Pressure-assisted sintering effectively promotes ceramic densification by providing additional sintering driving force, offering a feasible method for the fabrication of piezoelectric ceramics with high density and performance. Furthermore, Pressure-assisted sintering allows for flexible control of sintering temperature and time, which enables the preparation of high-density ceramics with varying grain sizes. Additionally, pressure-assisted sintering can combine with sintering atmospheres, electric currents, and other physical fields to engineer the defects. Uniaxial pressure can also introduce texture in some piezoelectric ceramics. Therefore, pressure-assisted sintering is expected to achieve synergistic regulation of density, grain size, defects, and grain orientation in piezoelectric ceramics, thereby comprehensively enhancing the mechanical, dielectric, piezoelectric, and optical properties of the material.

However, precisely controlling the process of pressure-assisted sintering is challenging due to limitations in sintering equipment and mold materials. The issue of reproducibility caused by inhomogeneous pressure or current distribution deserves concern. For fundamental scientific research, pressure-assisted sintering can bring novel physical properties to piezoelectric ceramics. The clear demonstration of the sintering behavior of piezoelectric ceramics is the prerequisite for the in-depth understanding of these properties. Therefore, developing in-situ sintering characterization may assist in fully understanding the relationships among pressure, sintering behavior, and physical properties. For practical applications, high cost is the most significant barrier to scaling up pressure-assisted sintering for piezoelectric ceramics. The improvement in equipment and optimization of control systems are in high demand to overcome these technical challenges.

ZnO inorganic white paint is composed of ZnO pigment and a potassium silicate binder as a type of thermal control coating of spacecraft. It is commonly used for spacecraft antennas, cabin sections, and radiator surfaces. In general, the solar absorptance of ZnO inorganic white paint is lower than that of other thermal control coatings. ZnO mainly exists in the form of wurtzite structure, and its zinc oxygen ratio often deviates from stoichiometry, generating intrinsic defects internally. Oxygen deficient compounds are generally generated due to the easy escape of oxygen in zinc oxide. During the in-orbit service of spacecraft, ZnO inorganic white paint is subjected to space environmental effects, such as electrons, protons, ultraviolet (UV), and atomic oxygen (AO), resulting in its performance degradation and affecting the stable service of spacecraft in orbit. It is thus necessary to investigate the degradation and damage mechanism of ZnO inorganic white paint to support the optimization of material properties.

ZnO pigment sample was a circular flake powder, which was pressed into a grinding tool with an inner diameter of 25 mm using a machine. The pressure parameter is 10 kN and the pressure time is 5 min. A circular sample with potassium silicate-coated zinc oxide (KSZ) inorganic white paint with a diameter of 30 mm, composed of K₂SiO₃ binder and ZnO pigment was prepared on an aluminum substrate by a spray coating process.

Space environmental irradiation experiments were conducted on ZnO pigments and KSZ coatings by environmental irradiation and low orbit AO equipment. The electron energy of ZnO pigment irradiation experiment is 50 keV, with a fluence of 1×10¹⁶ cm^-2, a proton energy is 50 keV, with a fluence 6×10¹⁵ p/cm² and the UV dose is 5000 ESH. The AO fluence is 1×10²¹ atoms/cm². The electron and proton energies of KSZ irradiation experiment are the same as those in the ZnO pigment experiment, with fluence of 2.25×10¹⁶ e/cm² and 3.15×10¹⁶ p/cm², respectively. The UV dose is 5000 ESH. The AO fluence is 2.0×10²² atoms/cm².

The spectrum reflectance of ZnO pigment and KSZ at wavelengths of 300-2200 nm was determined by a model Lambda 950 UV-Vis-NIR spectrophotometer (Perkin Elmer Co., USA), and the solar absorptance of KSZ was calculated. The fluorescence spectra of ZnO pigment were characterized by a model RF-5301PC spectrometer (SHIMADZU Co., Japan). The excitation source was a xenon lamp with an 220 nm wavelength. The electron spin resonance on ZnO pigments at a microwave frequency of 9.435 GHz was measured by a model JES-FE3AX electron spin resonance spectrometer (JEOL Co., Japan).

After environment irradiation experiment, the spectral reflectance coefficient of ZnO pigment increases sharply and reaches its maximum value in a wavelength range of 360-500 nm. The spectral reflectance coefficient decreases with increasing wavelength. The peak position of the optical absorption band of ZnO pigment is at 425 nm. Compared to other environments, the absorption peak changes most severely under proton irradiation.

Under electron, proton, and UV irradiation, the inherent V_Zn^2- in ZnO pigments undergoes ionization, generating single ionized zinc vacancies (V′_Zn^-). AO causes a very small portion of the inherent V_Zn^2- in ZnO pigments to be converted into V′_Zn^- due to its intense electron affinity and oxidizing properties, while the majority of V_Zn^2- is converted into V_Zn⁰.

The initial value of the solar absorptance of KSZ coating is 0.12. After the electron, proton, and UV combined irradiation experiment, the solar absorptance of the coating increases with the increase of environmental test fluence. The solar absorptance of the coating increases to 0.27 at electron fluence of 2.25×10¹⁶ e/cm², proton fluence of 3.15×10¹⁶ p/cm², and UV dose of 5000 ESH. The solar absorptance of KSZ coating increases within 2000 ESH UV irradiation, and then the solar absorptance stabilizes at 0.15. The KSZ solar absorptance is increased by 0.02 after 2.0×10²² atoms/cm² AO irradiation.

After space environment irradiation experiment, the spectral reflectance coefficient of ZnO pigment decreased, mainly concentrated in the visible and near-infrared regions, and the peak position of the optical absorption band was at 425 nm. The ionization of the inherent V_Zn^2- in ZnO pigments occurred, resulting in the formation V′_Zn^-, which was the main mechanism for the degradation of the optical properties of ZnO pigments under electron, proton, and UV irradiation. AO caused a very small portion of the inherent V_Zn^2- in ZnO pigments to be converted into V′_Zn^- due to its intense electron affinity and oxidizing properties, and the majority was converted into V_Zn⁰. After the combined experiments of electron, proton, and UV (i.e., electron fluence of 2.25×10¹⁶ e/cm², proton fluence of 3.15×10¹⁶ p/cm², UV dose of 5000 ESH), the solar absorptance of KSZ coating increased from 0.12 to 0.27. After 5000 ESH UV irradiation, the KSZ solar absorptance reached to 0.15. After AO irradiation of 2.0×10²² atoms/cm², the KSZ solar absorptance was increased by 0.02. Compared to UV and AO, the electron and proton irradiation had a greater impact on the optical properties of KSZ inorganic white paint.

Compared with the conventional moulded concrete process, the layer-by-layer stacking process produces a large number of interlayer interfaces in 3D printed concrete (i.e., 3DPC), leading to a low flexural and tensile strength of 3DPC and a poor structural integrity of 3D printed concrete structure, which restricts the development and large-scale application of concrete 3D printing technology. Reinforcement is considered as one of the most effective measures to improve the flexural and tensile properties of 3D printed structures and enhance the structural integrity. Various reinforcement techniques for 3DPC, such as fiber reinforcement and transverse reinforcement, are developed, but all of them fail to effectively reinforce the interlayers. A longitudinal reinforcing approach involving short-cut straight and n-shaped bars was firstly proposed in 2018, and some work showed that longitudinal reinforcing method of n-shaped bar could enhance the mechanical properties of 3DPC.

In this paper, the influences of the penetration degree of the n-shaped bars and the overlapping conditions on the flexural properties of 3DPC were investigated, and the damage process of 3DPC was analyzed via crack displacement and section morphology. The different reinforcing methods were compared.

Portland cement P·I 42.5 according to Chinese standard and natural river sand with a fineness modulus of 3.4 as a fine aggregate were used. Hydroxypropyl methylcellulose (HPMC) with an apparent viscosity of 198 Pa·s was selected as a viscosity enhancing agent and a bauxite-based powdered accelerator with a fineness (80 μm square-hole sieve residue) of 9% was used to adjust the thixotropy and buildability of 3DPC. A polycarboxylic acid-based high-performance superplasticizer with a solid content of 39% was used to adjust the flowability of the printed materials. In this work, a ratio of the layer number penetrated by n-shaped bar (L_P) to the total layer number (L_P) was defined as a penetration degree (P) including overlapped penetration degree P_ovr and nonoverlapped penetration degree P_non. Eight types of n-shaped bar layouts that meet the constraint requirements of the formula were investigated.

To ensure the accuracy of the n-shaped bar overlapping and positioning, a manual reinforcing method was used to accurately locate and reinforce the n-shaped bars. This process was repeated for ten layers of concrete, resulting in the completion of 3DPC specimens reinforced by n-shaped bars. For the unreinforced specimens, the preparation was completed via directly printing ten layers in a row.

After printing, the specimens with the sizes of 40 mm×200 mm×180 mm were placed in an indoor environment for 24 h and then transferred to a standard curing room for 28 d. In the curing period, the specimens underwent a three-point bending test using a CMT-300 universal pressure testing machine. The span between the specimen supports was 160 mm, and the loading was controlled using mid-span displacement mode at a loading speed of 0.01 mm/min. The flexural strength of each group with three specimens was obtained via calculating the average value.

When P_non=0.1, the addition of n-shaped bars fails to strengthen the 3DPC, and reduces the flexural strength of 3DPC. When P_non=1.0, the maximum flexural strength reaches 5.88 MPa, which is only slightly greater than that of the unreinforced 3DPC. Under the condition of nonoverlapped, the n-shaped bar is not effectively connected at the interface, and the n-bar has little enhanced effect.

P_ovr=0, Compared to the unreinforced 3DPC as P_ovr=0, the flexural strength of reinforced 3DPC increases when P_ovr=0.2, with an increase of 100%. As the penetration degree increases, the flexural strength increases by 128% when P_ovr=0.3 and by 152% when P_ovr=0.9. The flexural strength is highly improved by an overlapped n-shaped bar. The damage mode of overlapped n-shaped bar is mainly a ductile failure with obvious strain hardening characteristics.

The influence weights of f_t0, P and ρ of flexural strength of reinforced 3DPC are 0.896 2, 0.836 0 and 0.766 3, respectively. Based on the analysis of these influence weights, the advantages and disadvantages of the two reinforcement methods, i.e., the rivet method and chopped bar method, are evaluated. To ensure a fair comparison, each parameter is normalized to the same order of magnitude and unit, and transformed into equivalent flexural strength. The chopped bar and n-shaped bar (overlapped) methods both have the similar reinforcing effects, with equivalent flexural strengths of 27.74 MPa and 26.25 MPa, respectively, based on the proposed equivalent flexural strength formula.

The interface defects at the shoulder and bottom of the n-shaped bar were increased when the n-shaped bar was nonoverlapped, having little effect on the flexural strength. All the reinforced 3DPC exhibited a brittle damage. When the n-bar was overlapped, the flexural strength of reinforced 3DPC increased with the penetration degree, but there was a saturation value for the reinforcement of flexural strength by penetration degree, and there was a most economical penetration degree in the actual project. The damage mode of 3DPC reinforced by n-shaped bar under overlapped condition was a ductile failure with strain hardening characteristics. Based on the gray entropy system theory, the flexural strength of reinforced 3DPC (f_t) of the influence weight was obtained in a decreasing order, i.e., matrix flexural strength (f_t0)>penetration degree (P)>reinforcing ratio (ρ). To improve the flexural strength of current longitudinal reinforcement methods, the chopped bar and overlapped n-shaped bar were proven to be more effective rather than the rivet and nonoverlapped n-shaped bar techniques. It was essential to carefully choose an appropriate reinforcing method corresponding to the specific requirements of the printing project.

The MD simulation method can obtain the thermodynamic structure and mechanical properties of complex molecules, analyze the dynamic evolution of the system, and obtain the time-dependent dynamic properties of the system. This can favor exploring the corrosion mechanism more conveniently and intuitively, and improving the research efficiency. The MD simulation becomes an effective tool to investigate the mechanism and development of steel bar corrosion inhibitors and has broad application prospects. However, the existing MD simulation cannot simulate complex chemical reactions, so it is more commonly used in the study of organic corrosion inhibitors, and the simulation method is relatively simple, mostly adsorption of an inhibitor molecule in pure water environment. Some studies show that a variety of ions in concrete environment can affect the effect of inhibitor molecules, and the inhibition effect is affected by molecular concentration and environmental temperature. In future studies, OH^- can be added to simulate the alkaline environment, and Ca²⁺, K⁺, Na⁺ and other cations can be introduced according to the real situation when simulating the concrete environment. The interaction between cations and inhibitor molecules can be intense in the simulation of phenolic and metal cation complexation reaction, thus obtaining more accurate and scientific simulation results. The use of MD to simulate the dynamic process of adsorption of multiple corrosion inhibitor molecules to film formation and corrosion barrier needs a further study. The study of the coupling adsorption of various inhibitor molecules on the surface of steel bars can favor expanding the application range of MD simulation and providing the data support for future prediction methods such as machine learning and deep learning.

Geopolymer is a green cementitious material produced via the activation of silica-aluminate-rich precursor materials in an alkaline environment. It has outstanding mechanical qualities and durability. However, the different raw materials and complicated reaction process of geopolymers result in some parameters affecting mechanical qualities that are difficult to effectively manage and forecast. It is thus necessary to examine the variation rule of mechanical properties in geopolymer concrete (GPC) under various conditions via numerical simulation. Though the majority of existing numerical models are applicable to ordinary Portland cement (OPC), they cannot be directly applicable to GPC due to variations in the chemical characteristics as well as isomorphic model of mortar and interface transition zone (ITZ) in GPC and OPC. The existing studies lack clear criteria for material inhomogeneity in each phase of the model, and the majority of them neglect the impact of interior micropores on the model mechanical characteristics. In this work, an innovative method was used to account for non-homogeneity and determine the mechanical parameters and intrinsic model of geopolymer materials through experiment. In addition, the impacts of aggregate content, model size, and porosity on the mechanical properties of GPC were also investigated by the verified model.

The mechanical parameters and intrinsic model were determined via a geopolymer mortar test, and the model reliability was verified via a geopolymer concrete test. To prepare the specimens, slag and fly ash were used as precursors, and NaOH and water glass were used as alkaline activators. A coarse aggregate was a continuously graded gravel with a particle size range of 5-20 mm. A fine aggregate was a standard sand with a fineness modulus of 2.5. In this proportion, the alkali equivalent (in mass proportion of Na₂O to precursor) was 4%, the water-cement ratio was 0.50, the modulus of alkali solution (SiO₂/Na₂O) was 1.2, and the mortar was kept in the same proportion of mortar phase in the concrete.

At the simulation level, the Mesh-Placement-Identification-Assignment (MPIA) procedure was utilized to create a 3D GPC meso-scale model, and the model fundamental conditions were consistent with the experiments. It was assumed that the mechanical characteristics of each phase material inside the concrete could follow the Weibull distribution, and the mortar phase element non-homogeneous parameters could be solved using an equivalent unit approach. Also, initial flaws were introduced into the model, and multi-scale analysis was utilized to assess their impact on mechanical characteristics because the mixing process could form air bubbles in the concrete. After the model was constructed, the material constitutive model was determined via mortar experimental curve fitting, and the mortar test results and test algorithms were used to estimate the material mechanical characteristics. The model was imported into ABAQUS to set the boundary and loading conditions before being compared to the GPC test results obtained after the simulation. The results indicated that the damage patterns and mechanical characteristics of the test and simulation results could be nearly compatible, and the model was regarded as reliable.

Based on the validated model, GPC models with different aggregate volume fractions of 30%, 40%, and 50% are simulated under uniaxial compression. The simulation findings reveal that the compressive strength of GPC increases as the aggregate volume percent increases, as does the energy absorbed for damage. This is because the aggregate plays a role in preventing crack development, so as the aggregate content increases in a specific range, the crack morphology inside the model becomes more complex, the energy required for damage increases, thus improving the macroscopic performance of the mechanical properties.

Under uniaxial compression, GPC cubes with the side lengths of 70, 100 mm, and 150 mm are simulated, respectively. According to the simulation results, the compressive strength of GPC descends when the specimen volume grows. Compared to OPC, GPC displays the same pronounced size effect phenomena and aligns well with the Bažant theoretical formulation.

The GPC models with internal porosities of 1.9%, 4.3%, and 5.9% are simulated under uniaxial compression, having the compressive strengths of 49.10, 47.90 MPa, and 46.19 MPa, respectively. The mechanical characteristics of the geopolymer concrete decrease slightly as the model inside porosity increases. This is since the introduction of pores in this study takes into account both porosity and pore size distribution, and the number of small-sized pores increases as the porosity increases. The small pores have less influence on the mechanical properties, and the results show that the model mechanical properties are insensitive to the change of porosity.

A non-homogeneous meso-scale model of GPC was proposed, and the simulation results were similar to the experimental result. Also, an innovative approach to describe the non-homogeneity of GPC was proposed. The equivalent unit method was used to determine the non-homogeneity parameter, and the initial microporous defects were characterized via multiscale modeling and folding coefficients. The energy required to destroy GPC and the compressive strength increased with increasing aggregate content. As the model size increased, the mechanical characteristics of GPC decreased, aligning with Bažant's theory. The mechanical characteristics of geopolymer concrete diminished slightly as initial microporosity increased.

Cadmium magnesium telluride (CdMgTe) crystal is an ideal material for room-temperature radiation detection. However, there are still many defects in as-grown crystals, which degrade the detector performance. At present, a single annealing method is unable to achieve the effect of both eliminating defects and maintaining or increasing resistivity. The multi-step combined annealing method can improve the crystal quality and the detector performance. There are a few studies on the annealing of CdMgTe crystals. In this work, a multi-step combined annealing method was used, i.e., the CdMgTe crystals grown under Te-rich condition were annealed in Cd atmosphere and Te atmosphere. The effect of annealing temperature on the defects, mechanics, photoelectric properties and detector performance of CdMgTe crystals was investigated.

High purity (7N) cadmium (Cd) and tellurium (Te) were selected as raw materials. The selected slices for annealing were derived from In-doped Cd_0.95Mg_0.05Te ingot grown by a modified vertical Bridgman method under Te rich condition, with the size of 5 mm×5 mm×2 mm. The annealing in Cd atmosphere was mainly divided into two steps, i.e., high and low temperature annealing. In the high temperature annealing step, the temperatures of the slices were 873, 923 K and 973 K, respectively. The corresponding temperatures of Cd source were 901, 946 K and 986 K, respectively. The annealing time was 120 h. After high temperature annealing, the temperature of the slices and Cd source reduced to 623 K, and the annealing time was also 120 h. After annealing in Cd atmosphere, it continued annealing in Te atmosphere. The annealing temperature of both slices and Te source was selected to 773 K, and the annealing time was also 120 h.

The CdMgTe crystals before and after annealing were characterized by infrared transmission microscopy, scanning electron microscopy, ultraviolet-visible-near infrared (NIR) spectroscopy, infrared transmittance spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Vickers indentation analysis and current-voltage (I-V) measurement. Au/CdMgTe/Au planar detectors were fabricated by an evaporation method, and the energy spectrum was collected in ORTEC test system. The radiation source was ²⁴¹Am α particle source with a non-collimated energy of 5.48 MeV.

The band gap of the as-grown crystal is 1.510 eV, and the maximum band gap is increased by 0.021 eV after annealing. After annealing in Cd atmosphere, Te inclusions in the crystals are greatly reduced. At the annealing temperature of 973 K, Te inclusions can be completely eliminated. The IR transmittance and resistivity are reduced. The density of Te inclusions has little change after annealing in Te atmosphere. However, the IR transmittance and resistivity improve and are better than those of the as-grown crystals. The optimum IR transmittance and resistivity are 63% and 2.41×10¹⁰ Ω·cm, respectively. After multi-step combined annealing, there is no extra oxidation on the crystal surface, and the microhardness is increased by 16%. The Raman spectra show that the crystal quality is improved.

Au/CdMgTe/Au planar room temperature radiation detectors are fabricated with the crystals obtained under the optimum annealing conditions. The time-of-flight (TOF) measurements show a significant increase in electron mobility, up to 526.86 cm²/(V·s). The optimum energy resolution and electron mobility lifetime product of the detector are 15.6% and 1.89×10^-4 cm²/V, respectively, using ²⁴¹Am α particles with an energy of 5.48 MeV as a radiation source.

CdMgTe crystals were annealed by a multi-step combined method. After annealing, there was no additional oxidation on the crystal surface, and Te inclusions decreased significantly. The band gap, microhardness, IR transmittance and resistivity of the crystals were all improved, indicating the improvement of crystal quality. The optimum annealing conditions were in Cd atmosphere at high (i.e., 923 K) and low (623 K) temperatures for 120 h, and then in Te atmosphere at a low (i.e., 623 K) temperature for 120 h. The optimum performance of CdMgTe detector was electron mobility (μ_e) of 526.86 cm²/(V·s), the energy resolution of 15.6%, and the mobility lifetime product (μ_τ)_e of 1.89×10^-4 cm²/V.

The micro-structure evolution and macro-property development of cement-based composites are affected by the wall effect. For the cement paste, the formation of the interfacial transition zone (ITZ) is related to wall effect, i.e., the lower density of cement particles near aggregate induces the formation of ITZ. In addition, the aggregate density is low near the formwork during the concrete placement and molding, which results in a difference of properties between the wall effect zone and the material interior. Characterizing the influences of aggregate or cement particle shape and size-polydispersity on the thickness of wall effect layer and further on the volume fraction distribution of particles is important for controlling the performance of cement-based composite. Compared with theoretical and numerical simulation methods, the experimental method, such as concrete section cutting is time-consuming and labor-intensive. With the development of computer technology, it provided a technical basis for the study of wall effects in concrete through numerical simulation. In general, cement-based composite can be regarded as a particle packing system. However, most of the existing numerical and theoretical studies employs simple particle, such as circles, ellipses and spheres to model the aggregate or cement particles, which makes it difficult to accurately analyze the role of particle shape and size-polydispersity on the wall effect. It is thus of great significance to construct a complex particle packing system and develop a micro-structure characterization algorithm applicable to the particle packing system to investigate the influence of the particle shape and size-polydispersity on the wall effect.

This work used variable shaped ovoidal particles to construct aggregate and cement particles. A contact detection algorithm for ovoidal particles was proposed based on a geometric potential concept, and ovoidal particle packing systems were generated to simulate the packing state of aggregate and cement particles. Moreover, the cross-section analysis algorithm applicable to the ovoid particle packing system was proposed. Based on the cross-section analysis algorithm and the stereology unbiased estimation method, the influences of particle shape and size-polydispersity on the thickness of the wall effect layer were analyzed and a quantitative relationship between the thickness of the wall effect layer and particle shape and size-polydispersity was established. A prediction formula was proposed to determine the particle volume fraction distribution at different section planes considering the wall effect, and the accuracy of the prediction formula was verified via comparing it with the results of the existing studies. Finally, the modulus of elasticity of concrete at different locations was calculated by the quantitative model of wall effect layer thickness and particle volume fraction distribution based on the differential effective medium theory.

For monoshaped-monosized particle packing system, the relationship between volume fraction of particle (V_V) and distance from boundary plane to cross-section plane (h) is obtained. The results show that the V_V-h curves for different shaped particle packing systems are basically similar, indicating that the influence of particle shape on the thickness of wall effect layer T_P can be ignored.

For monoshaped-binarysized particle packing systems, the V_V-h curves move in the direction of increasing distance as the proportion of larger size particles increases. The results indicate that the thickness of wall effect layer T_P enlarges with increasing the proportion of larger size particles. For monoshaped-polysized particle packing system, the thickness of wall effect layer T_P enlarges with the increase of average equivalent diameter of particles.

Based on the obtained results, the prediction formulas are proposed to determine the wall effect layer thickness and particle volume fraction distribution, and the reliability of prediction formulas is verified via comparing the predicted data with the results from this work and other publications.

The relationship between elastic modulus E^MT and distance h is similar to that between volume fraction of particle (V_V) and distance h. The variation of E^MT-h curves can be divided into the increasing and stabilizing parts. The results indicate that the elastic modulus E^MT is small near the boundary plane, which is induced at a low particle volume fraction.

This work could provide a guidance for the optimization of the microfine structure of concrete and the regulation of macroscopic properties, and the following conclusions were summarized: 1) The influence of particle shape on the wall effect was not dominant; 2) The thickness of wall effect layer T_P increased with enlarging the average diameter of aggregate, and the thickness of wall effect layer T_P decreased with enlarging the aggregate volume fraction; 3) The relationship between elastic modulus E^MT and distance h was similar to that between volume fraction of particle (V_V) and distance h.

The rapid adoption of 3D printed cement-based materials (3DPC) in construction industry, particularly in sustainable buildings, highlights a critical importance of understanding its shrinkage characteristics. Different from conventional concrete, 3DPC is characterized due to the absence of coarse aggregates, the lack of templates support during printing, and immediately exposure to environmental conditions, which make it more susceptible to shrinkage-induced cracking. Shrinkage in cement-based materials leads to internal stresses and potential durability issues, affecting a long-term structural integrity. It is thus to investigate the shrinkage behavior of 3DPC. In particular, the effect of material compositions (i.e., water-binder ratio (W/B), sand-binder ratio (S/B), and supplementary cementitious materials such as fly ash (FA) and ground granulated blast furnace slag (GGBS)) on the shrinkage (total shrinkage, autogenous shrinkage, and drying shrinkage) is not fully clarified. This study was to investigate the shrinkage characteristics of 3DPC and compare it with other cement-based materials (i.e., UHPC and cast mortar).

To investigate the impact of material composition on 3DPC shrinkage, a series of laboratory experiments were conducted by a standardized contact shrinkage testing method. Four key factors evaluated were W/B, S/B, FA replacement ratio, and GGBS replacement ratio. These material variables were selected based on their known influence on the microstructure and shrinkage behavior of traditional cement-based materials.

Total shrinkage (TS), autogenous shrinkage (AS), and drying shrinkage (DS) were measured at specific intervals to capture early-age shrinkage behavior in 14 d. TS was calculated as a sum of AS and DS to represent the overall dimensional changes of the material for over time. The degree of influence of each material factor on the shrinkage was determined by grey relational analysis (GRA). GRA is a statistical method that enables the quantification of the relationship between multiple variables in complex systems, making it particularly suitable for determining the relative significance of material compositions in the context of 3DPC shrinkage.

For comparative purposes, the shrinkage characteristics of conventional cast mortar and UHPC were also analyzed under identical conditions, enabling a comprehensive evaluation of the shrinkage performance of 3DPC.

The results reveal the distinct shrinkage patterns based on the material composition of 3DPC. Increasing the W/B can significantly reduce AS. However, the influence of W/B on DS is less than that on AS. Also, the S/B has a measurable effect on drying shrinkage. After an initial curing period of 3 d, a higher S/B ratio leads to a marked reduction in DS likely due to the presence of fine sand particles, which restricts the movement of water within the matrix and limit capillary stresses.

Besides, the replacement of cement with FA has a beneficial effect on shrinkage reduction. The 14 d AS and DS both are significantly lower in 30% FA-modified 3DPC, compared to the control sample. Also, the incorporation of FA reduces the 14 d autogenous shrinkage and drying shrinkage of 3DPC in the same proportion. In contrast, the use of GGBS has a more complex influence on shrinkage. While GGBS replacement increases AS after 2 d, it reduced drying shrinkage before 5 d. The TS of 3DPC firstly decreases and then increases with the increase of GGBS dosage, and the TS of 3DPC is the minimum value when the dosage of GGBS is 10%.

The grey relational analysis shows the factors influencing 3DPC shrinkage in an order of significance, i.e., W/B, S/B, GGBS and FA. This highlights a predominant role of water content in governing both autogenous and drying shrinkage in 3DPC systems, which is consistent with those in other cement-based materials. However, the relative importance of sand and supplementary materials indicates that a further optimization of the mix design can lead to an improved shrinkage performance. Comparing the shrinkage behavior of 3DPC with that of UHPC and cast mortar, the shrinkage magnitudes of 3DPC are generally higher. These results indicate a necessity of tailored shrinkage control strategies in 3DPC, especially in large-scale applications where cracking risk is a major concern.

This study provided a comprehensive evaluation of the shrinkage behavior of 3D printed cementitious materials, especially the influence of material compositions such as W/B, S/B, FA, and GGBS. Increasing the W/B of 3DPC significantly reduced AS more than DS. The DS decreased with increasing S/B after 3 d. The replacement of FA proportionately decreased the 14 d AS and DS of 3DPC, whereas the replacement of GGBS replacement ratios increased AS after 2 d and DS before 5 d. The influence of material composition on 3DPC shrinkage was ranked in a decreasing order of W/B, S/B, GGBS, and FA. Compared to other cement-based materials like UHPC and cast mortar, 3DPC exhibited a higher shrinkage, indicating the need for more focused research and innovation in shrinkage reduction techniques for 3DPC. These findings contributed to a deeper understanding of the material behavior and provided a foundation for a future work on optimizing 3DPC for large-scale and crack-resistant applications.

C₃N₅ as a two-dimensional (2D) layered polymer material has advantages such as the unique structures, larger amount of nitrogen active sites, narrower band structure and excellent chemical stability. It is more favorable to the development prospects of C₃N₅ in various photocatalytic fields. This review summarizes the synthesis and molecular structure of C₃N₅, and the modification strategies developed to improve the photocatalytic performance of C₃N₅ nanomaterials in recent years, including morphology control, nitrogen vacancy creation, element doping and heterojunction construction. However, compared with the abundance of other semiconductor catalyst systems, a research on C₃N₅ still needs to be further explored in terms of rational preparation, construction of unique composite systems, elucidation of the intrinsic mechanism, and the application areas, etc.

In rational preparation, the eco-friendly and cost-effective large-scale preparation of C₃N₅ nanomaterials is still a challenge. The relationship between the reaction parameters in the thermal polymerization process and the resulting morphology structure is unclear, which hinders an ability to optimise and control the process further. Also, the synthesis of C₃N₅ inevitably results in the formation of toxic by-products, necessitating the development of supplementary follow-up treatment technology. The optimization of the rational synthesis of C₃N₅ for environmentally and large-scale preparation is more in line with sustainable development strategies.

In the construction of unique composite systems, the precise regulation is a pivotal concern, both in terms of industrial applications and fundamental scientific research. Until now, it becomes a challenge to elucidate the precise conformational relationship between vacancy/elemental sites density and photocatalytic performance, representing a significant obstacle to further performance optimization. The advancement of the precise regulation is instrumental in enhancing the catalytic performance of C₃N₅-based materials.

In the elucidation of the intrinsic mechanism, it is essential to investigate the atomic and electronic structures at the material/interface level in order to clarify the performance of the reaction process. This provides a theoretical foundation for the subsequent design and development of efficient photocatalysts. It is thus necessary to employ advanced characterizations like environmental transmission electron microscopy, aberration scanning transmission electron microscopy, synchrotron radiation and in-situ spectroscopy. The structural configurations for catalysts and the formation of intermediates during photoreaction can be monitored. The fine update is more beneficial to optimizing the properties of the catalyst material.

In the context of application areas,, the optimization of selectivity in catalytic reactions represents a crucial challenge for designing high-efficient photocatalytic systems. In the context of energy catalysis, such as methane conversion and CO₂/N₂ reduction, a detailed understanding of the intrinsic reaction mechanism and kinetic processes elucidated by theoretical simulation and in-situ monitoring is beneficial for the design and development of C₃N₅ catalysts with a high selectivity and a purpose of improving the yield of the target product. In the context of environmental remediation, the utilization of wastewater and seawater instead of purified water for hydrogen production from water is another way to achieve a sustainable development strategy. The design and development of C₃N₅-based materials with a high selectivity can facilitate their broader range of applications.

Titanium dioxide (TiO₂) is used as a photoelectric conversion material due to its non-toxicity, good photothermal stability and low cost. However, TiO₂ has a wide electron bandgap, and its photo-generated electrons and holes are easy to recombine. The photoelectric performance of TiO₂ is often improved via increasing the specific surface area, speeding up the axial transport of electrons. In this paper, titanium dioxide inverse opal photonic crystals (TiO₂ IOPCs) were selected as photoelectric conversion materials due to the advantages of large specific surface area and promoted light absorption. TiO₂ IOPCs were prepared by a template method and a sol-gel method. Also, TiO₂ IOPCs was modified with amorphous titanium dioxide (A-TiO₂).

Self-assembled opal photonic crystals on fluorine-doped tin oxide (FTO) were used as templates. The precursor liquid prepared with isopropyl titanate, ethanol, acetyl acetone, hydrochloric acid and deionized water was filled into the gap of opal photonic crystal. After the sol in the opal crystal gap was dried and gelled, the electrode was calcined at 450 ℃ to remove the opal photonic crystals template to obtain TiO₂ IOPCs. Afterwards, amorphous titanium dioxide modified titanium dioxide inverse opal photonic crystals (A-TiO₂/TiO₂ IOPCs) were prepared via placing TiO₂ IOPCs electrodes into TiCl₄ aqueous solution at a low temperature without stirring.

The phase composition, elemental composition, energy level structure and morphology of the electrode were analyzed by X-ray diffraction (XRD), X-ray electron spectroscopy (XPS), ultraviolet visible spectrophotometry (UV-VIS) and scanning electron microscopy (SEM). A three-electrode system was used to detect the photocurrent in the dark, and the LED with 390-410 nm was used as an excitation light source. The current-time (I-T) curve was recorded when the light source was switched.

The XRD patterns show that amorphous titanium dioxide modified anatase titanium dioxide electrode can be obtained. Based on the XPS analysis, the prepared material is TiO₂. The valence band value of the electrode is determined by the XPS valence band spectra. The indirect band gap of the electrode material is determined according to the UV-VIS analysis. The conduction band value of the electrode can be obtained by calculation. The SEM images indicate that A layer of amorphous titanium dioxide is deposited on the orderly arranged porous skeleton of TiO₂ IOPCs, which retains a porous structure and further increases the specific surface area. In the photocurrent test, TiO₂ IOPCs photocurrent reaches 100 nA, and the photocurrent of A-TiO₂/TiO₂ IOPCs reaches 600 nA, indicating that its photocurrent is increased by 6 times. The conduction band value of A-TiO₂ is more negative than that of anatase TiO₂ IOPCs. The photogenerated electrons of A-TiO₂ can be transferred to the conduction band of anatase TiO₂ IOPCs. The valence band value of anatase TiO₂ IOPCs is more positive than that of A-TiO₂, and the photogenerated holes can be transferred from the anatase TiO₂ IOPCs valence band to A-TiO₂ valence band, thereby reducing the recombination of photogenerated electrons and holes. In addition, the light absorption area increases due to the porous structure of A-TiO₂/TiO₂ IOPCs, thus increasing the photocurrent.

TiO₂ IOPCs were prepared by a template method and a sol-gel method, and then amorphous TiO₂ was deposited on TiO₂ IOPCs skeleton at a low temperature to prepare an amorphous titanium dioxide modified titanium dioxide inverse opal photonic crystal electrode. The porous structure of the electrode increased the light absorption area, and the modification of amorphous titanium dioxide reduced the recombination of photogenerated electrons and holes, and the photocurrent of the electrode was increased by 4 times. The results indicated that this method could be simple and feasible, which could obtain the superior photoelectric performance of the electrode, thus providing an effective approach for the development of green energy.