Hot-pressing Direction Research Articles

Effect of hexagonal boron nitride content on the anisotropy of hot-press-sintered AlN/BN composite ceramics

The AlN/BN composite ceramic, fabricated via hot-press sintering, exhibits obvious anisotropy due to the establishment of directional alignment of h-BN. However, the variation and influence mechanisms of bending strength (σ) and thermal conductivity (λ) are remain unclear in different directions, which greatly limit the application of AlN/BN composite ceramics. In this paper, the effects of h-BN content on phase composition, microstructure and anisotropy of AlN/BN composite ceramics are discussed in detail. The results reveal that AlN/BN composite ceramics have three directions: perpendicular (⊥), longitudinal parallel (∥), and lateral parallel (≡) to the hot-pressing direction. The content of h-BN is a crucial factor affecting the anisotropy of AlN/BN composite ceramics. h-BN enhances the thermal conductivity in the ∥ and hinders the thermal conductivity in the ⊥. With the increase in h-BN amount, λ∥ rises gradually, while λ⊥ gradually falls. When the mass percentage of h-BN and AlN is 25 % (AB25), the λ∥ is as high as 90 W m−1 K−1, which is about 2.25 times that of λ⊥. In terms of mechanical properties, σ≡ and σ⊥ first increase and then decrease, while σ∥ only decreases. This is attributable to h-BN being effective in enhancing mechanical properties by inhibiting crack propagation in the ⊥ and ≡ direction, while the promotion of crack propagation leads to a decrease in mechanical properties in the ∥ direction. σ≡ reaches the highest values (σ≡ = 460.2 MPa) at an h-BN amount of 10 % (AB10), which is about 1.19 and 2.04 times that of σ⊥ and σ∥, respectively, and 22.1 % higher than that of AB0.

Improvement of anisotropic thermoelectric performance in polycrystalline SnSe by metallic AgSnSe2 compositing

SnSe is an emerging star material in thermoelectric community owing to its attractive electronic band structure and ultralow intrinsic lattice thermal conductivity. By now, aliovalent doping is often employed to be a route strategy to optimize the thermoelectric properties of SnSe via improving its poor electrical conductivity. In this work, AgSnSe2, as a metallic composite, is introduced with the aim to increase the hole concentration of SnSe via metal-semiconductor contact. It is demonstrated that compositing of AgSnSe2 can effectively increase the hole concentration of polycrystalline SnSe hosts by two orders of magnitude (from 1017 to 1019 cm−3) without deteriorating the carrier mobilities. Therefore, a lager power factor of 5.72 μW cm−1 K−2 is achieved at 773 K for the SnSe + 0.020AgSnSe2 sample perpendicular to the hot-pressing direction. Combined with the intrinsic low thermal conductivity, the maximum zT value of the sample can be boosted to 0.68 at 773 K, which is increased by 124% compared to that of the uncomposited counterpart. This work proves that composite engineering should be an efficient alternative strategy in optimizing the thermoelectric performance of polycrystalline SnSe and can be synergistically used with other sophisticated approaches.

Effect of CNT addition and its orientation on thermal shock resistance of B4C/CNT composites fabricated by hot-pressing

ABSTRACT This paper describes the synthesis of B4C/carbon nanotube (CNT) composite by hot-pressing and shows the details for the effectiveness of CNT addition, additive amount and its orientation on the thermal shock resistance. CNTs tend to array perpendicular to the hot-pressing direction, which caused anisotropic mechanical and thermal properties of B4C/CNT composite. We investigated the residual bending strength of thermally shocked B4C/CNT composites and determined the critical temperature ΔTC that causes precipitous strength drop. In the case of B4C/15 vol% CNT composite, the residual bending strength almost unchanged at ~450°C; ΔTC increased by 150°C or more than that of monolithic B4C. Based on the thermal stress fracture-resistance parameter R’ of the composites, it was clarified that the excellent R’ is mainly due to the higher thermal conductivity and lower elastic modulus. The numerical results showed that CNT addition has a pronounced effect on the thermal shock residual bending strength and thermal stress fracture-resistance of B4C/CNT composite.

Preparation and characterization of graphitized polyimide film/epoxy resin composites with high thermal conductivities

The graphitized polyimide films (GPF) and the graphitized polyimide tapes (GPT) were coated with epoxy resin (EP), then their composites were prepared by hot-pressing the laminated GPF and stacked GPT, respectively. The crystal structure, morphology and optical texture of GPF and GPT as well as their epoxy adhered composites were characterized by X-ray diffraction, scanning electron microscopy and polarized light microscopy. The effects of volume fraction and dimension of GPF and GPT on the thermal conductivity of their composites were investigated. The thermal conductivity and thermal diffusion coefficient of GPT/EP composites increase with GPT volume fraction. Owing to the gap among GPT, the thermal conductivity perpendicular to the hot-pressing direction of GPT/EP composite stacked with 80vol.% GPT is changeable from 453.02 W/(m·K) to 615.15 W/(m·K). The GPF/EP composite laminated with 80vol.% GPF, showing a highly oriented sandwich structure, possesses the thermal conductivity of 894.36 W/(m·K). However, the thermal conductivity along the hot-pressing direction of GPT/EP and GPF/EP composites with 80vol.% of GPT and GPF is 1.82 W/(m·K) and 1.15 W/(m·K) respectively. The obvious differentia in the thermal conductivity at perpendicular and parallel to hot-pressing directions further confirms that the two composites are highly oriented.

Fracture toughness of Si3N4 ceramic composites: Effect of texture

Silicon nitride (Si3N4) ceramics with Al2O3, MgO and Y2O3 as sintering additives were fabricated via hot-pressing sintering and gas pressure sintering, respectively, focusing on the texture and fracture toughness. The hardness and fracture toughness on the plane perpendicular and parallel to the hot-pressing direction were different in the hot-pressing sintered samples, and similar values were obtained in the gas pressure sintering. The orientation distribution function (ODF) analyses via X-ray diffraction measurements revealed the presence of 112¯0〈0001〉 type texture in the hot-pressing sintered samples. This suggested that the 0001 planes of most rod-shaped β-Si3N4 grains were oriented in the hot-pressing direction or their c-axes were oriented towards the radial directions of the sintered cylinder-shaped samples. The presence of such a β-Si3N4 grain texture led to the anisotropy in fracture toughness, with its values changing from 9.3–11.3 MPa m1/2 with increasing indentation angles on the plane parallel to the hot-pressing direction.

Thermoelectric transport properties of the van der Waals-type layered rhombohedral SnAs-based compound, EuSn2As2

The thermoelectric transport properties of the van der Waals-type layered rhombohedral SnAs-based compound, EuSn2As2, have been investigated. A densified polycrystalline sample of EuSn2As2 with porosity (ϕ) of 2.4(9) vol.% exhibited a weak orientation to the c-axis for hexagonal coordination system; the weak orientation is parallel (P ∥) to the pressing direction of hot pressing. Measurements of electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) were conducted perpendicular (P ⊥) to the pressing direction. The experimental values of ρ and S exhibit metallic temperature dependence and p-type carrier polarity. The power factor (P) was 0.51(8) mW m−1 K−2 at 673(4) K. Using the Wiedemann–Franz–Lorenz law, the phonon thermal conductivity (κ ph) was estimated to be 0.4(6) W m−1 K−1 at 673(6) K. The dimensionless figure of merit, ZT, was 0.092(17) at 673(3) K.

Bi8Se7: Delocalized Interlayer π-Bond Interactions Enhancing Carrier Mobility and Thermoelectric Performance near Room Temperature.

Environmental heat-to-electric energy conversion provides a promising solution to power sensors used for wearable and portable devices. Yet the near-room-temperature thermoelectric materials are extremely rare. The natural heterostructure [Bi2]m[Bi2Q3]n family provides an important platform to search and develop the cheaper and less toxic of such materials. However, the bottleneck problem in this family is how to enhance the interlayer electrical conductivity (σ). Herein, we uncover for the first time that the delocalized π-bond interaction between the stacking layers in the [Bi2]m[Bi2Se3]n family effectively increases the interlayer carrier mobility (μH) and σ. Moreover, we propose an empirical index, F = Dpx,py(Bi0)/Dpx,py(Bi3+) along the kz direction in the Brillouin zone to evaluate the strength of the interlayer delocalized π-bond. F is optimized at a value of 1, under which μH is maximized. Interestingly, Bi8Se7 possessing an optimal F = 1.06 is predicted to have the best μH in the [Bi2]m[Bi2Q3]n family. Our subsequent experiments confirm the as-synthesized Bi8Se7 exhibiting n-type behavior with a μH value (33.08 cm2/(V s) at 300 K) that is higher than that of BiSe (26.19 cm2/(V s) at 300 K) and an enhanced σ value. Furthermore, the Te/Sb codoping, via varying the top of the valence band, significantly increases the Seebeck coefficient and eventually enhances the ZT value to ∼0.7 in Bi5.6Sb2.4Se5Te2 at 425 K along the hot-pressing direction, which is comparable to the optimized value of BiSe. According to the single parabolic band model prediction, the ZT of Bi5.6Sb2.4Se5Te2 may reach ∼1.2 at 425 K, suggesting a novel and promising n-type thermoelectric material near room temperature.

Enhanced thermoelectric performance in polycrystalline N-type Pr-doped SnSe by hot forging

SnSe has attracted significant attention for use in thermoelectric devices due to its reported ultrahigh figure-of-merit ZT in both p- and n-type single crystals, which inspired us to explore the thermoelectric properties of SnSe polycrystals for potential large-scale applications. Here, n-type Sn1-xPrxSe polycrystals are prepared by ball milling and hot pressing. A maximum ZT value of ~0.7 at 773 K is achieved in Sn0.97Pr0.03Se due to the enhanced electron concentration and electrical conductivity resulting from Pr doping at the Sn site. Through hot forging for a higher degree of orientation, a lowered thermal conductivity is obtained along the hot-pressing direction, contributing to an enhanced ZT value of ~0.9 at 773 K in this direction. This study paves a new way for optimization of n-type SnSe by cation-site lanthanide doping.

Microstructure and anisotropic mechanical properties of B6.5C-TiB2-SiC-BN composites fabricated by reactive hot pressing

B6.5C-TiB2-SiC-BN composite ceramics were prepared by a novel solid-state reaction using TiCN, B, and Si as raw materials. The final products obtained by hot pressing at 1950 °C possessed a fine microstructure, homogeneous distribution, and excellent mechanical properties. The obtained bulk B6.5C-TiB2-SiC-BN composite ceramic shows a high relative density (98.8 %). The mechanical properties of the composites are anisotropic because of the orientation growth and structural characteristics of TiB2 and h-BN grains. The values of hardness, bending strength, and fracture toughness measured along the hot-pressing direction were 19.6 GPa, 801 MPa, and 4.30 MPa m1/2, respectively, which were higher than those measured perpendicular to the hot-pressing direction. The formation of twin structures in B6.5C and SiC grains and the crack deflections induced by h-BN and TiB2 grains are beneficial for improving the mechanical properties of these composites.

Anisotropy of thermoelectric composites 0.7 Ca3Co4O9/0.3 Bi2Ca2Co2Oy

The anisotropy of thermoelectric composites 0.7 Ca3Co4O9/0.3 Bi2Ca2Co2Oy were investigated. The X-ray diffraction analysis and the micro-morphology characterization in different planes were conducted. The results show in the plane perpendicular to the hot press direction, the diffraction directions are all along (00l), and the diffraction intensity is the strongest. The composites shows layered structure due to the plat-like Ca3Co4O9 sheets stacking along the hot press direction. The electrical resistivity ρ, Seebeck coefficient S and thermal conductivity κ in different directions were measured. The results show the thermoelectric parameters are strong direction dependence. With the measured direction deviating from the hot press direction, the electrical resistivity ρ and the thermal conductivity κ decline monotonically. Meanwhile, the ρ (θ = 0)/ρ (θ = 90) (θ defined as the angle between the measured direction and the hot press direction) decreases as the temperature rises, however, the κ (θ = 0)/κ (θ = 90) almost keeps constant over the whole temperature range. The Seebeck coefficient reaches the maximum when θ = 22.5 or 67.5.

The effect of annealing temperature on flexural strength, dielectric loss and thermal conductivity of Si3N4 ceramics

Effect of annealing temperatures on flexural strength, dielectric and thermal properties of Si3N4 ceramics was investigated with different amounts of Yb2O3 (3.5 mol% and 7 mol%) as sintering additives. High density of >99% were achieved after hot pressing. The post annealing heat treatment promoted the crystallization of the grain boundary glass phase. Low dielectric loss of <4 × 10−4 were obtained for all samples, a low dielectric loss of 1.8 × 10−4 was achieved by annealing at 1500 °C for 24 h with 7 mol% Yb2O3 as sintering additives. A mild anisotropic microstructure was obtained due to merit of hot pressing method. High thermal conductivity of >75 W/(m·K) and >90 W/(m·K) were obtained in the direction parallel and perpendicular to the hot-pressing direction in all samples, respectively. The low flexural strength of ∼600 MPa may attribute to the large grain in the matrix and lack of elongated abnormal grains.

Effect of carbon fiber on the tribo-mechanical properties of boron carbide: Comparison with carbon nanotube reinforcement

Composites with boron carbide (B4C) as the matrix and short carbon fiber (Cf) as reinforcement in 2, 5 and 10 vol% were fabricated by hot pressing route. Microstructural analysis by scanning electron microscopy reveled that Cf got aligned perpendicular to the hot press direction. Agglomeration of Cf was observed at 10 vol% Cf. The addition of Cf improved the flexural strength of B4C-Cf composite and was superior to that of B4C-CNT composite. On other hand, the hardness and fracture toughness of B4C-Cf composite were inferior to that of B4C-CNT composite. The tribological studies revealed that B4C-CNT had superior wear resistance. Both B4C-Cf and B4C-CNT had lower coefficient of friction than monolithic B4C. The possible reasons for deviation of properties due to change in length scale of carbon reinforcement has been analyzed.

MAS-content dependence of the texture and fracture behavior of h-BN-MAS composite ceramics

Texturing was employed to tailor the mechanical properties of h-BN ceramic with MAS as liquid texturing aid. Effects of MAS content on the texture behavior, densification behavior, bending strength, fracture toughness and the fracture behavior of the h-BN ceramics were studied. Moderate MAS of 30 wt% can significantly facilitate the orientation of h-BN grains in the direction perpendicular to the hot-pressing direction. MAS is also in favor of the densification behavior of h-BN as sintering aid. In-situ mechanical testing shows that texturing can greatly improve the mechanical properties and avoid the catastrophic fracture of the h-BN ceramics, and the textured h-BN ceramics exhibit distinct rising R-curve behavior, which is primarily attributed to the effect of parallel-aligned h-BN grains to the crack propagation under loading.

Fabrication of fine-grained α/β Si3N4 by Hot Pressing Flowing Sintering at 1550 °C

Abstract In this paper, Hot Pressing Flowing Sintering (HPFS) was successfully applied to the consolidation of silicon nitride ceramics at low sintering temperature. Fine-grained α/β-Si3N4 with high density was fabricated by HPFS at 1550 °C. The low temperature densification process was based on the hot pressing flow enhanced particle rearrangement mechanism. The shear stress which generated in the flowing process helped enhance the particle rearrangement with the lubrication of the liquid phase, which lead to collapse of the pores. The densification process was finished with minimum grain growth. Meanwhile, the rod-like β-Si3N4 nuclei was textured during the flowing process. The (101)/(210) ratio of β-Si3N4 was 0.09 on the plane perpendicular to the hot-pressing direction. The Vickers hardness and flexural strength of the sintered sample was 19.59 ± 0.36 GPa and 1022 ± 138 MPa, respectively.

Anisotropic thermal expansion coefficient of multilayer graphene reinforced copper matrix composites

Multilayer graphene reinforced copper matrix (Cu-MLG) composites were fabricated via molecular-level mixing combined with vacuum hot-pressing (VHP). The MLG displayed a preferred orientation in the copper matrix, with the in-plane surface perpendicular to the hot-pressing direction, resulting in significant anisotropy in the thermal properties of Cu-MLG composites. Theoretical predictions were made using four models (Schapery, Kerner, Turner and mixture rule) to investigate the effect of the preferential orientation of MLG on the CTE anisotropy of Cu-MLG composites. The CTE anisotropy was further analyzed by carrying out molecular dynamics (MD) simulations to determine the variation in bond lengths for copper and MLG in different directions in the interface regions. The MLG bond length increased gradually with increasing temperatures in the in-plane direction, while in the out-plane directions, it increased significantly at first and then decreased at higher temperatures. The variations of bond lengths for MLG are consistent with the CTE anisotropy of the Cu-MLG composites in the same direction.

Fabrication and mechanical properties of Al2O3-SiCw-TiCnp ceramic tool material

Abstract Whiskers and nanoparticles are usually used as reinforcing additives of ceramic composite materials due to the synergistically toughening and strengthening mechanisms. In this paper, the effects of TiC nanoparticle content, particle size and preparation process on the mechanical properties of hot pressed Al 2 O 3 -SiC w ceramic tool materials were investigated. The results showed that the Vickers hardness and fracture toughness of the materials increased with the increasing of TiC content. The optimized flexural strength was obtained with TiC content of 4 vol% and particle size of 40 nm. The particle size has been found to have a great influence on flexural strength and small influence on hardness and fracture toughness. It was concluded that the flexural strength increased remarkably with the decreasing of the TiC particle size, which was resulted from the improved density and refined grain size of the composite material due to the dispersion of the smaller TiC particle size. SEM micrographs of fracture surface showed the whiskers to be mainly distributed along the direction perpendicular to the hot-pressing direction. The fracture toughness was improved by whisker crack bridging, crack deflection and whisker pullout; the TiC nanoparticles in Al 2 O 3 grains caused transgranular fracture and crack deflection, which improved the flexural strength and fracture toughness with whiskers synergistically. Uniaxial hot-pressing of SiC whisker reinforced Al 2 O 3 ceramic composites resulted in the anisotropy of whiskers’ distribution, which led to crack propagation differences between lateral crack and radical crack.

Boron carbide/graphene platelet ceramics with improved fracture toughness and electrical conductivity

Boron carbide/graphene platelet (B4C/GPLs) composites have been prepared with a different weight percent of GPLs as sintering additive and reinforcing phase, hot pressed at 2100°C in argon. The influence of the GPLs addition on fracture toughness (KIC) and electrical conductivity was investigated. Single Edge V-Notched Beam (SEVNB) method was used for fracture toughness measurements and the four-point Van der Pauw method for electrical conductivity measurements. With increasing amount of GPLs additives, the fracture toughness increased due to the activated toughening mechanisms in the form of crack deflection, crack bridging, crack branching and graphene sheet pull-out. The highest fracture toughness of 4.48MPa.m1/2 was achieved at 10wt.% of GPLs addition, which was ∼50% higher than the KIC value of the reference material. The electrical conductivity increased with GPLs addition and reached the maximum value at 8wt.% of GPLs, 1.526×103S/m in the perpendicular and 8.72×102S/m in the parallel direction to the hot press direction, respectively.

Low Thermal Conductivity and High Thermoelectric Performance in In4Se3−x with Phase-Separated Indium Inclusions

We report the thermoelectric properties of undoped hot-pressed In4Se3−x (x = 0.05). Stoichiometric imbalance due to selenium deficiency in In4Se3 was found to create a secondary phase of elemental indium in the host material. Heat treatment drove grain growth and increased the indium solubility in In4Se3. Indium-rich domains at grain surfaces/boundaries in untreated samples were found to redistribute inside the grains and their junctions after heat treatment. Due to enhanced phonon scattering by secondary phase of indium, very low values of thermal conductivity were observed for all samples, leading to a maximum thermoelectric figure of merit (zT) of 1.13 at 723 K along the hot-pressing direction for the heat-treated sample.

Microstructure and anisotropy of mechanical properties of graphene nanoplate toughened Al2O3-based ceramic composites

Graphene nanoplate (GNP)-toughened Al2O3-based ceramic composites were fabricated by the hot-pressing method. The ceramic composites prepared in this way exhibited anisotropy in their mechanical properties. The fracture toughness and flexural strength of the ceramic composites were measured in the vertical to hot-pressing direction (VHPD) and determined to be as high as 6.2MPam1/2 and 461MPa, respectively, with GNP contents of 0.75vol%, which were improvements of 38% and 39%, respectively, over the values measured in the parallel to hot-pressing direction (PHPD). Scanning electron microscope (SEM) observations indicated that GNPs parallel to each other and perpendicular to the hot-pressing direction are the primary explanation for the mechanical property anisotropy. The energy release rate of the crack deflection was calculated, demonstrating the effect of the GNP orientation on the mechanical properties.

Thermal conductivity of hot-pressed hexagonal boron nitride

The thermal conductivity parallel and perpendicular to the hot-pressing direction of hot-pressed h-BN was measured to investigate the effects of oxide additive species and compositions on the thermal conductivity. CaO, MgO, Y2O3, Yb2O3, and their mixtures were selected as oxide additives owing to their previously demonstrated ability to enhance the thermal conductivities of non-oxide materials such as AlN and Si3N4. The investigated additive concentrations were calculated to be 5, 10, and 15vol%. A thermal conductivity of approximately 210W/m·K was attained by hot-pressing mixed powders of h-BN and a Yb2O3–MgO or Yb2O3–CaO additive at a concentration of 15vol%.