Ceramic Transformer Research Articles

Pyrolysis mechanism of silicone rubber thermal protection system materials in service environment

The excellent oxidation and ablation resistance of silicone rubber thermal protection system (TPS) materials have made them extensively utilized in large-area thermal protection applications, such as ramjet engines and spacecraft reentry capsules, where air is present in the service environment. The ablation resistance is significantly influenced by the pyrolysis reactions, which serves as the foundation for subsequent ceramic transformation and the development of anti-erosion structures. The majority of previous research has been conducted in inert gas environments. To investigate the pyrolysis mechanism of silicone rubber TPS materials in realistic service environments, thermal analysis tests were performed using a range of analytical instruments including a thermal analyzer, mass spectrometer, infrared spectrometer, and X-ray photoelectron spectrometer from room temperature up to 1300 K in air atmosphere and compared with the results in argon atmosphere. The results demonstrate that silicone rubber TPS materials undergo both pyrolysis and oxidation reactions in air atmosphere. The primary pyrolysis of the matrix is attributed to cyclization reactions and side-chain cross-linking reactions. In comparison to argon atmosphere, the oxidation reaction produces a greater amount of organic gases, thereby diminishing the extent of cross-linking reaction and the thermal stability of the pyrolysis residue. The prolonged exposure to elevated temperatures allows for an extended duration of oxidation reactions, consequently diminishing the long-term thermal stability of TPS materials. The increase in mass of TPS materials at temperatures exceeding 1100 K can be attributed, in part, to the formation of new organic groups through oxidation reactions. The observed morphology of the residue, along with the weight gain from oxidation and SiO2 formation, promotes ceramic transformation of the materials at elevated temperatures.

Negative thermal expansion performance of (NaMg)xCr2-x(MoO4)3 (0≤x≤1) ceramics

Cr2Mo3O12 ceramics show negative thermal expansion (NTE) in a narrow high-temperature range due to the phase transition around 385 °C. To widen its NTE temperature range, a series of (NaMg)3+ co-doped Cr2Mo3O12 ceramics has been synthesized via a solid reaction process in this work. The effects of double cation (NaMg)3+ co-doping on the phase composition, morphology, phase transformation, and thermal expansion performance of Cr2Mo3O12 ceramics were studied using XRD, Raman, XPS, SEM, and TMA. Results indicate that double cation (NaMg)3+ can partially substitute Cr3+ in Cr2Mo3O12, which causes a phase transition from monoclinic Cr2Mo3O12 to hexagonal (NaMg)xCr2-xMo3O12. The single-phase (NaMg)xCr2-xMo3O12 ceramics with hexagonal structure is obtained when x = 0.5. (NaMg)3+ co-doping promotes the growth of grains and the density of hexagonal (NaMg)0·5Cr1·5Mo3O12 ceramics. (NaMg)3+ co-doping also successfully eliminates the temperature-induced phase transition of Cr2Mo3O12 at 385 °C. Hexagonal (NaMg)0·5Cr1·5Mo3O12 ceramics presents a better NTE with a coefficient of thermal expansion (CTE) of −3.97 × 10−6 °C−1 over a larger temperature range from 30 °C to 700 °C.

Catalytic Thermocuring and Synergistic Photothermocuring of Single-Component Acrylate-Grafted Liquid Oligosilazanes.

A novel thermosetting preceramic resin called acrylate-grafted liquid polysilazane (ALSZ) was readily synthesized. The curing behaviors of ALSZ were investigated by the techniques of differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and rheological tests. The catalytic thermocuring process was controlled by the addition of a polymerization accelerator composed of a radical initiator (cumene hydroperoxide) and a transition metal catalyst (nickel naphthenate or cobalt naphthenate). Photocuring at room temperature can proceed readily by the addition of photosensitizer 819 (phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide). By combining a radical initiator, a transition metal catalyst, and a photosensitizer, synergistic photothermocuring was achieved, demonstrating advantages such as material shaping at room temperature and low weight loss during curing. The ceramization of the solidified ceramic precursors in an Ar atmosphere was studied using TGA and tube furnace pyrolysis. ALSZs exhibited comparatively high ceramic transformation yields (71-75% at 800 °C). The resulting pyrolytic ceramics maintained their original shape without deformation or foaming expansion. Polysilazanes containing acrylate groups can directly form casting bodies, showing a high static glass transition temperature (>380 °C) by thermomechanical analysis (TMA). FT-IR analyses revealed that multiple reactions are involved in the curing of ALSZ. The results in this paper showed that ALSZ might find prospective applications in material processing, such as additive manufacturing and ceramic-matrix composites.

WO3/ZnWO4 microcomposites with potential application in photocatalysis

The paper presents the results of a comprehensive study of the structural, optical and strength features, changes in the morphology of grains of (1-x)WO3 – xZnO ceramics depending on the concentration of the initial components when they vary. Using the X-ray diffraction method, it was found that mechanochemical grinding of samples of (1-x)WO3 – xZnO ceramics, regardless of the concentration of the components, does not lead to the initiation of phase transformation processes associated with the formation of the monoclinic ZnWO4 phase, the formation of which is observed during thermal annealing of samples at a temperature of 1000 °C. Based on data obtained using scanning electron microscopy and X-ray diffraction methods, the dynamics of phase transformations in ceramics of the type WO3/ZnWO4 → ZnWO4 → ZnO/ZnWO4 was established. It was found that with a given ratio of WO3: ZnO components equal to 0.7 : 0.3, under selected conditions of mechanochemical grinding and an annealing temperature of 1000 °C, highly ordered single-phase ZnWO4 ceramics with a wolframite structure can be obtained. Analysis of the optical characteristics and changes in the band gap depending on the ratio of the components showed a direct dependence of changes in the band gap on the phase composition of the ceramics. An assessment of the strength characteristics made it possible to establish that the formation of two-phase WO3/ZnWO4 ceramics leads to strengthening of the surface layer, while for two-phase ZnO/ZnWO4 ceramics a decrease in hardness is observed, the decrease of which directly correlates with changes in the content of the ZnO phase in the composition of the ceramics.

Properties and ceramic transformation of Si–Zr–O–C precursor ceramics with porous structure

Properties and ceramic transformation of Si–Zr–O–C precursor ceramics with porous structure

Effect of electric field on the structure of SiCN pyrolyzed at 1600 °C

Application of electric field is known to influence the phase transformation and sintering conditions of ceramics. In this work, the effect of electric field on the phase structure evolution of SiCN pyrolyzed at 1600 °C was studied in comparison with conventional pressureless pyrolyzed SiCN. The structural information was characterized via X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and Raman spectroscopy. It was found that SiCN ceramic prepared via conventional pressureless pyrolysis was composed of SiC, Si3N4 and free carbon phases, while that produced via electric field-assisted pyrolysis consisted of Si3N4 and free carbon phases. No crystalline SiC was detected in SiCN ceramic pyrolyzed via the field-assisted method, indicating that the electric field affected the carbothermal reaction between Si3N4 and C phases and changed the formation mechanism of SiC. Moreover, the electric field promoted structural ordering rearrangement of the free carbon phase and increased the defect content therein.

Improvement of Phase Transformation and Densification of SiO2-ZrO2 Ceramics by Using Desert Sand as a SiO2 Source

Improvement of Phase Transformation and Densification of SiO2-ZrO2 Ceramics by Using Desert Sand as a SiO2 Source

Realizing reversible phase transformation of shape memory ceramics constrained in aluminum

Small-scale shape memory ceramics exhibit superior shape memory or superelasticity properties, while their integration into a matrix material and the subsequent attainment of their reversible tetragonal-monoclinic phase transformations remains a challenge. Here, cerium-doped zirconia (CZ) reinforced aluminum (Al) matrix composite is fabricated, and both macroscopic and microscopic mechanical tests reveal more than doubled compressive strength and energy absorbance of the composites as compared with pure Al. Full austenitization in the CZ single-crystal clusters is achieved when they are constrained by the Al matrix, and reversible martensitic transformation triggered by thermal or stress stimuli is observed in the composite micro-pillars without causing fracture in the composite. These results are interpreted by the strong geometric confinement offered by the Al matrix, the robust CZ/Al interface and the local three-dimensional particle network/force-chain configuration that effectively transfer mechanical loads, and the decent flowability of the matrix that accommodates the volume change during phase transformation.

Fabrication of high-performance Magnéli phase Ti4O7 ceramics by in-situ hot-pressed sintering in a single step

Magnéli phase ceramics have gained extensive attention in a variety of fields, including batteries, electrocatalysis, and wastewater treatment, due to their distinctive properties. In this study, high-performance Magnéli phase Ti4O7 ceramics were synthesized by in-situ hot-pressed sintering of titanium (Ti) and titanium dioxide (TiO2) in a single step. The effects of the sintering temperature and Ti content on the phase transformation and morphology of ceramics were investigated. Moreover, the mechanical properties and electrical conductivity of the samples were systematically characterized. Results show that the sample with the Ti/TiO2 weight ratio of 0.09 sintered at 1000 °C has impressive comprehensive properties, including a high bulk density of 4.281 g·cm−1, a mean value of flexural strength of 125.58 MPa, and an excellent conductivity of 1129 S·cm−1. Furthermore, an evolution mechanism of the in-situ sintering for Ti/TiO2 was discussed based on the experimental data, and the pattern of variation observed by samples can be reasonably explained.

The Thermal Stability, Ablation Properties, Creamization Mechanism of Graphite/Zirconia (ZrO2)/Silicone Rubber Composites Containing Carbon Multicomponent Systems

It is urgent to improve the long-term ablation resistance of silicone rubber composites in order to meet the application requirements in the field of aerospace thermal protection. The graphite/ZrO2/silicone rubber (G/ZrO2/SR) composites prepared in this study were for that purpose. Firstly, the thermal stability and ablation properties of the G/ZrO2/SR composites were studied. The results showed that with the increase of graphite content, the initial decomposition temperature of the composites increased from 456.5 °C to 478.6 °C. The linear and mass ablation rates of the G/ZrO2/SR composite with 3 phr graphite were 0.048 mm/s and 0.0,220 g/s, respectively, which were decreased by 14.3% and 9.1%, compared with the ZrO2/SR composite without graphite. Moreover, the phase composition transformation and ceramization mechanism at high temperature of the G/ZrO2/SR composites containing carbon multicomponent system were studied. The results showed that the SR matrix completed the ceramic transformation during the ablation process and -Si-O-C pyrolysis products were formed and served as the skeleton of the ceramic layers. Finally, SiO2 and ZrO2 reacted with the graphite individually and the in-situ SiC and ZrC ceramic products formed enhanced the ceramic layers.

The Influence of Alumina Airborne-Particle Abrasion with Various Sizes of Alumina Particles on the Phase Transformation and Fracture Resistance of Zirconia-Based Dental Ceramics.

The surface of zirconia-based dental ceramic restorations require preparation prior to adhesive cementation. The purpose of this study was to assess the influence of airborne-particle abrasion with different sizes of alumina particles (50 μm, 110 μm, or 250 μm) on the mechanical strength of zirconia-based ceramics' frameworks and on the extent of phase transformations. A fracture resistance test was performed. The central surface of the frameworks was subjected to a load [N]. The identification and quantitative determination of the crystalline phase present in the zirconia specimens was assessed using X-ray diffraction. The Kruskal-Wallis one-way analysis of variance was used to establish significance (α = 0.05). The fracture resistance of zirconia-based frameworks significantly increases with an increase in the size of alumina particles used for air abrasion: 715.5 N for 250 μm alumina particles, 661.1 N for 110 μm, 608.7 N for 50 μm and the lowest for the untreated specimens (364.2 N). The X-ray diffraction analysis showed an increase in the monoclinic phase content after air abrasion: 50 μm alumina particles-26%, 110 μm-40%, 250 μm-56%, and no treatment-none. Air abrasion of the zirconia-based dental ceramics' surface with alumina particles increases the fracture resistance of zirconia copings and the monoclinic phase volume. This increase is strongly related to the alumina particle size.

Effect of copper phosphate on the sintering and ablation resistance of novel ZrSiO4 based composites

ZrSiO4 based composites (ZSCC) were synthesized using copper phosphate as a sintering aid, via sintering at 1000 °C without pressure. The research aimed to investigate the microstructure, mechanical properties, thermophysical properties, and ablation resistance of ZSCC. This was done to enhance the understanding of the ceramic transformation mechanism and ablation mechanism of ZSCC at high temperatures. Results showed that the ZSCC obtained at low-temperature curing had oxide and phosphate phases at room temperature. As the temperature increased, the density of the sample improved, its porosity reduced, and its mechanical properties became more robust. Furthermore, a zirconium silicate ceramic phase was formed. The ZSCC showed remarkable ablation resistance due to the formation of dense oxide layers of ZrSiO4 and ZrO2. After oxyacetylene flame ablation lasting 60 s at 2000 °C, the mass and linear ablation rate of ZSCC were 5.38 mg/s and 8.38 μm/s, respectively.

Ab initio study of electric field effects on phonon vibrations in tetragonal ZrO2

The effects of external electric fields on phonon-associated phenomena, such as phase transformation and diffusion in ZrO2 ceramics, have been reported from recent experiments. This study examined the effects of external direct current (DC) electric fields on the phonon vibration properties in a tetragonal ZrO2 unit cell based on the density-functional perturbation theory. Phonon dispersions and densities of states were analyzed with optimized structures under varying external DC electric fields up to 45 mV/Å. The field sensitivities of phonon characteristics exhibited significant orientation dependence and were attributed to ionic polarization associated with symmetry breaking in dielectric properties. Optical phonons showed considerable field sensitivities particularly near the Brillouin zone boundaries, such as in the M (π/a, π/a, 0) to X (0, π/a, 0) and A (π/a, π/a, π/c) to R (0, π/a, π/c) directions, where doubly degenerate phonon frequencies exhibited splitting behaviors associated with the symmetry breaking between two unique oxygen atoms in the original unit cell. In contrast, transversal acoustic phonons demonstrated a softening trend with increasing field strengths around the Z (0, 0, π/c) point, where imaginary and splitting frequencies were obtained under electric fields of >40 mV/Å, indicating the potential phase transformation from the tetragonal to orthorhombic symmetries under strong external electric fields.

Effect of sintering temperature on phase transformation and energy storage properties of 0.95NaNbO3-0.05Bi(Zn0.5Zr0.5)O3 ceramics

Dielectric materials with excellent energy storage performance are crucial to the development of renewable energy. In this work, we prepared 0.95NaNbO3–0.05Bi(Zn[Formula: see text]Zr[Formula: see text])O3 (0.95NN–0.05BZZ) ceramics using solid state sintering and investigated the effect of sintering temperature on phase structure. We find that the phase transformation of ceramics occurs with the change of sintering temperature. The cubic phase (Pm-3[Formula: see text]) and the antiferroelectric phase (Pbma) coexist at 1150[Formula: see text]C, the ferroelectric phase ([Formula: see text]21ma) appears at 1200[Formula: see text]C and its phase proportion decreases with the sintering temperature increasing from 1200[Formula: see text]C to 1280[Formula: see text]C. Finally, we achieve the high recoverable energy storage density ([Formula: see text][Formula: see text]) 0.74 J/cm3and efficiency ([Formula: see text]) 71% (at 140 kV/cm) at 1150[Formula: see text]C.

Phase transformation of cold-sintered doped barium titanate ceramics during the post-annealing process

The cold sintering technique has been developed to densify electroceramics at significantly low temperatures to tackle high energy consumption as well as compatibility of electroceramic components in modern electronics. However, obstacles still exist since, to obtain desired electrical properties, a post-annealing procedure at elevated temperatures must follow to trigger compulsory phase formation and/or grain growth. This work investigates the BaTiO3 and (Ba,Sr)TiO3 ceramics via both the cold and conventional sintering routes, and then compares their phase transformation behaviors during the post-annealing process. Results suggest that, given similar particle and grain sizes, the cold-sintered ceramics tend to retain a substantial part of the pristine tetragonal phase at annealing temperatures of <800 °C, while the cubic phase tends to dominate when annealed at >800 °C. This indicates a process-stimulated phase transformation that may guide or limit the selection of annealing temperature in practice.

Study of the Mechanisms of Polymorphic Transformations in Zirconium Dioxide upon Doping with Magnesium Oxide, as Well as Establishing the Relationship between Structural Changes and Strength Properties

The aim of this work is to study the mechanisms of polymorphic transformations in ZrO2 ceramics doped with MgO with different concentrations during thermal isochronous annealing, as well as the effect of the phase composition of ceramics on the change in strength properties and resistance to mechanical stress. Solving the problem of polymorphic transformations in zirconium dioxide by doping them with MgO will increase the resistance of ceramics to external influences, as well as increase the mechanical strength of ceramics. According to the data of X-ray phase analysis, it was found that the addition of the MgO dopant to the composition of ceramics at the chosen thermal annealing temperature leads to the initialization of polymorphic transformation processes, while changing the dopant concentration leads to significant differences in the types of polymorphic transformations. In the case of an undoped ZrO2 ceramic sample, thermal annealing at a temperature of 1500 °C leads to structural ordering due to the partial removal of deformation distortions of the crystal lattice caused by mechanochemical grinding. During the study of the effect of MgO doping and polymorphic transformations in ZrO2 ceramics on the strength properties, it was found that the main hardening effect is due to a change in the dislocation density during the formation of a ZrO2/MgO type structure. At the same time, polymorphic transformations of the m—ZrO2 → t—ZrO2 type have a greater effect on hardening at low dopant concentrations than t—ZrO2 → c—ZrO2 type transformations.

Unveiling the dynamic instability mechanism of microstructure transformation in faceted oxide eutectic composite ceramics

Microstructure control is a great challenge in the high-temperature gradient directional solidification of eutectic composite ceramics due to the complex solidification behavior. Herein, the microstructure transformation of faceted Al2O3/Er3Al5O12 thermal emission eutectic composite ceramics is explored over wide ranges of compositions (13.5 mol%–22.5 mol% Er2O3) and solidification rates (2–200 μm/s). Entirely coupled eutectics with primary phases suppressed are fabricated and the coupled zone is broadened in a wide range of 15.5 mol%–22.5 mol% Er2O3 at low solidification rates. The competitive growth between eutectic and dendrite is evaluated on the basis of the maximum interface temperature criterion. In addition, the mechanisms of irregular eutectic spacing selection and adjustment under different solidification rates are revealed based on Magnin–Kurz model. A successful prediction of lamellar to rod-like eutectics is achieved associated with the dynamic instability of lamellar eutectic and the corresponding enlarged coexistence region is mapped based on the interface undercooling. According to the well microstructure tailoring, the flexural strength of Al2O3/Er3Al5O12 eutectic composite ceramics has improved from 508 MPa up to 1800 MPa due to the refined eutectic spacing with low fluctuation. The eutectic composite ceramics show strong selective optical absorption and the intensity increases with the refining microstructure. The as-designed Al2O3/Er3Al5O12 composites with microstructural tailoring have great potential as integrations of structural and functional materials.

Deposition and study of plasma sprayed Al2O3-TiO2 coatings on AZ31 magnesium alloy

In this study, Al2O3, TiO2 and Al2O3+ 3wt%TiO2 coatings were deposited on AZ31 Mg alloy substrate by plasma spraying. The coatings were structurally characterized by scanning electron microscopy and X-ray diffraction. Corrosion experiments were carried out in 3.5% NaCl solution. Adhesion tests were performed according to daimler benz VDI-3198 standard. A coating layer of approximately 70 microns in thickness was deposited. High plasma enthalpy caused phase transformations in alumina-based ceramics. As a result of electrochemical corrosion study, it was determined that the coatings increased the corrosion resistance of AZ31 Mg alloy. While the most corrosion resistant coating is Al2O3+ 3wt%TiO2, the weakest coating against corrosion is TiO2. The adhesion behavior of all coatings to the substrate was at an acceptable quality levels.

Study of Polymorphic Transformation Processes and Their Influence in Polycrystalline ZrO2 Ceramics upon Irradiation with Heavy Ions

The aim of this work was to study the mechanisms of polymorphic transformations in ZrO2 ceramics under irradiation with heavy ions, as well as to determine the nature of structural distortions in the case of t-ZrO2 → c-ZrO2 type transformations and associated anisotropic deformations. The samples of ZrO2 ceramics were irradiated with Kr15+ heavy ions with an energy of 150 MeV and fluences of 1011–1016 ion/cm2. During evaluation of the structural changes depending on the irradiation fluence, it was found that at low irradiation fluences (1011–1012 ion/cm2), the main role is played by deformation distortions of the crystal lattice, which have a pronounced anisotropic character. Meanwhile, at fluences above 1013 ion/cm2, the main role is played by polymorphic transformations of the t-ZrO2 → c-ZrO2 type, followed by amorphization of the damaged layer at fluences above 1015 ion/cm2. It was established that the anisotropic distortion of the crystal lattice is more pronounced along the crystallographic a axis, as well as the (011) texture orientation, which is characteristic of t-ZrO2. The polymorphic transformation processes of the t-ZrO2 → c-ZrO2 type occur at irradiation fluences of 1013–1014 ions/cm2, which are characterized by the formation of an overlap of local areas of defects that appear along the trajectory of ions in the material. The dependences of changes in the strength and thermophysical properties of ZrO2 ceramics on the irradiation fluence were obtained. The mechanisms of influence of the structural disorder and polymorphic transformations on the decrease in strength and crack resistance were established.

A further step toward the understanding of phase transformation mechanism between 10 wt. % dysprosia stabilized zirconia powders and ceramics

In this study, 10 wt. % dysprosia stabilized zirconia (10DySZ) ceramics were prepared by pressing and pretreatment method. The phase and structural evolution as well as the phase transformation behaviors of 10DySZ ceramics were investigated. Results show that the 10DySZ ceramics have the same phase composition as that of powders in final products. Differently, an asynchronous nucleation behavior of m- and c-DySZ can be recognized, the t→m martensite transformation was inhibited significantly during the incubation period of phase transformation. The phase transformation behaviors of 10DySZ ceramics were also confirmed to be followed the new phase nucleation and growth theory. In the early stage, the inhibition of phase transformation can be attributed to the comprehensive influence from steric hindrance, internal stress and critical size effect. However, in the later phase transformation explosion period, the rapid growth of new phases as well as the increased internal stress dominated the acceleration of phase transformation of 10DySZ ceramics.