Colloidal processing and partial sintering of high-performance porous zirconia nanoceramics with hierarchical heterogeneities

High-performance, porous, Y-TZP nanoceramics with hierarchical heterogeneities originating from uniform, intra- and inter-particle packing were prepared by colloidal processing and partial sintering of a mesoporous powder. The powder consisted of 100–150-nanometer-sized secondary particles initially composed of smaller, loosely aggregated, primary, nanoscale crystallites. Green bodies were prepared by centrifugal slip casting of weakly flocculated suspensions. During the initial stage of sintering, necking between the secondary particles was accompanied by intra-particle pore coalescence, while the pores originating from the secondary particle packing remained intact. Such microstructures with porosity levels between 46 and 18.3% and pore areas between 18 and 4 m²/g led to attractive properties, i.e., much reduced thermal conductivities of 0.63–1.88 W m^?1 K^?1, high bending strengths of 70–540 MPa and lowered elastic moduli of 32–156 GPa, making them potentially ideal as thermal insulators and/or load-bearing porous biomaterials owing to the possibility of further impregnation with bioactive ingredients.     

Polymerization of lactams, 95 Preparation of polyesteramides by the anionic polymerization of ε-caprolactam in the presence of poly(ε-caprolactone)

Preparation of polyesteramides-poly[(ε-caprolactam)-co-(ε-caprolactone)]s by anionic polymerization of ε-caprolactam in the presence of poly(ε-caprolactone) at 150 °C was studied in this paper. ε-Caprolactam magnesium bromide was used as an initiator of polymerization and polymeric materials containing 5-25 wt% ε-caprolactone units were obtained. Thermal methods (DSC and DMA) were employed for characterization of poly[(ε-caprolactam)-co-(ε-caprolactone)]s and their mechanical properties were also evaluated. By introducing the activator with N-acyllactam structure, the polymerization rate increased and it was possible to carry out the polymerization at 110 °C. Mechanical properties of polyesteramides were influenced by both the content of ε-caprolactone units incorporated into copolymer and polymerization temperature. The mechanism of incorporation of poly(ε-caprolactone) is discussed. The results show that it is not possible to restrict exchange transacylation reactions, progressing in the course of polymerization, by kinetic tools.     

Effects of Zn2SiO4 on the phase evolution,thermal expansion and mechanical properties of LiAlSiO4 ceramic

LiAlSiO₄ (abbreviated as LAS) ceramics doped with variable mass percent of Zn₂SiO₄ were prepared by conventional solid-state route. The effects of Zn₂SiO₄ on the phase evolution, microstructure, thermal expansion and mechanical properties have been fully investigated. The results show that Zn₂SiO₄ reacted with LAS matrix to produce Li₂Al₂Si₃O₁₀ and ZnAl₂O₄. Fine-grain ZnAl₂O₄ phase accumulated on the grain boundaries of the main phase, which was helpful to improve the density. Simultaneously, both of the flexure strength and Vickers hardness of the multiphase ceramics were significantly enhanced with the increasing mass percent of Zn₂SiO₄ for the reason of dispersion strengthening effect. In addition, when the content of Zn₂SiO₄ increased from 10?wt% to 22.5?wt%, the coefficient of thermal expansion (CTE) of the composite ceramics increased monotonously from ??5.24?×?10^?6/K to 1.49?×?10^?6/K. Typically, the LAS ceramic doping with 17.5?wt% Zn₂SiO₄ sintered at 1175?°C for 4?h possesses excellent properties: α?=?0.65?×?10^?6/K, H_v =?5.34?GPa, σ_s =?102.6?MPa, which is a promising material in laser gyroscope and precision machining fields.     

First principles calculations and synthesis of multi-phase (HfTiWZr)B2 high entropy diboride ceramics: Microstructural,mechanical and thermal characterization

First principles calculations were conducted on (HfTiWZr)B₂ high entropy diboride (HEB) composition, which indicated a low formation energy and promising mechanical properties. The (HfTiWZr)B₂ HEBs were synthesized from the constituent borides and elemental boron powders via high energy ball milling and spark plasma sintering. X-ray diffraction analyses revealed two main phases for the sintered samples: AlB₂ structured HEB phase and W-rich secondary phase. To investigate the performance of multi-phase microstructures containing a significant percentage of the HEB phase was focused in this study. The highest microhardness, nanohardness, and lowest wear volume loss were obtained for the 10 h milled and 2050 °C sintered sample as 24.34 ± 1.99 GPa, 32.8 ± 1.9 GPa and 1.41 ± 0.07 × 10^?4 mm³, respectively. Thermal conductivity measurements revealed that these multi-phase HEBs have low values varied between 15 and 23 W/mK. Thermal gravimetry measurements showed their mass gains below 2% at 1200 °C.     

Synthesis and characterization of layer-aligned poly(vinyl alcohol)/graphene nanocomposites

Layer-aligned poly(vinyl alcohol)/graphene nanocomposites in the form of films are prepared by reducing graphite oxide in the polymer matrix in a simple solution processing. X-ray diffractions, scanning electron microscopy, Fourier-transform infrared spectroscopy, differential scanning calorimetry and thermogravimetric analysis are used to study the structure and properties of these nanocomposites. The results indicate that graphene is dispersed on a molecular scale and aligned in the poly(vinyl alcohol) (PVA) matrix and there exists strong interfacial interactions between both components mainly by hydrogen bonding, which are responsible for the change of the structures and properties of the PVA/graphene nanocomposites such as the increase in T_g and the decrease in the level of crystallization.     

Influences of novel Si2BC3N antioxidant on the structure and properties of Al2O3-SiC-C castables: In air and coke bedded atmosphere

The ultra-low cement bonded Al₂O₃-SiC-C castables were prepared with the introduction of a novel Si₂BC₃N antioxidant. The microstructure evolution and the mechanical properties were evaluated in coke bed and air atmosphere. Besides, the thermal properties, including thermal shock, hot modulus of rupture, oxidation and refractories under load, were comparatively investigated. The results show that the Si₂BC₃N powder together with B₄C and Si can satisfy the oxidation resistance requirements over a full temperature range. Si₂BC₃N has mainly two effects depending on the treating temperature: 1) it protects the carbon from oxidation and increases the structure integrity when the specimens are treated below 1100?°C; 2) it stimulates the growth of SiC whiskers under 1400?°C due to the enhanced reaction between SiO and CO. Consequently, the CMOR and CCS of the Si₂BC₃N containing specimens have been improved attributing to the structural integrity and more SiC whiskers formation, regardless of the treating atmosphere. Besides, the thermal properties such as the hot modulus of rupture, thermal shock and refractories under load are also optimized with Si₂BC₃N addition.     

The significant influence of carbon content on mechanical and thermal properties of (VNbTaMoW)0.5Cx high entropy carbides

The high-entropy formation possibility of (VNbTaMoW)_0.5C_x was first analyzed by phase diagram and then a series of single-phase (VNbTaMoW)_0.5C_x was fabricated by SPS. We investigated the mechanical properties and thermal conductivity of (VNbTaMoW)_0.5C_x as C stoichiometry is varied. As C stoichiometry is increased from 0.345 to 0.35, multi-phase evolves into a single rock-salt phase. The high entropy phase formation depends on C atoms and vacancies, which increase the total number of microscopic states, and then increases the overall configuration entropy and lattice distortion energy. A highly carbon-deficient condition (C=0.345) leads to FCC structure disintegration and drives metal rich Mo₂C phase evolution. As the carbon stoichiometry increases from 0.35 to 0.5, the nanohardness and flexure strength of (VNbTaMoW)_0.5C_x first increase and reach the peak values of 48 GPa and 410 MPa ((VNbTaMoW)_0.5C_0.4), then decrease. The point defects concentration in (VNbTaMoW)_0.5C_x drastically change with carbon stoichiometry, which affects the structure, mechanical properties and thermal conductivity of high-entropy (VNbTaMoW)_0.5C_x. The thermal conductivity of (VNbTaMoW)C_x first decreases and then gradually increases with an increase in carbon content. Low carbon content (VNbTaMoW)C with primarily metallic bonding is electronically thermal conductivity. As the carbon content increase, the high-entropy (VNbTaMoW)C is covalent bonding and phonon contribution to thermal conductivity plays a great role.     

Recent advances and perspectives in carbon-based fillers reinforced Si3N4 composite for high power electronic devices

New advances in carbon-based fillers (CBFs) as reinforcing agents have gained worldwide attention due to their novel properties and promising applications to obtain advanced composite materials with superior electrical, mechanical and thermal performance. These CBFs (carbon nanotubes (CNTs), carbon nanofibers (CNFs), graphene, graphene oxide, and graphite) are important for ceramic materials to make them more attractive for modern industry. These materials in the ceramic matrix can enhance various properties, such as mechanical, thermal, and electrical conductivity, as a sensor material for pressure and other environmental changes. This overview introduces the latest developments in the fabrication of Si₃N₄ ceramics and the effect of CBFs as well SiC and SiC_w on structural, mechanical, and thermal properties of Si₃N₄ ceramics for next-generation electronic power devices. Moreover, we summarized the key aspects such as the fabrication approaches, influence of additive composition and concentration, and sintering parameters on the microstructure and overall performance of Si₃N₄ ceramics. In particular, design strategies for scientists and engineers concerned about the manufacture of Si₃N₄ substrate and active regeneration for the first time are proposed and discussed extensively.     

Effect of Ba concentration on phase stability and mechanical and thermal properties of La2Mo2O9

Ba-substituted La₂Mo₂O₉ ((La_1−xBa_x)₂Mo₂O_9−δ, x = 0–0.12) was prepared and the thermal and mechanical properties were evaluated. The thermal expansion coefficients (TECs) were determined from high-temperature X-ray diffraction (XRD) analysis. Phase transition in La₂Mo₂O₉ was suppressed via substitution of Ba for La, as demonstrated by differential scanning calorimetry (DSC) analysis. The mechanical properties, such as the bulk modulus, shear modulus, Young’s modulus, compressibility, and Debye temperature were evaluated from the measured sound velocities. The thermal conductivity was evaluated from the thermal diffusivity, heat capacity, and density in the temperature range from room temperature to 1073 K. The thermal conductivity decreased with increasing Ba content. Theoretical calculations based on the Klemens–Callaway model were performed to analyze the thermal conductivity, and the results suggest that the reduction of the thermal conductivity was mainly attributed to oxygen defects in the anion sublattice of La₂Mo₂O₉.     

Fabrication and characterisation of high-performance joints made of ZrB2-SiC ultra-high temperature ceramics

Joining is crucial for ultra-high temperature ceramics (UHTCs) to be used in demanding environments due to the difficulty in manufacturing large and complex ceramic components. In this study, ZrB₂-SiC composite UHTCs parts were joined via Ni foil as filler, and the mechanical properties and oxidation behaviour of the fabricated ZrB₂-SiC/Ni/ZrB₂-SiC (ZS/Ni/ZS) joint were investigated. Firstly, dense ZrB₂-SiC composites were prepared from nano-sized powders by spark plasma sintering (SPS). The ZrB₂-SiC parts were then joined using SPS. Furthermore, the elastic modulus, hardness, shear strength and high temperature oxidation behaviour of the ZS/Ni/ZS joint were examined to evaluate its properties and performance. The experimental results showed that the ZrB₂-SiC parts were effectively joined via Ni foil using SPS and the resultant microstructures were free from any marked defects or residual metallic layers in the joint. Although the elastic modulus and hardness in the joining zone were lower than those in the base ZrB₂-SiC ceramics, the shear strength of the joint reached ∼161 MPa, demonstrating satisfactory mechanical properties. Oxidation tests revealed that the ZS/Ni/ZS joint possesses good oxidation resistance for a wide range of elevated temperatures (800–1600 ^oC), paving the way for its employment in extreme environments.     

Effect of the Y2O3 additive concentration on the properties of a silicon nitride ceramic substrate

Silicon nitride (Si₃N₄) ceramics offer excellent thermal, mechanical and dielectric properties, which make Si₃N₄ a good candidate material for an application as electronic packaging material. For an application as a heat dissipation substrate, most studies focused on achieving a high thermal conductivity through long-time heat preservation and different kinds of heat treatments. Very few studies also considered the mechanical and dielectric properties. In addition, there have not been systematic researches about influence of additives concentration and type on the combination properties of Si₃N₄. Therefore, in this study, Si₃N₄ ceramic samples were prepared via hot pressing at 1800 °C with a relatively short heat preservation step (2 h), with different amounts of Y₂O₃ added as sintering additive. The effect of the initial concentration of the rare earth oxide on the chemical composition, microstructure, thermal conductivity, as well as the mechanical and dielectric properties of the Si₃N₄ ceramic samples was systematically studied.     

Mixture-proportioning of high-performance concrete

The paper presents a new approach to design concrete mixtures. It is based upon a set of models relating composition and engineering properties of concrete, to be implemented into software, linked with a material database. The principles underlying the various models are summarized, most of which focus on the granular structure of fresh/hardened concrete. A global approach to concrete is promoted, where performance specifications can be formulated in terms of fresh concrete (yield stress, plastic viscosity, slump and air content), hardening concrete (adiabatic temperature rise and autogenous shrinkage) and hardened concrete (compressive strength at any age, tensile strength, elastic modulus, creep and shrinkage). This approach is illustrated through the design of a special high-shrinkage high-performance concrete (HPC) for road application. To date, durability is lacking in the model and requires further research.     

Microstructures,thermophysical properties and thermal cyclic behaviors of Nd2O3 and Sc2O3 co-doped LaMgAl11O19 thermal barrier coating deposited by plasma spraying

In this study, La_1-xNd_xMgAl_11-xSc_xO₁₉ (x = 0.1, 0.2, 0.3; abbreviated as LNMAS-1, 2, 3) coatings which are supposed to possess better properties than LaMgAl₁₁O₁₉ (LMA) were plasma-sprayed and their high-temperature performance were comparatively investigated. Results show that addition of Nd³⁺ and Sc³⁺ as dopants to LMA endows corresponding coatings with reduced thermal conductivity and enhanced thermal expansion coefficient, while maintaining advantageous phase stability, although still being subjected to amorphization in plasma flame and following crystallization upon high-temperature service. Furthermore, the doping could cause adherence increasing between topcoat/bondcoat, benefiting from improved melting condition, especially in LNMAS-2 and LNMAS-3 coatings, which is related to the specific powder morphology and lowered melting point. During exposure to 1350°C, mechanical performance and structure integrity of doped free-standing LNMAS coatings can be well preserved even after 400 h aging. In thermal cyclic fatigue test, LNMAS-2 and LNMAS-3 coatings undertake thermal cycling lifetime of ～181 and 191 cycles at 1100°C, respectively, 40% durable than that of LMA coating. These preliminary results suggest that LNMAS-2, 3 might be promising candidates for advanced thermal barrier coating applications.     

Nanodiamonds as modifier of ethylene-1-octene copolymer structure and properties

The structure and stretching behavior of low density (0.86 g/cm³) ethylene-1-octene copolymer (EOC) modified with 0.015–1.44 vol.% (0.05–5 wt.%) of nanodiamonds (ND) particles were studied. Analysis of the structure of the EOC and nanocomposites by DSC, wide and small angle X-ray techniques revealed three phase structure in isotropic samples. In addition to the amorphous phase, there are two ordered phases: orthorhombic crystallites and hexagonal mesophase. ND particles display quite uniform distribution in EOC matrix up to 1.44 vol.%. The average size of ND particles is similar according to SAXS and WAXS data. It was found that introduction of ND particles into EOC matrix leads to the formation of the perfect orthorhombic phase crystallites, increases the rate of crystallization and crystallinity, resulting in increase in strength and Young modulus.     

Synthesis and thermal cure of high molecular weight polybenzoxazine precursors and the properties of the thermosets

High molecular weight polybenzoxazine precursors have been synthesized from aromatic or aliphatic diamine and bisphenol-A with paraformaldehyde. The precursors were obtained as soluble white powder. Molecular weight was estimated from the size exclusion chromatography to be several thousands. The structure of the precursors was confirmed by IR, ¹H NMR and elemental analysis, indicating the presence of cyclic benzoxazine structure. The ratio of the ring-closed benzoxazine structure and the ring-opened structure in the high molecular weight precursor was estimated from ¹H NMR spectrum and also from the exotherm of DSC, showing that the ratio of the ring-closed benzoxazine structure was 77–98%. The precursor solution was cast on glass plate, giving transparent and self-standing precursor films, which was thermally cured up to 240 °C to give brown transparent polybenzoxazine films. The toughness of the crosslinked polybenzoxazine films from the high molecular weight precursors was greatly enhanced compared with the cured film from the typical low molecular weight monomer. Tensile measurement of the polybenzoxazine films revealed that polybenzoxazine from aromatic diamine exhibited the highest strength and modulus. While, polybenzoxazine from longer aliphatic diamine had higher elongation at break. The viscoelastic analyses showed that the glass transition temperature of the polybenzoxazines derived from the high molecular weight precursors were as high as 238–260 °C. Additionally, these novel polybenzoxazine thermosets showed excellent thermal stability.     

Improvement on thermophysical and mechanincal properties of LaMgAl11O19 magnetoplumbite by Nd2O3 and Sc2O3 co-doping

LaMgAl₁₁O₁₉-type magnetoplumbite holds great promise to be used above 1300 °C as thermal barrier coatings (TBCs), but its practical application has been restricted because of inferior thermophysical properties. Herein, we focus on optimizing the thermophysical properties of LaMgAl₁₁O₁₉ by simultaneously substituting La³⁺ and Al³⁺ ions with Nd³⁺ and Sc³⁺ ions, respectively. Results show that the effects of co-substitution on reducing thermal conductivity are pronounced. The thermal conductivities of La_1-xNd_xMgAl_11-xSc_xO₁₉ (x = 0, 0.1, 0.2, 0.3) ceramics decrease progressively with dopant concentration and a lowest thermal conductivity of 2.04 W/(m·K) is achieved with x = 0.3 at 1000 °C, which is a value superior to pure LMA and even lower than YSZ. The mechanisms behind the lowered thermal conductivity are investigated. Increase of the thermal expansion coefficient is also realized (8.53 × 10⁻⁶ K⁻¹ for pure LMA, 9.07 × 10⁻⁶ K⁻¹ for x = 0.3, 1300 °C). Most importantly, Nd³⁺ and Sc³⁺ combination doping indeed facilitates mechanical properties of La_1-xNd_xMgAl_11-xSc_xO₁₉ solid solutions as well. It should be noted that Sc³⁺ doping at Al³⁺ site plays more effective role in improving thermal properties than Nd³⁺ does at La³⁺ site. This work provides a path to simultaneously integrate low thermal conductivity, good phase stability, moderate thermal expansion behavior and excellent mechanical properties on LMA for the next generation TBCs.     

Thermo-mechanical characterization of fumed silica-epoxy nanocomposites

DGEBA-based epoxy nanocomposites filled with various amounts of fumed silica nanopowders (10, 20 and 30 phr corresponding to 3.3, 6.4 and 9.2% by volume) were prepared by a solvent assisted dispersion procedure. The obtained nanocomposites were analyzed by means of differential scanning calorimetry, dynamic-mechanical thermal analysis, uniaxial tensile tests on un-notched samples and three-point bending tests on notched samples.

All the thermal and mechanical properties showed non-monotonic trends with relative minima as the silica content increased. A trend inversion in the physical properties was detected at the highest filler level tested, which was not previously observed for epoxy systems filled with comparable amounts of unmodified silica. The behavior could be explained by considering a reduction of cross-linking degree of the epoxy matrix due to the huge viscosity increase induced by the silica nanoparticles during composites preparation. On the other hand, the physical immobilization of the cross-linked matrix can be supposed to be responsible for the inversion of the properties trend at the highest filler level, which is presumably very close to the percolation threshold.     

Effect of polythiourethane hardener modification on the behaviour of epoxy resin

Polythiourethanes based on oligomeric polymercaptans were employed as curing agent of epoxy resin. The epoxy matrices, in the form of castings, were characterized for their mechanical properties such as tensile strength, elongation at break and unnotched Charpy impact strength as per ISO methods. Mechanical studies indicated that the incorporation of polythiourethane into epoxy resin improves the toughness and flexibility with reduction in tensile strength for samples cured at ambient conditions and influences the mechanical and thermal properties according to its percentage content for samples cured at 130°C. The high increase of impact strength was explained by the development of two-phase morphology during the cure process. The results of this study indicate that both the stoichiometry of the curing mixture and the initial thermal condition are of critical importance in governing the curing mechanism, structure of the network, morphology and the final properties of epoxy/polythiourethane compositions.     

Effect of Y2O3 content on the microstructural characteristics and the mechanical and thermal properties of Yb-doped SiAlON ceramics

SiAlON ceramics are primarily employed in ceramic cutting tools, which exploit their stable physical properties in high-temperature cutting environments due to their excellent mechanical properties. Here, Yb/Y-co-doped SiAlON ceramics are prepared by adding Yb and Y rare-earth (RE) ions in the RE_xSi_12-(m+n)Al_m+nO_nN_16-n (m = 0.4, n = 1.0) composition. Yb₂O₃, the RE oxide, is the main sintering additive. For RE_x composition design with (Yb_1-y + Y_y)_x, Yb₂O₃ is replaced by Y₂O₃ (y = 0.00, 0.25, 0.50, 0.75, 1.00). While Yb₂O₃ has excellent high-temperature stability, it is limited by its microstructural characteristics, that is, the β-SiAlON morphology and limited fracture toughness due to the small cation sizes. Thus, the changes in the above properties are investigated for various Y₂O₃ additive contents substituting for Yb₂O₃. The average grain width decreases, and the equiaxed β-SiAlON grains are elongated with increasing Y₂O₃ content. Regarding the mechanical properties, the hardness and fracture toughness are evaluated using the indentation fracture method. The hardness decreases with increasing Y₂O₃ content; however, the fracture toughness exhibits a significant increase (～53.6%) from 4.6 to 7.0 MPa?m^1/2. Regarding crack propagation, intergranular fracture is mainly observed in the Yb/Y-co-doped SiAlON ceramics, whereas transgranular fracture is primarily observed in the Yb-single-doped SiAlON ceramic. Y₂O₃ substitution increases the α/β-SiAlON phase ratio, and the grain boundary phase exhibits increasing vitrification with increasing Y₂O₃ content. Moreover, the thermal properties of the Yb/Y-co-doped compositions are analyzed and discussed regarding intrinsic properties such as phonon scattering. The microstructural characteristics and improved fracture toughness derived from the Yb/Y-co-doped system designed in this study suggest considerable potential for the future composition design of ceramic cutting tools.     

Thermal and Mechanical Performances of Diallyl Phthalate/Clay Nanocomposites

Diallyl phthalate (DAP) is one of the most versatile of the thermosetting resins, in practice usually it has been filled by mineral, glass, and synthetic fiber. However, no effects so far have been made toward the development of reinforced DAP by nano-size fillers. In this paper, DAP/clay nanocomposites were prepared, and their structure and properties such as thermal and mechanical properties were investigated in detail by x-ray diffraction (XRD), transmission electron microscope (TEM), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), and tensile measurement at first use. Results show that the incorporation of a small amount of clay can effectively decrease the curing temperature of DAP and improve the glass transition temperature and the dynamic mechanical properties of DAP resin as well as the tensile strength and elongation.