This review represents the research advancements in high-temperature insulation materials (i.e., water-resistant ceramic fiber insulation tiles, insulation blankets, and aerogels). Some strategies for enhancing their water resistance, such as the application of high-emissivity coatings, chemical vapor deposition, and modification with organic hydrophobic groups, are described. These materials possess a significant application potential at multiple fields due to their low density, robust water resistance, and efficient thermal insulation properties. Nevertheless, some challenges persist in maintaining water resistance at high temperatures, simplifying manufacturing processes, and developing novel materials. A future research should prioritize enhancing the water resistance of materials at elevated temperatures, streamlining production processes, and innovating novel materials. In addition, the stability and durability of these materials in practical applications also need to be further investigated to facilitate the technological advancements and industrial upgrading.

Ultra-high temperature ceramics include a series of high melting point materials, such as transition metal carbides, nitrides and borides, especially transition metal carbides with excellent high-temperature mechanical properties, stable physico-chemical properties, corrosion resistance and radiation resistance, showing broad prospects for application in the hypersonic aircraft, rocket engines, fourth-generation nuclear reactor, and other extreme environments. However, the traditional single-component transition metal carbide ceramics have been unable to meet the emerging requirements under extreme environments, there is an urgent need to develop a new high-performance material under ultra-high temperature. In recent years, the concept of multi-component "high entropy" has greatly expanded the scope of material composition design and property optimization. Compared with single-component carbides, multi-component ceramics perform better in terms of overall properties, including hardness, creep resistance, oxidation resistance and radiation resistance. These improvements mainly result from their complex component composition, electronic structure and lattice distortion. At present, the influence of elemental species on the microstructure evolution and mechanical properties of carbide high-entropy ceramics is not reported. In this paper, (TiZrNbTaMe)C (Me=V, Cr, Mo, W) high-entropy ceramics are prepared by hot press sintering, and the effects of Me elemental species on the physical phase, microstructure evolution, and mechanical properties of (TiZrNbTaMe) high-entropy ceramics are investigated.

In this work, (TiZrNbTaMe)C (Me=V, Cr, Mo, W) high-entropy ceramics with equimolar ratio were prepared by carbothermal reduction-assisted hot pressing using transition metal oxides and carbon black as raw materials. The oxides and carbon black were mixed using a planetary ball mill (Fritsch, model P4, Germany). Carbide powders were obtained by carbothermal reduction under vacuum using a pressureless sintering furnace (WS0404, Ningxia Sincere Co. Ltd., China) with a process of 1500 ℃/1 h. The synthesized carbide powders were loaded into graphite molds, and the ceramic samples were prepared by a two-step hot pressing method (AVS, model 1540, USA). The samples were held at 1850 ℃ for 1 h, and then at 2100 ℃ for 0.5 h under pressure of 30 MPa or 10 Pa. Phase analysis was carried out by X-ray diffractometry (XRD; D/max-B, Rigaku, Japan) using Cu-K_α rays. Scanning electron microscopy (SEM; Quanta 200FEG, USA) was used to analyze the microstructure and elemental content and distribution. Transmission electron microscopy (TEM; Talos F200X, USA) was used to analyze the microstructure, elemental content and distribution, and grain boundary characteristics of the materials. The relative densities of the samples were measured using image analysis software (Photoshop, Adobe, USA) based on the pores in the SEM photographs. Vickers hardness was measured using a Vickers hardness tester (HVS-30) at a load of 9.8 N with a holding time of 15 s. Fracture toughness was also measured using the indentation method. The interaction parameters between the metal elements were also calculated using DFT calculation.

The high-entropy ceramics are all characterized by an FCC crystal structure. Except for the (TiZrNbTaW)C sample, the porosity of the other three high entropy ceramics is low, and their density exceeds 98% and the element distribution is relatively uniform. Compared with the (TiZrNbTaMo)C sample, the grain sizes of the (TiZrNbTaV)C and (TiZrNbTaCr)C samples are significantly reduced, which are 2.69 μm and 5.39 μm, respectively. This shows that the addition of V and Cr elements helps to improve the sintering properties of the (TiZrNbTaMe)C system and inhibit grain growth. The element content analysis results of (TiZrNbTaCr)C show that the Cr element segregates significantly at the grain boundaries, while the other four metal elements are distributed more evenly. The segregation of Cr at the grain boundaries may be related to chromium carbides with low melting points. For example, the melting point of Cr₃C₂ is about 1810 ℃, while the sintering temperature is as high as 2100 ℃, suggesting that chromium carbides may form a liquid phase and aggregate at the grain boundaries during sintering. In addition, the complex composition of high-entropy ceramics may also affect the solid solubility of Cr. The interaction coefficients between Cr and each metal element in transition metal carbides show that the interaction parameter values of Cr and other metal elements are high. It is difficult for Cr to form a solid solution with other metal elements, thus tending to be enriched at the grain boundaries. Therefore, (Ti_xZr_0.4-xNb_0.2Ta_0.2Cr_0.2)C ceramics with different Ti and Zr contents (x=0, 0.2, 0.3 and 0.4) were designed and prepared. The effect of Ti content on the solid solubility of Cr in the (TiZrNbTaCr)C sample was investigated. As the x value increases from 0 to 0.4, the solid solubility of Cr in the grains increases from 3.18% to 8.68%. It shows that the increase of Ti content is beneficial to improve the solid solubility of Cr. In addition, for the (TiZrNbTaCr)C sample, the high lattice distortion leads to solid solution strengthening in the grains and the high density of the system increases its hardness, which has the best mechanical properties. Its Vickers hardness and fracture toughness reach 29.8 GPa and 3.71 MPa·m^1/2, respectively.

The four (TiZrNbTaMe)C (Me=V, Cr, Mo, W) high-entropy ceramics are all face-centered cubic structures, and the elements are uniformly distributed in the high-entropy ceramic systems except for the (TiZrNbTaCr)C sample. The (TiZrNbTaCr)C sample is also characterized by the presence of a significant Cr segregation at grain boundaries, resulting in a low Cr content inside the grains. The (TiZrNbTaCr)C sample has the best overall mechanical properties, with the Vickers hardness and fracture toughness reaching 29.8 GPa and 3.71 MPa·m^1/2, respectively. In-depth studies show that the solid solubility of Cr element in high-entropy carbide ceramics is closely related to the species and content of metal elements. Enhancing the content of Ti element helps to improve the solid solubility of Cr element in high-entropy carbide ceramics, in which the solid solubility of Cr element in the grain of (Ti_0.4Nb_0.2Ta_0.2Cr_0.2)C system reaches the maximum value of about 8.68%.

In recent years, the high-voltage and high-power multi-layer ceramic capacitors (MLCCs) have attracted extensive attention with the development of new energy vehicles, 5G communication and high-power pulse technologies. At present, the materials used in the MLCC inner electrode are mainly precious metal (Ag/Pd or Au) and base metal (Ni or Cu). Base metal materials have more cost advantages compared with the precious metal materials. In order to prevent the oxidation of the base metal Ni or Cu at high temperatures, it is necessary to sintering in a low oxygen or reducing atmosphere. In order to obtain MLCC products with excellent performance, it is necessary to strictly control the matching of the sintering atmosphere between the inner electrode and the ceramic body during the sintering process. The crystal structure and electrical properties of the ceramic will be significantly reduced in the reducing atmosphere or low oxygen atmosphere environment, mainly because the precipitation of metal lead will destroy the lattice structure, or produce lattice defects. Therefore, it is of great significance to study the properties of PbLaZrSnTiO₃-based (PLZST-based) antiferroelectric ceramics in low oxygen sintering atmosphere. In this paper, PLZST-based antiferroelectric ceramics were sintered in four kinds of low oxygen sintering atmosphere. The crystal structure, microstructure and electrical properties of PLZST-based antiferroelectric ceramics were studied. Moreover, the energy storage properties of all samples were studied.

Pb_0.95La_0.02Sr_0.02(Zr_0.50Sn_0.40Ti_0.10)O₃ (PLSZST) antiferroelectric ceramics doped with were prepared using the traditional solid-state method. High quality raw materials, namely Pb₃O₄(95%, Sinopharm), La₂O₃(99.99%, Acros), ZrO₂(99%, Aladdin), SnO₂(99.8%, Sinopharm), TiO₂ (99.8%, Sinopharm) and SrCO₃(99%, Sinopharm), were weighted 500 g according to stoichiometric and ball milling for 12 h with ethyl alcohol as a solvent and dried at 70 ℃. These raw materials were calcined at 850 ℃ for 2 h in a closed crucible. Then, 0.6% BaCO₃-B₂O₃-SiO₂-K₂CO₃(BBSK, in mass) was added to the mixed powder according to the solid weight ratio as the calcination powder. After ball milling for 12 h, the powder was dried. Next, the powder was pressed into pellets at 20 MPa with 5% Polyvinyl Alcohol (PVA, in mass) as the binding agent 2 mm thick and 10 mm in diameter. Finally, the green ceramics with BBSK addition were sintered at 1020 ℃ and low oxygen sintering atmosphere (5.3×10^-10, 2.1×10^-9, 2.1×10^-8, 1.3×10^-7 atm) and air for 2 h in closed crucibles filled with the calcination powder.

In order to study the effect of sintering atmosphere on the crystal structure of PLSZST-0.6% BBSK (in mass) antiferroelectric material, XRD were conducted as shown in Fig. 1. It can be seen that all samples have perovskite structure, and a small amount of second phase is generated, which may be pyrocholate phase produced by Pb volatilization at high temperature. From the splitting peak near 44.2°, it can be seen that the intensity and Angle of the two splitting peaks do not change under the condition of low oxygen atmosphere, which indicates that the tetragonal structure of the ceramic body does not change. The peaks of (200) and (002) move to the lower Angle at lower oxygen content, indicating that the low oxygen atmosphere leads to the lattice distortion of the material. Then, the microstructure was studied. As can be seen in the surface micrograph, there are no metal lead or other substances precipitated from the ceramic body as observed in those samples treated at low oxygen sintering atmosphere. It can be seen from the f dielectric temperature spectra that all samples show the same variation characteristics. With the increase of temperature, the dielectric constant first increases to the maximum value and then slowly decreases. This dielectric anomaly peak is caused by the transformation of tetragonal antiferroelectric phase into paraelectric phase. With the increase of frequency, the dielectric constant and the peak value of dielectric anomaly do not change significantly, which indicates that the ceramics are a diffuse phase transition, and does not change with the change of oxygen content, which indicates that the oxygen content does not change the phase transition characteristics of antiferroelectric. With the decrease of oxygen content, the breakdown field strength and maximum polarization strength of the ceramic decrease, resulting in the gradual decrease of the saturated energy storage density and recoverable energy storage density of the ceramic body. The ceramic sintered at 5.3×10^-10 atm has a recoverable energy storage density of 2.76 J·cm^-3 which is lower than that of the ceramic sintered at air (3.56 J·cm^-3), showing a discharge energy density of 2.21 J·cm^-3.

Multi-layer ceramic capacitors with base metal inner electrodes have been widely concerned because of its lower cost advantages. In order to prevent the oxidation of the base metal inner electrodes (Ni or Cu) at high temperatures, it needs to be sintered in low oxygen or reducing atmosphere. The effect of low oxygen atmosphere on the electrical properties of PLSZST antiferroelectric ceramics was studied. With the decrease of oxygen content, the insulation characteristics and energy storage characteristics of antiferroelectric ceramics were reduced. Under the oxygen content of 5.3×10^-10 atm, the ceramic has a recoverable energy storage density of 2.76 J·cm^-3, which is lower than that of sintered sample under air (3.56 J·cm^-3), has a discharge energy storage density of 2.21 J·cm^-3, and has a discharge current of 28.6 A. This work provides understanding of anti-reduction characteristics of PLZST-based antiferroelectric ceramics, and further promotes the development and production of PLZST-based antiferroelectric multilayer ceramic capacitors with internal electrodes in base metals.

As a high-quality light source, the performance of phosphor-converted white light emitting diodes (pc-WLEDs) is closely related to the performance of the selected luminescent materials. The traditional method of combining blue LED chips and yellow phosphors to generate white light has problems of low color rendering index and high color temperature. To address the pressing requirements of lighting and display technologies, the development of novel red phosphors with high efficiency and excellent thermal stability is urgent. Perovskite tellurites have attracted extensive attention due to their elemental diversity and structural tunability. Based on the double perovskite Ca₃TeO₆, a series of NaLaCaTeO₆:xEu³⁺ phosphors that can be effectively excited by near-ultraviolet light have been designed and synthesized by elemental design. Through a co-substitution strategy of [Na⁺-La³⁺] substitution of [Ca²⁺-Ca²⁺] ion pair, the problem of low Eu³⁺ doping concentration in Ca₃TeO₆:Eu³⁺ is effectively improved by regulating the matrix structure. Meanwhile, the luminous intensity and thermal stability of the sample were also significantly improved.

Series of NLCT:xEu³⁺ (0.05 ≤ x ≤ 1.0) samples were successfully synthesized by a high-temperature solid-state reaction. The stoichiometric amounts of raw materials, including Na₂CO₃ (105.99 g/mol, 99.9%), CaCO₃ (100.09 g/mol, 99.9%), TeO₂ (159.6 g/mol, 99.99%), La₂O₃ (325.84 g/mol, 99.99%) and Eu₂O₃ (351.93 g/mol, 99.99%) purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., were weighed and grounded into homogeneous powder for 30 min. Afterward, the obtained powder was put into crucibles and heated at 1473 K for 12 h in a muffle furnace. Eventually, the target substances were obtained after cooling down and grinding again. A red LED device was assembled using the NLCT:0.6Eu³⁺ phosphor with a 395 nm chip. The WLED device was fabricated by connecting tricolor phosphors including BaMgAl₁₀O₁₇:Eu²⁺ (BAM:Eu²⁺, blue), (Ba, Sr)₂SiO₄:Eu²⁺ (BSS:Eu²⁺, green), along with the prepared NLCT:0.6Eu³⁺ (red) with a 395 nm chip.

The results of XRD patterns showed that [Na⁺-La³⁺] substituted the [Ca²⁺-Ca²⁺] ion pair, which did not affect the structure of the sample. The diffraction peaks of the samples with x = 0.1-0.6 did not show obvious impurity peaks, indicating that the synthesized samples were pure phases. Eu³⁺ ions tended to occupy the site of La in the matrix rather than Te. DRS results also confirmed that Eu³⁺ ions were successfully introduced into the NLCT matrix. The dominant absorption peak at 395 nm matches well with InGaN-based LED, and the emission intensity reaches the maximum at x = 0.6. The primary mechanism among Eu³⁺ ions in NLCT:xEu³⁺ samples was proved to the dipole-dipole interaction.

The luminous intensity of the prepared NLCT:0.6Eu³⁺ red phosphor is about 1.98 and 3.97 times than that of CT:0.2Eu³⁺ and Y₂O₃:Eu³⁺, confirming the feasibility of the co-substitution strategy. The internal quantum yield and color purity of NLCT:0.6Eu³⁺ are 67.6% and 99.87%, respectively. The CIE color coordinates of this sample (0.658 1, 0.341 0) are closer to the ideal red point (0.670, 0.330) than that of the purchased commercial red phosphor Y₂O₃:Eu³⁺. Compared with CT:0.2Eu³⁺ (about 77.5%), the thermal stability of NLCT:0.6Eu³⁺ is significantly improved. When the temperature rises to 420 K, the emission intensity at 616 nm remains 91.7% of that at 300 K. Furthermore, the sample exhibits excellent color stability by the CIE coordinates shifting only slightly from (0.658 2, 0.340 8) to (0.646 6, 0.351 9) with increasing temperature. The CCT and R_a of the prepared WLED device are 6215 K and 80.6, respectively, and the CIE coordinates (0.317 6, 0.334 0) are located in the white light region.

A series of innovative NaLaCaTeO₆:xEu³⁺ phosphors were synthesized using a high temperature solid-state reaction method. Under 395 nm excitation, the NaLaCaTeO₆:Eu³⁺ samples exhibit a strong red emission at 616 nm (⁵D₀→⁷F₂) of Eu³⁺ ions. Through a co-substitution strategy where [Na⁺-La³⁺] replaced the [Ca²⁺-Ca²⁺] ion pair, the optimized doping concentration of the NaLaCaTeO₆:xEu³⁺ is elevated to 60% (molar fractior), leading to exceptional color purity and high internal quantum efficiency reaching 99.87% and 67.60%, respectively. Furthermore, compared to the Ca₃TeO₆:0.2Eu³⁺ sample pre-substitution, the luminous intensity and thermal stability of NaLaCaTeO₆:0.6Eu³⁺ phosphor witness a notable increase. Even at 420 K, the emission intensity remains at 91.7% compared to 300 K. The resultant WLED displays a color rendering index R_a of 80.6 and CIE chromaticity coordinates of (0.317 6, 0.334 0), underscoring the broad application potential of NaLaCaTeO₆:0.6Eu³⁺ in solid-state lighting.

Based on the composition of 2MgO-2Al₂O₃-5SiO₂, a series of Mg_2-xNa_xAlSiO_18-xQ_x:1%Eu²⁺ (Q=Cl, F) fluorescent powders (abbreviated as 1Eu²⁺, 1Eu²⁺-xNaCl and 1Eu²⁺-xNaF) were prepared by introducing NaCl (A.R.), NaF (A.R.), and Eu₂O₃ (A.R.), and glass crystallization after quenching the reactants in ice water. Doping concentration: NaCl are 0, 1%, 2%, 3%, and 5% molar fraction and NaF are 1%, 2.0%, 2.5%, and 3.0%. Weigh according to the set reactant stoichiometry, and grind the mixture thoroughly in an agate mortar for 30 minutes to ensure even mixing. Transfer it to a graphite crucible, place it in a well furnace, heat it up to 1550 ℃ at a heating rate of 5°/min, and keep it in an argon atmosphere for 30 minutes. Then, pour the molten reactants into ice water for rapid cooling, dry and grind to obtain the precursor glass sample PG. Finally, the precursor glass sample PG will be kept in a reducing atmosphere at 1100 ℃ for 1 hour, and then ground into a crystalline sample after cooling in the furnace.

When single doped with Eu²⁺, Eu²⁺ occupies the Mg²⁺ and channel sites in the α- cordierite structure, forming and luminescent structural units. When NaF is introduced, Eu²⁺ occupies the Na⁺ site to form a new luminescent structural unit . Introducing NaCl/NaF can enhance the occupation of channel sites by Eu²⁺. When F^-/Cl^- replaces O^2- and partially opens the hexagonal ring of the channel, the channel sites can accommodate more Eu²⁺, which makes it easier to explain the increased structure formed when NaCl/NaF is introduced. When NaCl is introduced, Eu²⁺ hardly occupies the Na⁺ sites, while when NaF is introduced, as the amount of NaF introduced increases, Eu²⁺ occupies more Na⁺ sites. That is because the stronger electronegativity of O^2- than Cl^-, replacing O^2- with Cl^- can introduce more covalences in the lattice, that means Eu²⁺- Cl^- has stronger contradiction and polarization than Eu²⁺-O^2-. Moreover, the ionic radius of Cl^- differs significantly from that of O^2-, making the formation of Eu²⁺-Cl^- more difficult. Therefore, when Cl^- is introduced, Eu²⁺ does not occupy the Na⁺ sites. Similarly, when F^- is introduced, Eu²⁺ is more likely to occupy Na⁺ sites. In addition, the amount of Eu²⁺ introduced is constant, and when Na⁺-Cl^- is introduced, Eu²⁺ does not occupy the Na⁺ sites. As the structure increases, the corresponding generated structure will decrease. As a ligand, compared to Mg²⁺-O^2-, the introduction of Na⁺-F^- has a smaller steric hindrance and greater bonding ability due to the smaller radius and greater electronegativity of F^- (compared to O^2-) ions. Eu²⁺ will preferentially occupy Na⁺ sites with similar radii and then occupy sites. As the amount of NaF introduced increases, more structures are formed while the corresponding structures decrease.

The main conclusions of this paper are summarized as following. This article mainly studies the effect of introducing Na⁺ sites and Cl^-/F^- substituted O^2- on the occupancy of Eu²⁺ in the α- cordierite structure. Exploring the mechanism of distinguishing subgrain structure evolution for luminescent structural probes. By introducing Na⁺ sites and anionic Cl^-/F^- substituted O^2- to regulate the selective occupancy of Eu²⁺, temperature sensitive emission from blue white to yellow light is achieved. Cl^-/F^- substituted O^2- opens the hexagonal ring structure of the channel, which facilitates the occupation of channel sites by Eu²⁺. The proportion of the formed structure increases from 33.32% to 64.89%. When introducing Na⁺ sites, the presence of F^- increases the proportion of Eu²⁺ occupying Na⁺ sites to form structures to 18.63%. This series of fluorescent powders can significantly improve their thermal quenching performance when NaF is introduced, and have superior thermal stability at 150 ℃. This series of fluorescent powders can be widely used in the fields of adjustable luminescence, detection, and anti-counterfeiting.

The extensive use of fossil fuel resources has a serious impact on the environment, and hence energy conservation and CO₂ emission reduction have become the focuses of current development. Solid oxide fuel cells (SOFCs) have attracted the research attention because of the outstanding energy conversion efficiency, wide range of fuel choices, no pollution, and high quality waste. H₂ is the ideal fuel for SOFCs, but its low volumetric energy density and the requirement of high pressure to be liquefied, making it difficult to store and transport. At ambient temperature and pressure, methanol is liquid and its volumetric energy density is much higher than that of hydrogen, which makes methanol easy to storage and transport as a good hydrogen carrier. Furthermore, methanol has been quite mature production process. The industrial methanol production was hydrogenation of CO and CO₂. SOFCs can generate power with direct internal methanol reforming. However, the main problem of direct methanol fed SOFCs is carbon deposition. The formation of carbon deposition not only reduces the cell performance, but affects the anode structure as well. In this study, the anode surface of flat-tube anode supported SOFCs was modified by Barium. The cell performance, durability, and methanol conversion of the unmodified and modified SOFCs were compared and evaluated under methanol atmosphere with a steam/carbon ratio of 0.75.

The anode support was composed of nickel oxide with 3%Y₂O₃-stabilized ZrO₂ (NiO-3YSZ, in mole), and the anode functional layer was made up of nickel oxide with 8%Y₂O₃-stabilized ZrO₂ (NiO-8YSZ). The electrolyte, barrier layer, and cathode layer were 8YSZ, Gd_0.1Ce_0.9O_2-δ (GDC), and La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ-GDC (LSCF-GDC), respectively. The active cathode area was 60 cm². The cell is denoted as NiO-3YSZ|NiO-8YSZ|8YSZ|GDC|LSCF-GDC. Using 15.68 g Ba(NO₃)₂ powder dissolved in 200 mL deionized water, being stirred at 70 ℃ for 2 h, to obtain 0.3 mol/L barium nitrate precursor solution for wet impregnation. After sealing one end of the Ni/YSZ cell with wax, the prepared barium nitrate precursor solution (70 ℃) was added into the fuel channels of through the sampler, and the other end of the cell was sealed with wax. The cell was kept in an oven at 50 ℃ for 24 hours to ensure that the Ba(NO₃)₂ solution was fully immersed in the anode channels. Then, the cell was dried at 90 ℃ for 1 h to remove the wax, and the cell obtained as above preparation processes was recorded as Ba-Ni/YSZ cell. After the cell was reduced at 750 ℃ by H₂, 0.60 L/min (SLM) H₂ was used as fuel for the initial cell performance test. When methanol was used as fuel for the test, the methanol solution with a steam/carbon ratio of 0.75 was added to the anode channels, and the temperature of the water steam generator was set at 130 ℃. At this time, the corresponding methanol flow rate was 0.30 g/min, and 5 L/min air was passed into the cathode. Fuel cell testing equipment (SOFC-100W, Wuxi Lead Equipment Co., LTD.) and electro-chemical workstation (VMP3B-20, France BioLogic Co., LTD.) were used to test the electro-chemical performance and durability of the cell under different operating condition. The frequencies of electrochemical impedance spectroscopy (EIS) tests were ranging from 20 mHz to 30 kHz. The AC amplitude of EIS testing was 10 mV.

The scanning electrical microscopy (SEM) with energy dispersive spectrometer analysis proved that Ba element was effectively loaded on the surfaces of Ni/YSZ anode fuel channels. Under H₂ fuel, the current voltage (I-V) characteristics of Ni/YSZ and Ba-Ni/YSZ cells at 750 ℃ indicate that the open-circuit voltage (OCV) of Ni/YSZ and Ba-Ni/YSZ cells were 1.12 V and 1.07 V, respectively, which revealed that the cell air tightness was good. At 750 ℃, the maximum power density (P_max) of Ni/YSZ and Ba-Ni/YSZ cells were 555.66 mW/cm² and 507.03 mW/cm², respectively, indicating that Ba element had no obvious influence on the cell output power of flat-tube SOFC fueled by H₂. The electro-chemical performance of Ni/YSZ and Ba-Ni/YSZ cells in methanol atmosphere [r(S/C) = 0.75] showed that the power density of Ni/YSZ and Ba-Ni/YSZ cells at 750 ℃ and 0.8V were 326.72mW/cm² and 452.87mW/cm², respectively. It can be seen that the power density of Ba-modified cell was significantly improved when fueled by methanol. Under OCV condition, the methanol conversion rates of Ni/YSZ and Ba-Ni/YSZ cell were 89.40% and 92.70%, respectively, because Ba on the surfaces of the fuel channels promoted methanol conversion within the cell. The hydrophilicity of the Ba-modified cell was enhanced, and the water cracking into OH^- groups that were adsorbed on the Ni surface was promoted. The durability of Ni/YSZ and Ba-Ni/YSZ cells was tested at 750 ℃ under methanol [r(S/C) = 0.75] and 200 mA/cm². The results showed that the electro-chemical performance of Ni/YSZ cell deteriorated sharply shortly after 3 h to 4 h, while the voltage of Ba-Ni/YSZ cell kept stable. After running for more than 500 h, the OCV value of the Ba-Ni/YSZ cell remained stable, indicating that the air tightness of cell was good, and the voltage was 0.89 V with a deterioration rate of 0.01 (%)/h. The gas composition analysis of the Ba-Ni/YSZ cell during the durability test showed that the conversion rate of methanol in the modified cell during the test was stable, and the conversion rate was larger than 90%. According to the SEM images of the Ba-Ni/YSZ cell before and after the durability test, the average Ni particle size of anode functional layer of the Ba-modified cell lowered from 0.97 μm to 0.92 μm. This deduced that the Ni particle lost in the anode function layer of the Ba-Ni/YSZ cell was obvious after the durability test, leading to the performance deterioration of the cell. Raman spectra showed that Ni/YSZ cell has detectable carbon deposition after the durability test, while Ba-Ni/YSZ has no carbon deposition. This showed that Ba content on the surface of the anode support of flat-tube SOFC could effectively inhibit carbon deposition.

The performance and durability of the flat-tube Ba-Ni/YSZ cell were investigated in this study. The Ba-Ni/YSZ cell was prepared by wet impregnation. The power density of the Ba-Ni/YSZ cell with methanol under r(S/C)=0.75 at 750 ℃ was 452.87 mW·cm^-2 at 0.8 V, larger than that of Ni/YSZ cell. The durability of Ba-Ni/YSZ with methanol under r(S/C)=0.75 at 750 ℃ reached 500 h, while the Ni/YSZ cell revealed a sudden performance degradation. The Ni particle agglomeration was detected for the Ba-Ni/YSZ cell after the durability test, causing the performance degradation during the test. No carbon was observed on the surface of the Ba-Ni/YSZ after the durability test.

Alkali-activated fly ash cementitious materials are characterized by several advantageous properties, including corrosion resistance, high-temperature resistance, and the ability to solidify heavy metals. Additionally, these materials demonstrate a high utilization rate of fly ash and are produced through an environmentally friendly process. Despite these benefits, a significant limitation is their slow solidification and hardening at room temperature, which restricts their practical engineering applications. Consequently, alkali-seed composite activated solutions were used to prepare alkali-seed composite excitation fly ash (ASAFA) specimens, and their properties and structural evolution were explored. The results indicated that N-A-S-H gel and zeolite (sodalite and chabazite-Na) were the main components of alkali-seed composite activated solution, and their proportion played a crucial role in determining the excitation effect. Through adjustments in the activity of raw materials, reaction temperature and duration, it is possible to increase the total content and proportion of N-A-S-H gel and zeolite in the seed, thereby encouraging accelerated and uniform growth of the reaction products within the ASAFA system, and reducing the proportion of harmful pores in the hardened structure. As a result, the setting and hardening process and compressive strength of ASAFA was promoted.

The study utilized first-class fly ash (FA1) with a specific surface area of 430 m²/kg as the primary raw material. FA1 was subjected to sorting-roller grinding to achieve specific surface areas of 715 m²/kg and 1014 m²/kg, resulting in the production of FA2 and FA3, respectively. The activity of the three types of fly ash was assessed in accordance with GB/T 1596-2005 "Fly Ash Used in Cement and Concrete." The results indicated that the mechanical activation from the grinding process significantly enhanced the activity of FA2 and FA3 at all ages compared to FA1, with activity values at 28 days reaching 92% and 114%, respectively. An 8 mol/L NaOH solution was employed as the alkali activator for the synthesis process.

The alkali-seed composite activated solution was prepared by treating fly ash with the NaOH solution at elevated temperatures. During the preparation, four technological parameters were systematically adjusted: ① Type of fly ash used; ② Mass ratio of fly ash to NaOH solution; ③ Synthesis temperature; And ④ synthesis time. The ASAFA specimen was created using the alkali-seed composite activated solution in conjunction with FA1. For comparative purposes, a reference specimen (GB) was prepared using a mass ratio of 0.5:1.0 of NaOH solution and FA1. Cubic specimens measuring 40 mm × 40 mm × 40 mm were fabricated and cured at a temperature of (20 ± 3) ℃ and 60% relative humidity. Following the initial curing period, the specimens were demoulded and subjected to additional curing for 7 and 28 days prior to the compressive strength testing.

In the preparation of the alkali-seed composite activated solution, a significant amount of sodium hydroxide solution is utilized, with a mass ratio of 100:4 to fly ash. The simultaneous application of heating and stirring notably accelerates the geopolymerization process. After a synthesis duration of 0.5 h, a compound activator is produced, characterized by a zeolite phase that includes sodalite and sodium-chabazite, exhibiting a markedly higher content than that observed in conventional polymerization methods. As the synthesis time is extended to 1 hour, the sodalite content within the composite activator continues to rise, with the zeolite phase predominantly existing at the nanometer scale. However, upon further extending the synthesis time to 2 hours, a slight decrease in sodalite content is observed, accompanied by an increase in the N-A-S-H gel content. This phenomenon may be attributed to the interaction between the zeolite phase and the strong alkali present in the sodium hydroxide solution, which facilitates the dissolution of aluminum ions from the zeolite phase into the alkali solution. Consequently, the N-A-S-H gel is generated through a polymerization reaction, a process further evidenced by the blurring of grain corners of the zeolite phase. Overall, in the sample designated as AD0.5, a greater coexistence of N-A-S-H gels and zeolite is observed. In contrast, AD1 contains only a limited number of unreacted fly ash particles, while the N-A-S-H gel undergoes a secondary reaction, resulting in the formation of smaller zeolite grains. In AD2, the content of zeolite particles decreases slightly, yet the structure becomes denser, and the particle size increases, alongside a rise in the N-A-S-H gel phase content. The seeds present in the alkali-seed composite activator primarily consist of N-A-S-H gel and zeolite crystal phases. These nano-products serve as nucleation sites that facilitate the geopolymerization of fly ash. Additionally, they fill the spaces between fly ash particles, promoting uniform growth of the induced polymerization products throughout the system. Notably, an increased content of crystal seeds in the composite activator, particularly a higher proportion of small zeolite crystal phases, significantly enhances their positive impact on the advancement of geopolymerization.

The alkali-seed composite activated solution primarily consists of N-A-S-H gel and zeolite phases, with their content and proportions being influenced by several factors, including the activity and dosage of fly ash, as well as the synthesis temperature and duration. By optimizing these parameters, it is possible to achieve the final setting of ASAFA pastes within as little as 2 h. Furthermore, these optimized specimens exhibit significant compressive strength, reaching 4.5 MPa at 7 d and 13 MPa at 28 d. In contrast, alkali-activated fly ash paste demonstrates a considerably longer final setting time of nearly 100 h, along with a much lower compressive strength of only 0.2 MPa at 7 d. This comparison underscores the advantages of using alkali-seed compounds in enhancing the performance of ASAFA pastes.

The pore structure of cement-based materials significantly influences their compressive strength. Studies have shown that the nanoscale pore structure significantly changes with variations in water content. Traditional pore measurement methods like mercury intrusion porosimetry and gas adsorption require the drying of specimens, potentially altering or even destroying the pore structure. The accuracy and representativeness of the compressive strength-pore structure relationship model established on this basis are therefore questionable. To accurately describe the important correlation between compressive strength and pore structure of cement-based materials, low-field magnetic resonance relaxation technique is utilized to carry out in-situ, nondestructive testing of saturated cement mortar doped with air-entraining agent. This is then combined with compressive strength measurement results to validate and refine the model describing the relationship between compressive strength and pore structure characteristics.

(1) Specimen preparation To avoid the ambiguous effects of ferromagnetic substances when using by low-field nuclear magnetic resonance (LF-NMR) technique, white Portland cement with low Fe₂O₃ content was used to prepare cement mortars with water-to-cement ratio of 0.4. Cement to sand ratio is 1∶2. And air-entraining agents (SJ-2 type) were incorporated. Chemical composition of white cement were 0.50% Fe₂O₃, 64.60% CaO, 21.71% SiO₂, 4.60% Al₂O₃, 2.80% SO₃, 2.43% MgO, 0.48% R₂O. The mass ratio of air-entraining agents to cement was 0%, 0.05%, 0.10%, and 0.15%. Considering different mass ratios, they are named as WP-BLK, WP-SJ05, WP-WP-SJ10, and WP-SJ15, respectively.

According to the designed proportions, the white cement, sand, air-entraining agents and water were mixed and casted into prisms of size 20 mm×20 mm×50 mm. After demounting at 24 h, they were all cured in saturated lime water at (20±2) ℃ for 180 d. After reaching the specified age, ten specimens were taken from each group of mortar and put into the high-pressure water saturation equipment for 3 d to ensure that the specimens were completely saturated and set aside.

(2) Pore structure test The pore structure of saturated mortars were tested by LF-NMR technology. Utilising a 2 MHz NMR analyser manufactured by Limecho Ltd., China, transverse relaxation test was performed through the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence at controlled room temperature about 20 ℃. In detail, the echo time τ_e (s) was adopted as short as 60 μs to detect the water confined in nanoscale pores of fast relaxation. Preliminary experimental tests had revealed that echo time τ_e ≤100 μs was short enough to make the contribution of diffusion relaxation negligible due to the controlled low concentration of Fe₂O₃ in white cement pastes. Moreover, A long sequence of N = 40 000-60 000 echoes generated at time τ= nτ_e (n=1, 2, …, N) (s) was recorded to capture the water stored in large capillary pores and even air voids with long relaxation time. 256-1024 scans with enough long repetition time 5-15 s were averaged. Except for the echo time, the rest of the parameters were flexibly adjusted according to water content of specimen. One can see detailed steps for testing pore structure by low-field magnetic resonance techniques in the relevant literature. (3) compressive strength test The compressive strength test of specimens were carried out by YAW-300 microcomputer automatic cement folding testing machine, which was produced by Jinan Hengruijin Testing Machine Co., Ltd., with a maximum capacity of 100 kN. The size of the compression surface of the specimen is 20 mm×50 mm, and the loading rate is fixed at 0.2 kN/s.

LF-NMR technology accurately captures the pore structure characteristics of saturated mortar. Empirical models using total porosity or graded porosity as variables are generally effective, but the empirical parameters obtained from model fitting are not stable enough, limiting the model's applicability. An improved empirical model, based on the pore size distribution curve and assuming a linear relationship between the weights of different pores on compressive strength and their logarithmic pore sizes, offers a concise form, significantly improves fitting accuracy, and accurately reflects the differences within the specimen groups. Furthermore, building on Griffith's fracture theory, the theoretical model, which uses total porosity and logarithmic mean radius as independent variables, exhibits complex expressions and low predictive accuracy. If pores with a radius above 10 nm are regarded as defects, their influence can be explained by Griffith's fracture theory. Meanwhile, pores with a radius of 10 nm or less are regarded as internal pores of the gel, and its effect on the strength of the matrix is approximated using the model description proposed by Powers. The resulting modified Griffith fracture theory model is very high in prediction accuracy, stable, and interpretable, accurately reflecting how minor differences in pore structure lead to small changes in compressive strength.

The incorporation of air-entraining agents mainly increases the capillary pores above 10 nm in radius but also decreases the matrix pores below 10 nm in radius. LF-NMR technology can obtain the full-scale pore structure of cement mortar, aiding in the description of the fundamental correlation between pore structure characteristics and compressive strength.

Salt crystallization plays an important role in the physical weathering process of materials, and is generally considered to be one of the main problems affecting the durability of materials, and most of the weathering damage of porous materials is related to salt crystallization. Since the permeation and crystallization of salt solutions are unavoidable. Therefore, it is of great value to explore the crystallization process and mechanism of salt in pores for the study of material durability in the fields of construction engineering, historical building protection engineering, traffic engineering, underground and geotechnical engineering, etc, and is of great significance for improving the disaster prevention and mitigation capabilities of historical buildings and infrastructure, and reasonably predicting the service life of structures. At present, the supersaturated crystallization pressure hypothesis is generally accepted as the cause of salt crystallization failure of porous materials, which holds that the crystallization pressure exerted by crystal on the pore surface is the main cause of the failure of cement-based materials. However, existing study has found that the actual crystallization pressure produced by NaCl evaporation crystallization is about 0.46 MPa, which is much lower than the tensile strength of concrete, and the concrete material still subjected to damage. Therefore, in this paper, the finite element method is used to study the crystallization process of NaCl in the micro pore of hardened cement paste, and the mechanism of crystallization pressure during the crystallization process is revealed. In addition, the effects of pore and crystal geometry, crystallization pressure and filling degree on the maximum tensile stress inside the hardened cement paste are also discussed.

The micro pore model and NaCl crystal model of hardened cement paste are established respectively, and the NaCl crystal model is horizontally put into the micro pore model of hardened cement paste to obtain the NaCl crystallization process model in the micro pore of hardened cement paste. In order to accurately distinguish the crystallization process of NaCl crystal, a two-dimensional spatial filling degree is introduced. The model of NaCl crystallization process in micro pore of hardened cement paste is established in three steps. The first step is to determine the geometric properties of the crystal. The size change of the crystals in the NaCl crystallization process model is controlled by the filling degree, and the crystals without pore wall confinement are obtained, and the crystals in the pore confined state are obtained by Boolean operation. The second step is to split the model. The NaCl crystallization process model is divided into two parts: the hardened cement paste micro pore model and the NaCl crystal model. The third step is to apply crystallization pressure. The stress distribution diagram of hardened cement paste and NaCl crystal is obtained by applying crystallization pressure to the contact parts between the crystal and the pore wall in the micro pore model and the NaCl crystal model. Results and discussion During the crystallization process of NaCl in the micro pore of the hardened cement paste, the tensile stress region is distributed within 2 μm of the pore radius, and the maximum tensile stress in the crystallization process is about 3 times of the crystallization pressure, which appears at the apex of the long axis of the pore, while the NaCl crystal is mainly compressed and the tensile stress is small. NaCl crystal do not fail before hardened cement paste. The pore axial ratio, filling degree and crystallization pressure are the main factors influencing the maximum tensile stress in the cement-based material during the crystallization process. The maximum tensile stress of hardened cement paste increases with the increase of pore axial ratio, filling degree and crystallization pressure during the crystallization process, while the crystal axial ratio has no obvious effect on the maximum tensile stress.

There are free crystallization and crystallization restricted stages in the crystallization process of salt in micro pore. The crystallization pressure generated by the crystallization restricted stage is a major factor causing damage to cement-based materials, and the tensile stress concentration caused by crystallization pressure is the direct cause of the damage of hardened cement paste. The crystallization pressure leads to the tensile stress concentration in the hardened cement paste, and the maximum tensile stress is about 3 times of the crystallization pressure, which appears at the apex of the long axis end of the pore. NaCl crystals will not be damaged before hardened cement paste. In addition, the pore axial ratio, filling degree and crystallization pressure are the main factors affecting the maximum tensile stress in the hardened cement paste during the crystallization process.

Mineral carbon sequestration is a crucial area of research in both environmental protection and industry. While much of the existing research has focused on the direct carbon sequestration of minerals to enhance cementation properties, there is a gap in understanding the interface micro-structure evolution of mineral powder particles during the carbonization process for the use of supplementary cementing materials. This study delves into the investigation of three calcium silicate minerals found in steel slag, examining the single-particle carbonization reaction rate and the micro-evolution process of the reaction interface at a micron scale. The goal is to uncover the differences and mechanisms behind the carbon fixation reaction rates of these three calcium silicate minerals.

Three types of calcium silicate minerals, namely γ-C₂S, β-C₂S, and C₃S, were prepared using the sintering method. A time gradient experiment was conducted on these minerals, and the evolution of multi-scale products was observed and analyzed through various microscopic techniques. Quantitative analysis using XRD and TG-DTG was employed to compare the carbon sequestration rates of the three minerals, and reaction equations were constructed through fitting analysis. The morphology, crystal structure, and growth characteristics of calcium carbonate products during the carbon sequestration reactions of the three minerals were analyzed using SEM, TEM, and crystal size calculations. The correlation between the differences in carbon sequestration rates among the three minerals and the evolution of micro-structure was elucidated.

γ-C₂S had the highest carbon sequestration rate, which was 47.40% after 2880 min of carbon sequestration, reaching 92.65% of the theoretical carbon sequestration, and still had an exposed reaction surface at the later stage of the reaction. During the carbon sequestration process of β-C₂S, the carbon sequestration rate increases and the particle size, grain size and structure distribution of the product calcium carbonate aggregate are relatively stable. C₃S has the highest carbon sequestration rate. Due to its high reactivity, C₃S stagnates rapidly after 5 min, and then the carbon sequestration rate increases slowly, but the crystal shape of surface calcium carbonate products changes with time.

The carbon sequestration reaction processes of γ-C₂S and β-C₂S conform to BoxLucas1 model. The first 60 min is the rapid carbon sequestration stage of γ-C₂S and β-C₂S, and the inflection point of reaction rate reduction occurs in 180 min and 360 min respectively, and the reaction plateau begins at 360 min. The carbon sequestration reaction of C₃S is consistent with the nuclear shrinkage reaction model, and almost stops after 5 min. The calcite produced by the three minerals tend to grow on the (104) rhomboid plane, and the growth of calcite is controlled by the competition between grain growth and grain size reduction.

γ-C₂S and β-C₂S carbon fixation product calcium carbonate is mainly calcite, C₃S carbon fixation product calcium carbonate also contains relatively more aragonite. The growth of calcium carbonate products on the surface of γ-C₂S particles shows a transition from epitaxial growth to mantle growth. The epitaxial growth in the early stage leads to more reaction surfaces and increases the grain size of calcite. The growth of calcium carbonate products on the surface of β-C₂S particles is from mantle growth to epitaxial growth. The mantle growth in the early stage leads to denser nucleation, and the reaction ability is poor in the later stage, but the grain size development is more stable. On the surface of C₃S, calcite and aragonite are mixed, and the composite structure of acicular aragonite and brick calcite hinders the gas-solid contact reaction.

The development rules of carbon sequestration rate of γ-C₂S and β-C₂S are similar, and both conform to BoxLucas1 model; The calcium carbonate crystals on the surface of the two C₂S particles are dominated by calcite, and tend to grow more preferentially on the (104) thermo-dynamically stable surface. The calcium carbonate on the surface of γ-C₂S particles grows from mantle to epitaxial. The epitaxial growth in the early stage leads to a larger reaction contact surface, and it still has reactive activity in the late stage, and the calcite grain size is larger. However, β-C₂S epitaxial growth to mantle growth, the early mantle growth led to denser nucleation, the late reaction ability is poor, the grain size development is more stable. Due to its high calcium content, C₃S has the fastest carbon sequestration reaction at the initial stage, and quickly stops after 5 minutes of reaction, which conforms to the nuclear shrinkage reaction model. C₃S presents different product evolution rules: In the early stage of carbon sequestration reaction, acicular aragonite and cubic calcite on the surface of C₃S are mixed, and its dense structure causes premature stagnation of carbon sequestration reaction. As the reaction proceeds, aragonite is transformed into calcite.