Thermoelectric properties of the textured Bi1.9Gd0.1Te3 compounds spark-plasma-sintered at various temperatures

Elemental composition, crystal and grain structures, specific electrical resistivity, Seebeck coefficient, thermal conductivity, and thermoelectric figure-of-merit of n-type grained Bi_1.9Gd_0.1Te₃ compounds, spark-plasma-sintered at T_S = 690, 720, 750, 780 and 810 K, have been studied. All the samples are highly textured along the 001 direction parallel to the pressing direction. The average grain size measured along the pressing direction is much less as compared to the average grain size measured in the perpendicular direction. A strong anisotropy in the transport properties measured along directions parallel and perpendicular to the pressing direction was found within the 290 ÷ 630 K interval. Electrical resistivity decreases and thermal conductivity increases for parallel orientation as compared to these properties for perpendicular orientation. The T_S - effect on thermoelectric figure-of-merit of textured Bi_1.9Gd_0.1Te₃ compounds has been found and analyzed. Highest thermoelectric figure-of-merit (∼0.75) was observed for sample with T_S = 750 K at perpendicular orientation.     

Gliding arc plasma reforming of toluene for on-board hydrogen production

Hydrogen is considered as a clean and promising fuel, and hydrogen production on-board has attracted widespread research attention. In this work, a gliding arc discharge (GAD) plasma reactor was utilized to reform toluene at room temperature and atmosphere pressure. The performance of hydrogen production through oxidative reforming with toluene as raw material under different input power, oxygen to carbon molecular ratio (O/C), residence time and argon addition was investigated. The optimal yields of H₂ and CO (48.6% and 44.3%) were obtained under the condition of the input power of 32 W, the O/C of 0.68, the residence time of 18.4 s and 10 vol% Ar addition. By analyses of spectrum lines and GC-MS, the plasma reforming mechanism of toluene was proposed. It is believed that N₂(B³Πg) and Ar* could increase the formation of reactive oxygen species (O⁺, O (¹D), O and so on), and N₂(B³Πg) could impact directly the reforming of toluene.     

Role of MgO on densification and mechanical properties in spark plasma sintered Os0.9Re0.1B2 ceramic

To promote the densification and therefore the mechanical properties of boride-based ceramics, MgO was added as sintering aid into Os_0.9Re_0.1B₂ powders for densification by using spark plasma sintering (SPS). The Os_0.9Re_0.1B₂ powders were synthesized by mechanochemical method from powder mixture of Os, Re and amorphous B. The role of MgO on densification, phase composition, microstructure and mechanical properties (hardness, fracture toughness and wear behavior) were studied by using X-ray diffraction (XRD), scanning electron microscope (SEM) with energy-dispersive spectroscopy (EDS), micro indentation and ball-on-disk tribometer. The results show that, with the introduction of MgO as sintering aid, the relative density of the Os_0.9Re_0.1B₂ ceramic samples increased. When the MgO content reached 9 wt%, the as-sintered sample is almost fully dense. No obvious regularity was found from the samples with the addition of different content of MgO. Vickers hardness values of the samples with 0, 3 wt% and 9 wt% MgO are found to be very close with each other within the experimental error (~30 GPa), while the sample with the addition of 6 wt% MgO exhibits the highest hardness of ~35 GPa. The fracture toughness of the samples is decreased slightly with the addition of MgO. The friction coefficient and wear rate of the sample with the addition of 6 wt% MgO was also found to be the lowest among all samples, which indicate best wear resistance. As a whole, with the addition content of 6 wt% MgO, the Os_0.9Re_0.1B₂ ceramic sample performs relatively excellent mechanical properties among four groups of samples.     

Fabrication and mechanical properties of Al2O3–TiC ceramic composites synergistically reinforced with multi-walled carbon nanotubes and graphene nanoplates

In this study, Al₂O₃–TiC composites synergistically reinforced with multi-walled carbon nanotubes (MWCNTs) and graphene nanoplates (GNPs) were prepared via spark plasma sintering (SPS). The effects of the MWCNT and GNP contents on the phase composition, mechanical properties, fracture mode, and toughening mechanism of the composites were systematically investigated. The experimental results indicated that the composite grains became more refined with the addition of MWCNTs and GNPs. The nanocomposites presented high compactness and excellent mechanical properties. The composite with 0.8 wt% MWCNTs and 0.2 wt% GNPs presented the best properties of all analysed specimens, and its relative density, hardness, and fracture toughness were 97.3%, 18.38 ± 0.6 GPa, and 9.40 ± 1.6 MPa m^1/2, respectively. The crack deflection, bridging, branching, and drawing effects of MWCNTs and GNPs were the main toughening mechanisms of Al₂O₃–TiC composites synergistically reinforced with MWCNTs and GNPs.     

Influence of CeO2 addition on the microstructure and mechanical properties of Zirconia-toughened alumina (ZTA) composite prepared by spark plasma sintering

In this study, zirconia-toughened alumina (ZTA) samples with different amounts of CeO₂ were prepared by the spark plasma sintering method. The phase composition and microstructure of the samples were examined by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The addition of CeO₂ results in grain refinement and density increase; moreover, CeO₂ stabilises the high-temperature metastable phase. As the amount of CeO₂ reaches 7 wt%, a new CeAl₁₁O₁₈ phase appears. The Vickers hardness, modulus, and fracture toughness of the samples depend to a large extent on the grain size, relative density, and existence of the second phase. Among the composites, that with 5 wt% CeO₂ shows the best performance with the highest values of relative density, Vickers hardness, and fracture toughness: 96.51%, 1688 HV, and 9.91 MPa.√m, respectively.     

The effects of adding CNTs and GNPs on the microstructure and mechanical properties of hexagonal-boron nitride

This present work investigated the mechanical properties and microstructure of h-BN based ceramic composites reinforced with CNTs and GNPs. Accordingly, two different batches of pure h-BN, h-BN/0.1 wt%CNTs and h-BN/0.1 wt% GNPs were prepared through a high energy mixer mill to gain a uniform dispersion of reinforcement with the initial stable CNTs or GNPs solution in ethanol. After drying the mixtures, the pure h-BN and also, two different composite components were directly inserted into the graphite mold and the sintering process was performed with the initial and final pressure of 10 and 50 MPa, respectively, at 1900 °C, under the vacuum condition of 15–35 Pa. The relative density of the samples was calculated based on the Archimedes principle. The densification behavior of the samples showed the maximum amount of 98.31% for the theoretical density of the h-BN/GNPs composite. On the other hand, the minimum relative density of 96.41% was obtained for the h-BN/CNTs composite. The microstructure studies of the prepared sample showed the uniform distribution of GNPs in the h-BN layers; however, when the CNTs were added, some agglomerated area was found. Moreover, the fracture surface of all samples showed a laminar fracture as a result of the layer-by-layer structure of h-BN. The investigation of the mechanical properties of the prepared specimens also revealed the highest bending strength, fracture toughness and Vickers hardness of 199 MPa, 1.26 GPa and 3.62 MPa m^−1/2, respectively, which belonged to the h-BN/GNPs composite. In the case of CNTs, this trend exhibited lower amounts, probably due to the agglomeration of CNTs.     

Evolution mechanism of the microstructure and mechanical properties of plasma-sprayed yttria-stabilized hafnia thermal barrier coating at 1400 °C

Yttria stabilized hafnia (Hf_0.84Y_0.16O_1.92, YSH16) coatings were sprayed by atmospheric plasma spraying (APS). The effects of thermal aging at 1400 °C on the microstructures, mechanical properties and thermal conductivity of the coatings were studied. The results show that the as-sprayed coating was composed of the cubic phase, and the nano-sized monoclinic (M) phase was precipitated in the annealed coating. The presence of M phase effectively constrained the sintering of the coating due to its superior sintering-resistance. The Young's modulus kept at a nearly same level of ~78 GPa even after annealing, and the coating annealed for 6 h yielded a maximum value of hardness but revealed a declining tendency in the Vicker's hardness with prolonged sintering time. The thermal conductivity increased from 0.8-0.95 W m^-1 K^-1 at as-sprayed state to 1.6 W m^-1 K^-1 after annealing at 1400 °C for 96 h. The dual-phase coating is promising to serve at temperatures above 1400 °C due to its excellent thermal stability and mechanical properties.     

Beneficial role of carbon black on the properties of TiC ceramics

This investigation aimed to study the influence of carbon black on the qualifications of TiC-based materials. For this objective, two samples, namely monolithic TiC and TiC-5 wt% carbon black were sintered by spark plasma sintering (SPS) method at 1900 °C. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) were used to characterize the as-sintered samples. Introducing carbon black enhanced the relative density of TiC significantly, reaching a near fully dense substance. Phase analysis and microstructural studies manifested the formation of non-stoichiometric TiC_x in both ceramics. Although the introduction of carbonaceous additive considerably increased the thermal conductivity and flexural strength of TiC, standing at 25.1 W/mK and 658 MPa, respectively, its influence on the Vickers hardness was trivial (both ~ 3200 HV_0.1 kg). Finally, the composite specimen presented a lower coefficient of friction (~ 0.31) on average compared to the undoped TiC (~ 0.34).