Co2+:MgAl2O4 saturable absorber transparent ceramics fabricated by high-pressure spark plasma sintering

Single-stage processing of high-quality transparent functional polycrystalline ceramics is desirable but challenging. In the present work, spark plasma sintering (SPS) was employed for fabrication of Co²⁺:MgAl₂O₄ saturable absorbers for laser passive Q-switching. Densification of commercial MgAl₂O₄ powders, doped via co-precipitation, was carried out by conventional SPS and high-pressure SPS (HPSPS) under pressures of 60 and 400 MPa, respectively. The presence of LiF, a common sintering additive, was detrimental to optical properties as it promoted reaction of cobalt with sulfur impurities and the formation of Co₉S₈ inclusions. Densification by HPSPS without LiF allowed to obtain highly transparent Co²⁺:MgAl₂O₄. The optical properties of samples, with doping concentrations varying between 0.01 and 0.1 at.% Co²⁺, were assessed and saturable absorption was demonstrated at ~1.5 µm wavelength, exhibiting ground-state (σ_gs) and excited (σ_es) cross-sections of 3.5×10^-19 and 0.8×10^-19 cm², respectively. Thus, it was established that HPSPS is an effective method to fabricate transparent Co²⁺:MgAl₂O₄ ceramics.     

Low temperature synthesis and spark plasma sintering of a boron carbide with a low residual carbon content

Using spark plasma sintering (SPS), >98.5 % dense boron carbide (B₄C) samples were made from commercially available and lab-synthesised powders made via a low temperature synthesis (LTS) process. The work showed that the LTS powder can be produced in batches of tens to hundreds of grams whilst maintaining a high purity material with lower levels of residual free carbon (20.6–21.3 wt.% C) than commercially available samples (22.4 wt.% C). The LTS material was seen to exhibit higher hardness values (37.8 GPa) than the commercial grade material (32.5 GPa) despite featuring a coarser average grain size (10.8 μm and 2.4 μm respectively). This is largely thought to be due to the influence of ZrO₂ and AlB₂ impurities introduced to the material during micronising milling of the powder after synthesis, as opposed to the influence of the materials lower carbon content.     

Ultra-high temperature ceramics: Aspiration to overcome challenges in thermal protection systems

Ultra-high temperature ceramics (UHTCs) have played a significant role in fulfilling demands for the thermal protection system (TPS) in the aerospace sector, however, a promising candidate has not emerged yet. This critical review provides typical inconsistencies and new perspectives related to UHTCs in terms of: (i) material and processing: i.e., sinterability, reinforcements, microstructural evolution, (ii) properties and performance correlation with the processing conditions and resulting microstructure, and (iii) outlook on the most promising ZrB₂-HfB₂-SiC-based composites as potential candidates for hypersonic leading edge and re-entry structures. An optimal selection of the content, size and reinforcing phase (such as silicides, refractory carbides, and carbon-based, etc.) is mandated in upgrading the thermo-mechanical performance of UHTCs to sustain elevated temperature (1700 °C), exhibiting flexural/fracture strength of >300 MPa, high thermal conductivity >14.5 Wm^?1K^?1, and high oxidation resistance (<80 gm^?2 over 2 h at 1400 °C). From emphasis on the powder purity, and sintering additives on affecting the densification, mechanical properties and high temperature oxidation, improvements in the functional performance of UHTCs are carried forward with emphasis mainly on borides and carbides. Emergence of SiC as most promising sintering additive with optimal content of ～20 vol%, and with supplemented HfB₂ addition in ZrB₂-HfB₂-SiC based UHTCs have exhibited higher oxidation resistance and may serve as conceivable entrants for hypersonic vehicles. Further, the review leads the reader to developing new materials (including silicides, MAX phases, and high entropy UHTCs), incorporating novel strategies like designing layered structures or functionally graded materials (FGM), and effective joining to allow the integration of smaller components into scaled up structures. On one hand, where plasma arc-jet exposure mimics high heat-flux exposures, the utilization of multi-length-scale computational modeling (such as finite element methods, density functional theory, ab initio etc.) allows assessing the material performance under dynamic changes (of variable partial pressure, temperatures, gradation, etc.) towards perceiving new insights into the structural stability and thermo-mechanical properties of UHTCs. This review critically underlines the present state of the art and guides the reader towards the futuristic development of new class of high-temperature materials for TPSs.     

Combined hot extrusion and spark plasma sintering method for producing highly textured thermoelectric Bi2Te3 alloys

Hot extrusion is a promising method for producing high-performance thermoelectric bismuth telluride alloys because of its ability to create textured microstructures. However, hot extrusion is less favourable for scaling-up because of temperature and strain gradients along the radial direction, and only <110> -textured thermoelectric legs can be obtained because of the fibre-like texture. We suggest a way to overcome these disadvantages by implementing an additional spark plasma sintering process on a stack of extrudates. Using this combined process, we demonstrate the fabrication of 12 × 15 × 13 mm³ p-type (Bi_0.2Sb_0.8)₂Te₃ samples from extrudates that had originally been 3 mm in diameter. The evolution of sheet-like texture revealed by SEM, XRD, and EBSD allows us to obtain both <110> - and <001> -textured thermoelectric legs from a single specimen that are desirable for low- and high-temperature applications, respectively. Our results demonstrate the combined method as an industry-friendly process for fabricating high-performance thermoelectric materials.     

Gel-casting of transparent magnesium aluminate spinel ceramics fabricated by spark plasma sintering (SPS)

In this paper, a transparent magnesium aluminate spinel ceramic was fabricated through the newest colloidal gel casting method, using a synthetic powder with the average particle size of 90 nm and Isobutylene-Maleic Anhydride (ISOBAM) additive. ISOBAM served as both a dispersant and a gelation agent to achieve a dense body. Also, the suspension rheological behavior was optimized by the solid loading of 85 wt%, the additive content of 0.7 wt%, and the gelation time of 350 s. This led to a green body with a density equal to 65% of theoretical density and the green strength of 14.48 MPa. The results revealed that the reduction of porosity and the uniform distribution of pores in the green body (smaller than half of the initial powder particle size, 35 nm), as accompanied by spark plasma sintering (SPS), resulted in the final body density of 99.97%, as well as the high in-line transmittance of 86.7% at the wavelength of 1100 nm.     

Influence of the heating rate on grain size of alumina ceramics prepared via spark plasma sintering (SPS)

The dependence of grain size on the heating rate has been investigated for alumina ceramics prepared via spark plasma sintering (SPS). For this purpose, the local grain size has been determined via position-dependent microscopic image analysis, using two independent grain size measures (mean chord length and Jeffries grain size). For alumina ceramics prepared with heating rates between 5 and 100 °C/min (pressure 80 MPa, maximum temperature 1300 °C) it is found that for higher heating rates the grain size is smaller. However, the microstructural non-uniformity is so large that any grain size determination that does not take this non-uniformity into account becomes meaningless, because grain size gradients from the specimen periphery to the center are larger than the differences in grain size due to different heating rates. Temperature and pressure gradients are discussed as the most plausible reasons for the microstructural non-uniformity.     

Densification,microstructure and properties of ultra-high temperature HfB2 ceramics by the spark plasma sintering without any additives

Ultra-high temperature HfB₂ ceramic with nearly full densification is achieved by using gradient sintering process of SPS without any additives. The effect of the sintering temperature on the densification behavior, relative density, microstructure, mechanical and thermionic properties is systematically investigated. The results show that the fast densification of HfB₂ ceramic occurs at the heating stage, and the highest relative density of 96.75% is obtained at T =1950 °C, P = 60 MPa and t =10min. As the temperature is increased from 1800 to 1950 °C, the grain size of HfB₂ increases from 6.12 ±1.33 to 10.99 ± 2.25 μm, and refined microstructure gives the excellently mechanical properties. The highest hardness of 26.34 ±2.1GPa, fracture toughness of 7.12 ± 1.33 MPa m^1/2 and bending strength of 501 ±10MPa belong to the HfB₂ ceramic obtained at T =1950°C. Moreover, both the Vickers hardness and fracture toughness obey the normal indentation size effect. HfB₂ ceramic also exhibits the thermionic emission characterization with the highest current density of 6.12 A/cm² and the lowest work function of 2.92 eV.     

The synergistic role of hierarchical preferential oxidation in the enhanced ablation performance of multi-phase multicomponent ultra-high temperature ceramics

Designing ablation-resistant multi-phase multicomponent ultra-high temperature ceramics (UHTCs) remains challenging due to the absence of the understanding toward their oxidation mechanism. Herein, through the investigations on the ablation behaviors of a multiphase multicomponent UHTC, the mechanism of hierarchical preferential oxidation was elucidated. We found that, during ablation, the intraphase preferential oxidation of elements and the inter-phase preferential oxidation of thermodynamically favored phase would occur synchronously. In this hierarchical preferential oxidation process, the intraphase preferential oxidation led to the formation of nanoscale self-healing oxide skeletons in carbide phase. More importantly, the synergy of the hierarchical preferential oxidation induced an inter-phase migration of preferentially oxidized elements (i.e., Hf/Zr) to the preferentially oxidized phase (i.e., silicide), which facilitated the development of micrometer-sized Hf/Zr-rich oxide frameworks. These frameworks and nano-sized skeletons together constituted a multi-scale hierarchical oxide layer, offering an effective protection against scouring of the high-speed hot gas and enhancing the ablation performance.     

Effect of sintering temperature on the structural and magnetic properties of MgFe2O4 ceramics prepared by spark plasma sintering

Spark plasma sintering (SPS) is a powerful technique to produce fine grain dense ferrite at low temperature. This work was undertaken to study the effect of sintering temperature on the densification, microstructures and magnetic properties of magnesium ferrite (MgFe₂O₄). MgFe₂O₄ nanoparticles were synthesized via sol–gel self-combustion method. The powders were pressed into pellets which were sintered by spark plasma sintering at 700–900 °C for 5 min under 40 MPa. A densification of 95% of the theoretical density of Mg ferrite was achieved in the spark plasma sintered (SPSed) ceramics. The density, grain size and saturation magnetization of SPSed ceramics were found to increase with an increase in sintering temperature. Infrared (IR) spectra exhibit two important vibration bands of tetrahedral and octahedral metal-oxygen sites. The investigations of microstructures and magnetic properties reveal that the unique sintering mechanism in the SPS process is responsible for the enhancement of magnetic properties of SPSed compacts.     

Oxidation of TaB2-SiC coatings prepared by spark plasma sintering and effect of pre-oxidation treatments

To suppress the oxidation of TaB₂-SiC coatings, the effects of pre-oxidation temperature on the oxygen hindering properties of TaB₂-SiC coatings were investigated to prepare TaB₂-SiC coatings with enhanced oxidation behavior. The addition of 40 wt% TaB₂ made the oxygen permeability of the coating decrease by 62.16%. However, excessive TaB₂ weakened the oxygen hindering ability of the coating due to the large ion complex ability of Ta⁵⁺. The pre-oxidation temperature at 1500 °C led to a homogeneous dispersion of Ta-oxide nanocrystal particles in the Ta-B-Si-O complex-phase glass layer. In contrast with the untreated samples, the active factor and inert factor values of the TaB₂-SiC coating after pre-oxidation treatment at 1500 °C decreased by 43.12% and 17.33%, respectively, which improved the dynamic stability of the coating during oxidation.     

Mechanical properties and microstructures of TiCN/nano-TiB2/TiN cermets prepared by spark plasma sintering

The effects of TiN and nano-TiB₂ additions to titanium carbonitride (TiCN-WC-Cr₃C₂-Co)-based cermets processed by spark plasma sintering (SPS) are identified. The TiN and nano-TiB₂ additions were varied from 0 to 15?wt% to ascertain their combined effects on the mechanical properties. Scanning electron microscopy (SEM) revealed the combined chemical composition of the new phases formed during sintering. The hardness and fracture toughness values were recorded. Increase in the fracture toughness value with TiN addition was more compared with the nano-TiB₂ addition. In contrast, the hardness values were higher for the cermets formed with the nano-TiB₂ addition. Sintered bodies were made as tool inserts that meet SNGN120408 standard tool configuration. Using these tools, EN24 work-piece was turned at different cutting speeds of 11.87, 29.68, 71.46, 163.88?m/min under conditions of dry cutting. The performance was evaluated. Cutting force as well as surface roughness of the work-piece after machining was measured. For all cutting tools, initially the cutting force was high but it tended to decrease at higher cutting speeds. In addition, for all the tools, at higher cutting tools the surface roughness values were uniformly minimal. The cermet with a composition 55TiCN-15WC-10Co-5Cr₃C₂–15nanoTiB₂ (all in wt%), in particular, showed a balanced enhancement in both fracture toughness (6.8?MPa?m^1/2) and Vickers hardness (18?GPa) values. The surface finish of the work-piece was also the best after machining when a tool of the above composition was used.     

Fabrication of Y6MoO12 molybdate ceramics: From synthesis of cubic nano-powder to sintering

The family of cubic yttrium molybdate (Y₆MoO₁₂) has been chosen due to the possibility of substituting Y³⁺ cations by luminescent trivalent rare earth (RE³⁺) ones and to obtain transparent optical ceramics. The goal of the paper is to find a route towards the fabrication of dense Y₆MoO₁₂ ceramics from nano-powders and using well-known ceramic sintering techniques such as Spark Plasma Sintering (SPS) and Hot Isostatic Pressing (HIP). We have investigated in detail process of the powder preparation by a combustion method as well as the sintering processes taking into account all important sintering parameters. Evaporation of MoO₃ was found to be correlated to reductive conditions or heating rate. Sintering was optimized in terms of phase composition, microstructure and porosity.     

Chemical and microstructural characterisation of HfO2-Y2O3 ceramics with high amount of Y2O3 (33, 40 and 50 mol. %) manufactured using spark plasma sintering

Hafnia based ceramics are potential promising candidates to be used as thermal barrier coatings (TBC) for applications in the field of propulsion. In this study, Spark Plasma Sintering (SPS) of fully stabilised hafnia with yttrium oxide (yttria) was investigated to provide a better understanding of the effect of manufacturing parameters, on the crystallography, chemistry and microstructure of the material. Several hafnia powders, containing different amounts of yttria (33 mol. %, 40 mol. % or 50 mol. %), were sintered by SPS at different temperature levels ranging from 1600 °C to 1850 °C. On these materials, X-ray diffraction patterns associated with scanning electron micrographs have highlighted the influence of both the sintering temperature and the amount of yttria on the final composition, the lattice parameter and the microstructure of hafnia-based materials. In the end, it is established that, for all quantities of yttrium employed, the main phase is Y₂Hf₂O₇ with very high densification levels.     

Spark plasma sintering of Sm3+ doped Y2O3 transparent ceramics for visible light lasers

Transparent Sm:Y₂O₃ ceramics were fabricated by spark plasma sintering (SPS). The effects of LiF additive and sintering temperature on the microstructure and optical transmittance of the Sm:Y₂O₃ ceramics were investigated. The optimal content of LiF additive and sintering temperature was found to be 0.3 wt% and 1500 ℃. The transmittance of Sm:Y₂O₃ ceramics with a thickness of 1.7 mm reached 75.3% at 609 nm, which is about 94% of the theoretical value. The average grain size of the sample was about 50 µm.     

Unveiling enhanced oxidation resistance and mechanical integrity of multicomponent ultra-high temperature carbides

The development of a new class of multicomponent ultra-high temperature ceramics (MC-UHTCs), often referred to as high-entropy UHTCs, has gained increased interest due to the possibility of improved thermomechanical and oxidation properties. In this study, a systematic approach by gradual addition in the UHTC components ranging from a binary to a dense quaternary (Ta,Nb,Hf,Ti)C is synthesized using spark plasma sintering (SPS). The solid solutioning was the critical factor in homogenizing the composition in the multicomponent system. The segregation of NbC and HfC was seen in binary and ternary UHTC systems, while a single-phase homogeneity was observed in the quaternary UHTC improving its hardness up to 34.8 GPa. The presence of closely spaced slip lines in the MC-UHTCs enhances resistance to indentation damage up to 72% at an applied load of 200 N. The formation of complex mixed oxide phase of Hf₆Ta₂O₁₇ ensued in the lower to negligible oxidation even up to 3 min of plasma exposure with temperature exceeding 2800°C. In sum, though the entropy remains medium (0.96R) for the selected system, the quaternary UHTC system undoubtedly has significantly better thermomechanical performance when compared to established baseline UHTCs. This raises the debate on the justification for calling a multicomponent system a “high entropy” to be seen in a new light. The developed MC-UHTCs elicits the paradigm of this new class of UHTCs expanding their potential in thermal protection systems for hypersonic applications.     

Structural properties and mechanical behavior of SWCNTs and MWCNTs reinforced Al2O3 fabricated by spark plasma sintering

Carbon nanotubes due to their structural and mechanical properties are good candidates as the second phase to improve the mechanical properties of alumina-based ceramics. In the present study, the effects of single wall and multi-wall carbon nanotubes on structural and mechanical properties of alumina were investigated. SWCNTs and MWCNTs were dispersed in alumina powder via a conventional method using 1 wt % PVA water solution as media. Sintering process for two different composite powders, alumina-2 wt. % SWCNTs and alumina 2 wt % MWCNTs was performed by spark plasma sintering technique at 1500 °C and 20 MPa for 10 min. Results showed that the presence of CNTs in alumina caused a considerable amount of porosity in final bodies. SEM images of fracture surfaces revealed the agglomeration of SWCNTs which played a dominant role in the deterioration of mechanical properties. MWCNTs reinforced alumina obtained higher Vickers hardness and bending strength values (12.91 GPa and 291 MPa, respectively) compared to that of SWCNTs (9.18 GPa and 276 MPa, respectively), due to sever agglomerate of SWCNTs throughout sintered composites. Typical load-displacement (P/h) curves were obtained from bending strength test and discussed. It was concluded that the addition of MWCNTs to alumina represented better densification and mechanical properties compared to SWCNTs.     

The effect of slip casting and spark plasma sintering (SPS) temperature on the transparency of MgAl2O4 spinel

MgAl₂O₄ bulk samples were fabricated by two different approaches to investigate the effect of slip casting and sintering temperature on their transparency. Three MgAl₂O₄ samples containing 1 wt% LiF, as the sintering aid, were prepared by the spark plasma sintering process (SPS) at 1400 °C and 1500 °C, under 100 MPa, for 15 min. Also, another MgAl₂O₄ sample was prepared by slip casting followed by SPS under similar conditions. It was observed that utilizing slip casting led to more transparency (10% in the visible region and 20% in the IR region) due to the more homogeneous structure. It was also observed that by reducing the SPS temperature from 1500 °C to 1400 °C, the transparency increased (20% in the IR region) because of the lower grain growth rate at the lower temperature.     

Ce3+ doped Lu3Al5O12 ceramics prepared by spark plasma sintering technology using micrometre powders: Microstructure,luminescence, and scintillation properties

Ce³⁺ doped Lu₃Al₅O₁₂ (Ce:LuAG) ceramics were fabricated by the solid-state reaction method through spark plasma sintering (SPS) from 1350 °C to 1700 °C for 5 min at a pressure of 50 MPa using micro powders. The average grain size of the SPSed ceramics gradually grew from 0.42 µm (1400 °C) to 1.55 µm (1700 °C), which is nearly one order of magnitude lower than that of vacuum sintered (VSed) Ce:LuAG ceramics (~24.6 µm). Characteristic Ce³⁺ emission peaking at around 510 nm appeared and 92% photoluminescence intensity of room temperature can be reserved at 200 °C revealing excellent thermal stability. The maximum radioluminescence intensity reached around 3 times of VSed Ce:LuAG ceramics and 7.8 times of BGO crystals. The maximum scintillation light yield under γ-ray (¹³⁷Cs) excitation reached 9634 pho/MeV @ 2 μs. It is concluded that SPS technology is a feasible way to develop Ce:LuAG ceramics and further optical enhancement can be expected.     

Effect of LaB6 addition on compressive creep behavior with simultaneous oxide scale growth of spark plasma sintered ZrB2-SiC composites

A study has been carried out to examine the effect of LaB₆ addition on the compressive creep behavior of ZrB₂-SiC composites at 1300–1400°C under stresses between 47 and 78 MPa in laboratory air. The ZrB₂-20 vol% SiC composites containing LaB₆ (10% in ZSBCL-10 and 14% in ZSBCL-14) besides 5.6% B₄C and 4.8% C as additives were prepared by spark plasma sintering at 1600°C. Due to cleaner interfaces and superior oxidation resistance, the ZSBCL-14 composite has exhibited a lower steady-state creep rate at 1300°C than the ZSBCL-10. The obtained stress exponent (n ∼ 2 ± 0.1) along with cracking at ZrB₂ grain boundaries and ZrB₂-SiC interfaces are considered evidence of grain boundary sliding during creep of the ZSBCL-10 composite. However, the values of n ∼ 1 and apparent activation energy ∼700 kJ/mol obtained for the ZSBCL-14 composite at 1300–1400°C suggest that ZrB₂ grain boundary diffusion is the rate-limiting mechanism of creep. The thickness of the damaged outer layer containing cracks scales with temperature and applied stress, indicating their role in facilitating the ingress of oxygen causing oxide scale growth. Decreasing oxidation-induced defect density with depth to a limit of ∼280 μm, indicates the predominance of creep-based deformation and damage at the inner core of samples.     

High-temperature oxidation behavior of monolithic Zr3[Al(Si)]4C6 and its composite incorporating 30vol% ZrB2-SiC: Providing a basis for broadening the service temperature

To provide a basis for the high-temperature oxidation of ultra-high temperature ceramics (UHTCs), the oxidation behavior of Zr₃[Al(Si)]₄C₆ and a novel Zr₃[Al(Si)]₄C₆-ZrB₂-SiC composite at 1500 °C were investigated for the first time. From the calculation results, the oxidation kinetics of the two specimens follow the oxidation dynamic parabolic law. Zr₃[Al(Si)]₄C₆ exhibited a thinner oxide scale and lower oxidation rate than those of the composite under the same conditions. The oxide scale of Zr₃[Al(Si)]₄C₆ exhibited a two-layer structure, while that of the composite exhibited a three-layer structure. Owing to the volatilization of B₂O₃ and the active oxidation of SiC, a porous oxide layer formed in the oxide scale of the composite, resulting in the degradation of its oxidation performance. Furthermore, the cracks and defects in the oxide scale of the composite indicate that the reliability of the oxide scale was poor. The results support the service temperature of the obtained ceramics.