_w) in water that combines with surface Ga atoms (Ga_s) to form Ga_s-O_wH_w, and H combines with surface oxygen atoms (O_s) to form O_s-H_w. The effect of annealing temperature on the surface quality and bonding strength was also investigated. The results indicate that the bonding strength increases with the increase of annealing temperature. When the annealing temperature increases to 500 ℃, the bond strength increases to 7.31 MPa. However, this process leads to the cracking of the β-Ga₂O₃ substrate. Some cracks in β-Ga₂O₃ occur due to the thermal stress resulting from the different thermal expansion coefficients between SiO₂ and β-Ga₂O₃. β-Ga₂O₃ and SiO₂ surfaces are atomically bonded with no interfacial voids or cracks. The bonding interface's elements O, Ga, and Si are fully diffused during annealing.

In this study, a high-strength bonding of β-Ga₂O₃ and SiO₂ substrates was achieved at low temperatures via O₂ plasma activation. The FTIR analysis showed an increase in the hydroxyl density of the β-Ga₂O₃ and SiO₂/Si surfaces with the increase in plasma treatment time. The XPS and FTIR analysis demonstrated the changes in the hydrophilic functional groups on the surfaces before and after the plasma treatments. The bond strength gradually increased with the annealing temperature and reached 7.31 MPa, but unbonding and cracking also occurred at the edges of the bonded regions. The SEM and TEM images of the bonding interface demonstrated the existence of tight bonding between β-Ga₂O₃ and SiO₂ with full diffusion of elements Ga, O, and Si. In addititon, a model for direct bonding of oxygen plasma-activated β-Ga₂O₃/SiO₂/Si was also proposed.

Silicon is considered as one of the most promising lithium anode materials due to its high lithium storage capacity. However, the volume expansion effect and semiconductor properties of silicon hinder its practical application. Two effective strategies exist for mitigating this issue, i.e., reducing the size of silicon to nanoscale; and introducing non-active matrices to improve stability and conductivity. In recent decades, the incorporation of iron silicide (FeSi₂) with silicon (Si) has attracted much attention due to its high conductivity, mechanical hardness and environmental friendliness. Unfortunately, the Si/FeSi₂ nanocomposites are mainly prepared by high-energy ball milling, which has some drawbacks such as low purity, large particle size, and uncontrollable morphology, making it difficult to achieve the commercial application. Porous silica derived from clay minerals is extensively investigated for the preparation of silicon nanoparticles by magnesiothermic reduction. In addition, porous silica has a large specific surface area, which facilitates the effective dispersion and loading of iron oxides. The reduction of iron oxide and silica dioxide to Fe and Si can be achieved by magnesium. In this paper, a natural montmorillonite (Mnt) with a high content of silica (65%, in mass) was utilized as a raw material. Silicon/iron silicide (Si/FeSi₂) nanocomposites were synthesized via modification of Mnt (including acid etching and iron loading) and molten salt-assisted magnesiothermic reduction.

A porous silica (SiO₂) was prepared via leaching Mnt at a high concentration of HCl. Subsequently, a iron oxide was loaded on SiO₂ via precipitation in different Fe/Si mass ratios (i.e., 0, 10%, and 20%). Modified SiO₂ (Fe-SiO₂) was mixed with Mg and NaCl in a mass ratio of 1.00∶0.85∶3.00. Afterwards, the mixture was heated in an inert atmosphere at 650 ℃ for 5 h in a heating rate of 5 ℃/min. After cooling naturally, the product was washed with 1 mol/L HCl and 1% (in mass) HF in sequence to remove the by-product. Finally, it was rinsed with ultra-pure water until neutral and dried at 80 ℃. The sample were named as Si, Si/FeSi₂-1, and Si-FeSi₂-2, corresponding to the Fe/Si mass ratio of 0, 10%, and 20%, respectively.

The XRD patterns show that Si/FeSi₂ nanocomposites are consisted of Si and ɑ-FeSi₂. An increase in Fe content in precursors leads to a higher α-FeSi₂ content in the product. Based on the results of RIR value method and elemental composition analysis, the mass ratio of FeSi₂ to Si in Si/FeSi₂-1 and Si/FeSi₂-2 is 5% and 10%, respectively. The average grain sizes of Si, Si/FeSi₂-1, and Si/FeSi₂-2 are 20, 40 nm, and 48 nm, respectively, according to Scherrer's equation,. The grain size of silicon increases with the augmentation of Fe content in the precursor. This phenomenon analyzed by both transmission electron microscopy and scanning electron microscopy images is since the melting temperature of silicon nanoparticle is lower than that of bulk silicon (~1 410 ℃), the addition of iron greatly increases the local temperature (ΔH_Fe2O3= -309 kJ/mol vs. ΔH_SiO2= -130 kJ/mol), and the addition of Fe can effectively reduces the melting point of nanocrystalline grains in Fe/Si eutectic alloys. Moreover, the specific surface areas of Si, Si/FeSi₂-1, and Si/FeSi₂-2 are 89, 42 m²/g, and 33 m²/g, respectively. The gradual decrease in specific surface area coincides with the concurrent increase in grain size.

As anode electrodes, Si/FeSi₂ nanocomposites exhibit a higher initial coulombic efficiency, compared with Si due to the smaller specific surface area. The rate performance show that Si exhibits higher capacity than Si/FeSi₂-1 and Si/FeSi₂-2 at a low current density of 0.1 A/g. However, when the current density increases gradually, the capacity of Si is lower than that of Si/FeSi₂-1 and Si/FeSi₂-2. This phenomenon indicates that the incorporation of ɑ-FeSi₂ enhances both electron conduction and ion diffusion rates. After undergoing 500 cycles at a current of 1 A/g, the capacities of Si, Si/FeSi₂-1, and Si/FeSi₂-2 are 417, 781 mA·h/g, and 561 mA·h/g, respectively. The corresponding capacity retention rates are 35%, 77%, and 50%, respectively. The cycling performance demonstrates that Si/FeSi₂ nanocomposites have a more robust structure, compared to the Si electrode. In addition, the cyclic stability of Si/FeSi₂-1 is inferior to that of Si/FeSi₂-2, potentially due to the reduced structural stability resulting from a larger particle size.

The Si/FeSi₂ nanocomposites were prepared via modification of montmorillonite and molten salt-assisted magnesiothermic reduction. This method utilized magnesium powder to reduce Fe₂O₃ and SiO₂ for the in-situ formation of Si/FeSi₂ nanocomposites. The composition of Si and FeSi₂ nanocomposites could be adjusted via altering the ratio of Si/Fe in the precursor. Moreover, the reduction of Fe₂O₃ resulted in an increase in grain size and a decrease in specific surface area of silicon nanoparticles. As anode materials, Si/FeSi₂ composites demonstrated the superior rate performance and cycle stability, compared to Si. This phenomenon could be due to the incorporation of ɑ-FeSi₂, effectively enhancing the kinetics of lithium ions and electrons on the electrode. In addition, as an inert substance, FeSi₂ could reduce the volume expansion effect of silicon, thereby maintaining the stability of electrode structural. The structural stability of the Si/FeSi₂-2 electrode was relatively weaker than that of the Si/FeSi₂-1 electrode. Therefore, Fe content in the precursor could be of paramount importance for achieving high-performance silicon-based nanocomposite anodes.

The thermoelectric cooling technology has a potential in the field of refrigeration for 5G chips, infrared detector, and temperature controlling, etc.. To improve the efficiency of thermoelectric refrigeration and enhance the competitiveness of thermoelectric refrigeration technology, it is of great significance to develop high-performance thermoelectric refrigeration materials. However, the refrigeration temperature of the first-stage refrigerator is 230 K based on commercial Bi₂Te₃-based alloy, and the refrigeration temperature of the multi-stage refrigerator is as low as 150 K, while the thermoelectric performance of the commercial Bi₂Te₃-based alloy is sharply reduced at below room temperature, which is difficult to meet the needs of low-temperature refrigeration. It is thus necessary to develop high-performance low-temperature thermoelectric cooling materials to meet the low-temperature refrigeration in the future.

The gyroscope operation relies on the electrostatic actuation and capacitance detection between the resonator and the planar electrode, with a typical gap ranging from 20-150 µm. When organic impurities adhere to planar electrode's surface, gap's capacitance changes, leading to fluctuations in the excitation detection signal and thereby affecting the accuracy and reliability of the gyroscope. TiO₂ thin film can effectively degrade organic pollutants through photocatalytic reactions. However, little research on its application on the surface of high-precision devices has been reported yet. The thickness of the TiO₂ thin film affects the generation and effective separation of photo-generated electron-hole pairs, thereby influencing the photocatalytic performance of the TiO₂ thin film. It is thus necessary to determine the thickness of the TiO₂ thin film on the surface of the planar electrode through experiments. In this paper, a TiO₂/Au composite thin film was designed to reduce the adsorption of organic substances on the surface of planar electrode and improve the thin film quality on the planar electrode surface.

TiO₂ thin films with different thicknesses were prepared on fused quartz substrates via radio frequency magnetron sputtering. The films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), UV-visible spectrophotometry (UV-Vis), and photocatalytic reaction analysis. The crystalline structure, grain size, surface morphology, and optical absorption characteristics of the TiO₂ thin films were analyzed to assess their impacts on the photocatalytic performance. The optimal thickness of the TiO₂ thin film suitable for the surface of the planar electrode was determined to be 240 nm. Finally, the planar electrode with a TiO₂/Au composite thin film was fabricated. The surface film layer of the planar electrode was characterized by nanoindentation, nano-scratching, and photocatalytic reaction equipment. The exceptional performance of the TiO₂-coated planar electrode in terms of adhesion, friction resistance, and photocatalytic self-cleaning properties was validated. The final step involved assembling the prepared flat electrode with a hemispherical resonator and using a capacitance meter to validate the superior performance of the TiO₂/Au composite film structured planar electrode for reducing the impact of organic impurities on the gap capacitance between the resonator and the planar electrode.

The XRD patterns show that the crystalline integrity and grain size of the film gradually increase as the thickness of TiO₂ thin film increases. The SEM and AFM images reveal that the surface roughness increases with the enlargement of grains as the film thickness increases. Based on the analysis of the properties of the TiO₂ thin film, TiO₂ thin film with a thickness of 240 nm exhibits the smallest bandgap width and the optimal photocatalytic performance. For the mechanical properties of the surface film layer on the planar electrode, the abrasion resistance of the planar electrode coated with the TiO₂ thin film is higher than that of the electrode uncoated with the thin film. Also, the adhesion between TiO₂ thin film and Au film on the surface of the planar electrode is comparable to the adhesion between Au film on the surface of the uncoated planar electrode and the substrate. The planar electrode undergoes photocatalytic self-cleaning experiments, TiO₂/Au composite film structured flat electrode can reduce the impact of organic impurities on the capacitance uniformity between the resonator and the flat electrode, thereby contributing to the enhanceed performance of the hemispherical resonant gyroscope.

The crystallinity, grain size, and surface roughness of the TiO₂ thin film increased with increasing TiO₂ film thickness. The TiO₂ thin film with a thickness of 240 nm exhibited the smallest bandgap width and appropriate grain size, thus demonstrating the optimal photocatalytic activity. The experimental verification of mechanical, photocatalytic, and capacitive performance of planar electrodes coated with TiO₂/Au composite films revealed that the adhesion of the TiO₂ thin film on the planar electrode met practical application requirements. Leveraging the excellent hardness and elastic modulus of the TiO₂ thin film significantly enhanced the abrasion resistance of the surface film layer on the planar electrode. Also, the efficiency of photocatalytic degradation of organic impurities on the planar electrodes coated with TiO₂ thin films was 1.6 times higher than that of the electrodes uncoated with the thin film. For assembling the prepared flat electrode with a hemispherical resonator and conducting capacitance meter testing, the TiO₂/Au composite film structured flat electrode could degrade organic impurities and reduce their impact on the gap capacitance between the resonator and the flat electrode under UV light exposure. It was evident that the TiO₂/Au composite film could have a promising practical application in the fabrication of resonant gyroscope flat electrodes.

Relying on their advantages of high water reducing rate, low dosage, and strong molecular structure design ability, polycarboxylate superplasticizers (PCEs) have become the most widely used admixture in the field of concrete admixtures. Adsorption of PCEs onto the surfaces of cement particles is the prerequisite for the dispersing performance of PCE molecules. Therefore, the influences of PCE molecules with different structures on the dispersion properties of cement slurry can be understood by revealing the adsorption process of PCE and the conformation characteristics of PCE after adsorption.

It should be noted that both the adsorption process and the conformation of the adsorbed PCE involve interfacial interactions between the organic and inorganic phases, which is the dominant factor determining the overall properties of the material. Recently, molecular dynamics (MD) simulations have been applied to the PCEs to elucidate its working mechanism. However, the molecular size of these model PCEs was much smaller than those typically used for industrial applications in order to reduce computation cost. Furthermore, only pure water was used, or only Na⁺, Ca²⁺ and Cl^- were additionally added to mimic a cement pore solution. The above methods make the results unable to accurately reflect the adsorption mechanism of PCE. Therefore, to better understand and effectively control the performance of PCE, more in-depth studies at the atomic and molecular scales are needed. Our research team innovatively used MD simulations in which the molecular size of the model PCE is consistent with that of PCE commonly used for industrial applications and simulates cement pore solutions. In addition, the adsorption mechanism of sulfonation modification of PCE was studied by combining the structure, dynamics and stability of interfacial connections.

The MD simulations used the Gromacs-4.6.7 software package and a conventional oplsaa force field with RESP charges. The UFF force field was used for the substrate. The equilibrium MD simulation was conducted under the NPT ensemble for a total run time of 20 ns at a relaxed liquid configuration (25 ℃). The solution environment simulations were performed using 3000 water molecules. A relaxation system was selected and the energy was minimized by the steepest descent method with a termination gradient of 100 kJ/(mol∙nm) before the relaxation. The system temperature maintained constant through a Nosé-Hoover thermostat, and periodic boundary conditions were applied to all three dimensions. Long-range electrostatics within a relative tolerance of 1×10^-6 is calculated by the particle mesh Ewald method and a cut-off distance of 1 nm was used as the Ewald interaction and van der Waals interactions. The bond lengths of hydrogen atoms were constrained by the LINCS algorithm. A leap-frog algorithm was used with a time step of 1 fs.

Under cement solution conditions, the side chains of PAA-g-PALS molecules were not entangled with the backbone (present a network-like conformation), while a small part of the side chains of PAA-g-HPEG molecules will curl onto the backbone (most of the side chains curled into a mushroom shape). In addition, the rotation radius of PAA-g-PALS (7.8 nm) was significantly larger than PAA-g-HPEG (7.1 nm) due to the large amount of —SO₃^- with strong electrostatic repulsion.

Part of the backbone of PAA-g-HPEG was adsorbed to the particles (the side chains curl and wrap the backbone), while PAA-g-PALS was adsorbed to the substrate surface by the "flat" mode of the individual side chains (the backbone and other side chains extend into the solution). Compared with PAA-g-HPEG, the binding point of sulfation modified PCE and the substrate is relatively scattered and increases the binding surface with the substrate, which is consistent with the calculation of binding force PAA-g-PALS (174.3 kJ/mol)> PAA-g-HPEG (134.5 kJ/mol).

The vertical heights of the individual PAA-g-HPEG and PAA-g-PALS molecules on the substrate surface were 6.8 nm and 19.7 nm, respectively, and the coverage rate was 26.4% and 21.2%, respectively. When considering the polymer cross-dimensions parallel and perpendicular to the particle surface, the penetration volume formed by PAA-g-PALS on the substrate surface increased by 132.64% relative to PAA-g-HPEG.

The main conclusions of this paper are summarized as following. PAA-g-HPEG and PAA-g-PALS tend to be comb and network conformations in cement slurry environment, respectively. Sulfonated modification dispersed the distribution of binding points between PCE and the substrate, improved the binding force between PCE and the substrate, extended the polymer far away from the substrate, and significantly increase the permeability volume (by about 132.64%). The present results not only provide new insights into the action mechanism of PCE at the atomic level, but also provide a theoretical basis for promoting the development of high performance cement-based materials.

As a kind of high-energy electromagnetic radiation, gamma rays can lead to radiation effect and environmental pollution. To ensure the reliability and safety utilization of gamma radiations in different areas, materials with reliable gamma shielding performances should be developed. The existing efforts are to use glasses as alternative for the conventional radiation shielding materials. As a typical window material, glass itself has attracted wide attention due to its unique chemical and physical advantages. Among them, lead-based radiation shielding materials are widely used in the field of ionizing radiation protection due to their reliable physical properties. In the nuclear industry and high energy physics laboratories, different materials can be used for temporary or permanent shielding. The lead-based glass doped with heavy metal oxide PbO is widely used in the field of radiation shielding for a long time with reliable light transmission properties, which facilitates the observation and monitoring of radiation areas. In this paper, three kinds of transparent silicate glass with different PbO contents were prepared by a high-temperature melting forming method. The gamma-rays protection abilities of the three kinds of glass were investigated through experimental and thermotical methods. The chemical and thermal stability of the glass were also analyzed. In addition, the influence mechanism of the structure and performance differences of the three kinds of glass was also discussed.

Analytical pure quartz sand (SiO₂, purity 99.5%, in mass, the same below), lead monoxide (PbO, purity 99.0%), potassium nitrate (KNO₃, purity 99.0%) and sodium nitrate (NaNO₃, purity 99.0%) were selected as raw materials for preparation of three kinds of gamma-rays shielding glass materials by a high-temperature melting method. An appropriate amount of raw materials was thoroughly mixed and added to 3 L quartz crucible and heated in air at 1 200 ℃. Then the raw materials were melted at 1 250 ℃ for 14 h. The uniform melt was quickly poured into the 150 mm×150 mm×50 mm stainless steel mold preheated at 750 ℃. Finally, the quenched glasses were annealed at 450 ℃ for 12 h and cooled naturally to room temperature.

The density of the three glass samples was tested based on the Archimedes principle. The thermal expansion coefficient of glass samples was measured by a model Netzsch DIL 402 expansion coefficient tester. The infrared spectra were measured by a model Perkinelmer Spectrum 100 FTIR Spectrometer. The Raman spectra were tested by a model HORIBA LabRAM HR800 high-resolution Raman spectrometer. The gamma ray shielding test was completed in China Academy of Engineering Physics. The chemical stability test was completed in Chengdu Guangming Glass Co., LTD. The acid stability and moisture stability of the glass samples were tested according to the national standards GB/T 7962.14-2010 and GB/T 7962.15-2010, respectively. The thermal stability of the galss was also determined.

Glass structure becomes more compact with the exchange of Pb alternatives of Si, K and Na. Increasing the density of glass is conducive to improving its shielding performance, but this leads to significant changes in the optical properties, thermal properties and chemical stability of glass due to the increase of heavy metal content. The preparation of high transmittance glass is more difficult. The coefficient of thermal expansion of glass samples gradually decreases from 103.6 × 10^-7 ℃^-1 to 89.2 ×10^-7 ℃^-1, and the values of T_g and T_f also decrease with the increase of mass fraction of PbO in glass components. The change of the thermal properties of the three glass samples is a result of the joint action of PbO, Na₂O and K₂O. The FTIR spectra show that the added PbO is transformed into [PbO₄] units with 4 coordination numbers into the network under the condition of sufficient free oxygen as PbO content in the glass increases. These [PbO₄] units participate in the formation of silicate networks. The Raman spectra indicate that the silicate network structure is de-polymerized as PbO content is increased, and the enhancement of the characteristic vibrational peaks corresponding to the Pb-Si-O indicates an increase in the degree of participation of the [PbO₄] units in the formation of silicate network. The ion packing density of the glass sample gradually increases with the increase of PbO content, which shows that the compressible free volume per unit volume of the glass sample becomes smaller. PbO with a high concentration (x = 0.50 and x = 0.66) is used as a glass-forming agent, and the [PbO₄] structural units can form a connection structure with the silicon oxide tetrahedron, which plays a role in repairing the glass network.

The permeability of the glass can be improved via strictly controlling the iron content of the introduced raw materials and making full use of the double alkali effect of the glass. The actual shielding performance of glass is not a result of simple addition of various components. This can be affected by the actual composition content of the glass, melting process and endoplasmic uniformity. The influence of glass expansion coefficient on the thermal stability is much greater than that on its chemical stability.

PbO with a high concentration (x = 0.50 and x = 0.66) was used as a glass-forming agent, and the [PbO₄] structural units could form a connection structure with the silicon oxide tetrahedron, playing a role in repairing the glass network. The measured shielding rate of ⁶⁰Co gamma-ray could be 50% and the glass with 50% PbO content (in mass fraction) exhibited a linear transmission rate of more than 85% in the range of 400-800 nm. In addition, the reliable thermal and chemical stabilities were also reflected in the glass.