Diamond-like carbon sintered compacts formed by spark plasma sintering

A spark plasma sintering (SPS) method was utilized for the novel production of diamond-like carbon (DLC) compacts. Two amorphous carbon powders with different particle sizes (45 μm and 24 nm diameter) were employed as starting materials for the sintering experiments. The carbon powders were sintered using a SPS system at various sintering temperatures and holding times. The structural properties of the sintered compacts were evaluated using X-ray diffraction (XRD) analysis and high-resolution transmission electron microscopy (HRTEM). Disk-shaped compacts were obtained by sintering the powder with a particle diameter of 45 μm, although the compacts were very brittle and easily broken. However, sintering of the 24 nm diameter powder particles at temperatures of 1473 to 1573 K with a holding time of 300 s led to the successful production of sintered compacts without breakage. Reflection peaks related to graphite structure were observed in XRD patterns of the compacts sintered from the 24 nm diameter particles. HRTEM analysis revealed that the compacts sintered at 1473 K with a holding time of 300 s had an amorphous structure and consisted of 34% sp³ carbon bonding. Evaluation of the structural properties indicated that sintered compacts with DLC structure could be created by the SPS method with 24 nm diameter amorphous carbon particles.     

Synthesis of structure-controlled carbon nano spheres by solution plasma process

Structure-controlled carbon nanospheres (CNSs) were synthesized by an innovative plasma in liquid method, termed solution plasma processing (SPP). CNSs were formed by using benzene as a carbon precursor. Typically, 500 mg of CNSs were obtained from 100 ml of benzene with 20 min of treatment. The average diameters of CNSs increased from 20 to 100 nm when the pulse frequency of the bipolar power supply adjusted from 25 to 65 kHz. The TEM images showed that CNSs synthesized at 25–50 kHz consisted of amorphous carbon, while CNSs generated at 65 kHz were composed of continuous short range graphite with turbostratic structure. X-ray diffraction (XRD) patterns and TEM images showed that CNSs synthesized at 25–50 kHz had a low graphitization degree, while CNSs synthesized at 65 kHz consisted of graphite sheets with regular and ordering structure in basal planes. The increase of D/G intensity ratio in Raman spectroscopy confirmed the transition from amorphous carbon to nanocrystalline-graphite (NCG) with increasing pulse frequency. The resistivity of CNSs also increased with increasing pulse frequency. CNSs synthesized at 65 kHz have shown similar degree of resistivity as other commercial carbon black materials.     

Structure of copolymer films created by plasma enhanced chemical vapor deposition

The interface structure in copolymer films made using plasma enhanced chemical vapor deposition (PECVD) has been probed for the first time using X-ray reflectivity. Copolymer films made from comonomers benzene (B), octafluorocyclobutane (OFCB), and hexamethyldisiloxane (HMDS) show extremely sharp interfaces and scattering length density depth profiles that are uniform with depth, making them useful for optical applications. The polymer/air interface has an rms roughness (∼5 Å) that is only slightly larger than that of the supporting substrate (∼3 Å). Addition of either benzene or HMDS as a comonomer in the deposition of OFCB alters a transient deposition behavior at the silicon oxide interface that occurs when using only OFCB. For the B-OFCB copolymer films, a facile control of refractive index with monomer feed composition is achieved. A nonlinear variation in the X-ray scattering length density with composition for the HMDS-OFCB copolymer films is consistent with the nonlinear visible light refractive index (632.8 nm) variation reported earlier.     

Spark plasma sintered ZrC-Mo cermets: Influence of temperature and compaction pressure

The microstructure analysis and mechanical characterisation were performed on a ZrC-20 wt%Mo cermet that was spark plasma sintered at various temperatures ranging between 1600 and 2100 °C under either 50 or 100 MPa of compaction pressure. The composite reached ~98% relative density for all experiments with an average grain size between 1 and 3.5 µm after densification. The nature of SPS technology caused a faster densification rate when higher compaction pressures were applied. The difference in compaction pressures produced different behaviors in densification and grain structure: 1900 °C, 100 MPa produced excessive grain growth in ZrC; 1600 °C, 50 MPa revealed a very clear ZrC grain structure and Mo diffusion between carbide grains; and 2100 °C, 50 MPa exhibited the highest overall mechanical properties due to small clusters of Mo phases across the microstructure. In fact, this particular sintering regime gave the most optimal mechanical values: 2231 HV10 and 5.4 MPa*m^1/2, and 396 GPa Young's modulus. The compaction pressure of SPS played a pivotal role in the composites’ properties. A moderate 50 MPa pressure caused all three mechanical properties to increase with increasing sintering temperature. Conversely, a higher 100 MPa pressure caused fracture toughness and Young modulus to decrease with increasing sintering temperature.     

Densification of plasma sprayed YSZ electrolytes by spark plasma sintering (SPS)

Solid oxide fuel cells (SOFC) are promising candidates for alternative power generation systems due to their high-energy conversion efficiencies, and low emissions of environmentally hazardous by-products. Plasma spray (PS) is an effective, and relatively inexpensive process for fabricating high performance yttria stabilized zirconia (YSZ) electrolyte for SOFC. Yet, because of the numerous inter-granular defects introduced to the electrolyte by the plasma spray process, the electrolyte is not gas tight and consequently, the energy efficiency of the cell is severely curtailed. In order to improve the performance of the SOFC, spark plasma sintering (SPS) is introduced as a post-spray treatment to enhance the density of the PS YSZ electrolyte rapidly, and effectively. In this study, spark plasma sintering (SPS) was performed at 1200, 1400 and 1500 °C. Each sintering cycle had a holding time of 3 min. Single and multiple SPS cycles (3 min at preset temperature per cycle) were used to treat the plasma sprayed yttria stabilized zirconia (PS YSZ) electrolytes. The microstructure of as-received and SPS treated electrolytes as examined by scanning electron microscopy (SEM) demonstrated a microstructure transition above 1200 °C, where the typical plasma sprayed lamella structure transformed to a granular-type structure. The porosity of as-received and SPS post-treated electrolytes, which were determined by a mercury intrusion porosimeter (MIP) revealed a significant reduction in pores at 1500 °C. Average pore size reduced from 0.2 to 0.08 μm. The ionic conductivity of the electrolytes is evaluated by AC impedance spectroscopy to characterize the effect of SPS on enhancing the ionic conductivity of the electrolytes.     

Fabrication of 4H-SiC piezoresistive pressure sensor for high temperature using an integrated femtosecond laser-assisted plasma etching method

The processing, particularly, etching of brittle, hard, and anti-corrosion materials represented by the third-generation wide bandgap semiconductor silicon carbide (SiC), is a significant challenge. Although SiC has excellent electrical, mechanical, and chemical properties, the difficulty of processing limits its application in various sensor devices. To solve this problem, in this study, an integrated processing method of femtosecond laser-assisted SiC dry etching is proposed, which realizes high surface quality and high rate etching of the SiC microstructure. Specifically, the effects of different laser processing parameters on the processing effect were first studied through orthogonal experiments. Experiments indicate that compared with laser power and laser scan times, laser processing speed has a more obvious impact on the processing effect. Subsequently, considering the elastic modulus anisotropy of SiC, a 5 MPa piezoresistive pressure sensor chip was designed. Using the proposed composite processing method, a chip sensitive diaphragm was obtained. The diaphragm thickness and diameter are 76 μm and 1700 μm respectively. The overall sensor chip dimension was 4000 μm × 4000 μm × 350 μm. Static tests demonstrated that the sensor have excellent performance with sensitivity of 6.8 mV/MPa, linearity of 0.69% FS, and repeatability of 0.078% FS. In addition, by designing high-temperature packaging, the sensor achieved a pressure test at 400 °C. This study verifies the feasibility of the composite processing method, realizes the fabrication and measurement of high-temperature pressure sensors, and provides a reference for the micro-and nanostructure processing of various SiC sensors.     

Combustion synthesis and spark plasma sintering of apatite-tricalcium phosphate nanocomposites

A processing route consisting of Spark Plasma Sintering (SPS) of precursor powders prepared by Solution Combustion Synthesis (SCS) is proposed for the first time for the fabrication of bulk nanostructured biphasic calcium phosphates. The apatite phase content in the product obtained by SCS was maximized using a fuel to oxidizer ratio of 1.1. After a post-synthesis air-annealing step conducted a 700 °C/3 h, powders consisted of 83 wt.% of carbonated apatite, with average crystallite size less than 70 nm, and β- and α-TCP (tricalcium phosphate), as secondary phases. Detailed structural analyses evidenced that the original nanostructure was retained after sintering at 900 °C, with the obtainment of nearly 91% dense, apatite-rich, biphasic bioceramics, with grains size of about 100 nm. The developed nanostructured biphasic material is expected to possess a higher resorption rate than standard microcrystalline hydroxyapatite, which makes it preferable for bone tissue regeneration.     

Silicon carbide ceramics: Mechanical activation,combustion and spark plasma sintering

Single-stage fabrication of SiC ceramics by a combination of self-propagating high temperature synthesis (SHS) and spark plasma sintering (SPS) is reported. SHS+SPS is demonstrated to be an efficient method for production of SiC ceramics with density 3.1 g/cm³, hardness of 24 GPa and toughness of 5 MPa m^1/2. The starting material for the process is fine (50–300 nm in size) highly reactive powder, which involves composite particles of elemental carbon and silicon. This powder was prepared using a high-energy ball milling (HEBM). To optimize precursor preparation conditions, the structure transformation in nano-composite Si/C particles at different HEBM stages is also investigated.     

Alumina/molybdenum nanocomposites obtained by colloidal synthesis and spark plasma sintering

Alumina/molybdenum nanocomposites were prepared by colloidal synthesis from alumina powder and molybdenum (V) chloride using ethanol as dispersion medium. Modified alumina was calcined at 450 °C in air atmosphere to remove chlorides, and then treated in a tubular furnace at 850 °C under Ar/H₂ to reduce the MoO₃ formed in the previous stage and obtain Al₂O₃ with molybdenum nanoparticles on the surface. Three different molybdenum contents were proposed (1, 5 and 10 wt % Mo), and pure alumina was used as reference, that were sintered by spark plasma sintering (SPS) under vacuum atmosphere at 1400 °C for 3 min with an applied pressure of 80 MPa. Composites were characterized by microstructure, hardness, toughness, and three-point bending test. The presence of molybdenum nanoparticles resulted in a fine-grained structure promoted by the presence of molybdenum at grain boundaries and triple points, as well as by the utilization of the SPS equipment. Hardness is at least a 20% greater and fracture toughness 30% larger in the composites than in the monolithic alumina.     

Synthesis of magneto-dielectric coatings in electron-beam produced plasma in medium vacuum

Using sequential electron-beam evaporation of high-temperature dielectric (alumina ceramic) and magnetic (iron) targets in various gas atmospheres (helium, air, and oxygen) in medium vacuum (5–8 Pa), magneto-dielectric coatings with thickness of around 2 μm were deposited from a multicomponent beam plasma at a deposition rate of 0.2–0.3 μm/min. The coating magnetic properties were explored by the ferromagnetic resonance technique, revealing that their effective magnetization depends on the type of operating gas and varied from 4.2 to 6.8 kGs (for deposition in helium) to 0.3 kGs (in oxygen), which is characteristic of oxide ferromagnetic materials and is considerably lower than the corresponding value (∼22 kGs) for thin iron films formed by vacuum arc deposition in high vacuum. X-ray structural analysis of coatings deposited in medium vacuum in helium showed that the magnetic layer has a magnetite (Fe₃O₄) structure. The alumina ceramic layer provides the dielectric properties of the magneto-dielectric coating; a relative dielectric constant of 6.0 and a conductivity of 8.4 mS/m were achieved.     

Synthesis of amorphous KAlSi3O8 for cesium radionuclide immobilization into solid matrices using spark plasma sintering technique

An effective sorption material for cesium radionuclides immobilization in highly safe and reliable solid-state matrices was proposed. Prepared aluminosilicate (КAlSi₃O₈) adsorbent had amorphous mesoporous structure and Cs⁺ ions sorption capacity of ～3.7 mmol/g. The physical-chemical characteristics of (Cs, К)AlSi₃O₈ sample saturated with Cs ⁺ ions were studied using XRD, FT-IR, SEM-EDX, and DTA-TG methods. Firstly, solid-state aluminosilicate matrices were obtained using spark plasma sintering (SPS) technology with high values of relative density (up to 99.9%), compressive strength (31.3–79.2 MPa), and Vickers microhardness (0.9–5.3 GPa). The sample obtained at 1000 °C had a low value of Cs⁺ leaching from matrices (R_Cs within the range of 10^-7 g cm^-2·day^-1) and cesium diffusion coefficient (D_e 9.07 × 10^-14 cm²/s). It was shown that prepared aluminosilicate cesium matrices comply with regulatory requirements of GOST R 50926-96 and ANSI/ANS 16.1.     

Spark plasma sintering of Sm2O3-doped aluminum nitride

The high sintering temperature required for aluminum nitride (AlN) at typically 1800 °C, is an impediment to its development as an engineering material. Spark plasma sintering (SPS) of AlN is carried out with samarium oxide (Sm₂O₃) as sintering additive at a sintering temperature as low as 1500–1600 °C. The effect of sintering temperature and SPS cycle on the microstructure and performance of AlN is studied. There appears to be a direct correlation between SPS temperature and number of repeated SPS sintering cycle per sample with the density of the final sintered sample. The addition of Sm₂O₃ as a sintering aid (1 and 3 wt.%) improves the properties and density of AlN noticeably. Thermal conductivity of AlN samples improves with increase in number of SPS cycle (maximum of 2) and sintering temperature (up to 1600 °C). Thermal conductivity is found to be greatly improved with the presence of Sm₂O₃ as sintering additive, with a thermal conductivity value about 118 W m⁻¹ K⁻¹) for the 3 wt.% Sm₂O₃-doped AlN sample SPS at 1500 °C for 3 min. Dielectric constant of the sintered AlN samples is dependent on the relative density of the samples. The number of repeated SPS cycle and sintering aid do not, however, cause significant elevation of the dielectric constant of the final sintered samples. Microstructures of the AlN samples show that, densification of AlN sample is effectively enhanced through increase in the operating SPS temperature and the employment of multiple SPS cycles. Addition of Sm₂O₃ greatly improves the densification of AlN sample while maintaining a fine grain structure. The Sm₂O₃ dopant modifies the microstructures to decidedly faceted AlN grains, resulting in the flattening of AlN–AlN grain contacts.     

Fabrication and characterization of 8YSZ ceramic based abradable seal coatings by atmospheric plasma spraying

8 wt% yttria-stabilized zirconia (8YSZ) powders are fabricated as high-temperature based materials via a solid-state reaction method and ground into spheres in this paper. Following that, 4 wt% Nickle (Ni), 4 wt% Hexagonal Boron Nitride (hBN) and 4 wt% PHB (Polyphenyl ester) are added to 8YSZ for getting 8YSZ ceramic-based abradable seal powders (8YSZ CAS_p). Then, the 8YSZ CAS_p are sprayed on the stainless steel substrate with a NiCoCrAlY transition layer by an atmospheric plasma spraying (APS) technology. The phase structure, surface morphology and the cross-section topography of the fabricated are analyzed, the indentation hardness and nano-indentation test are conducted. The experiments of 8YSZ ceramic-based abradable seal coatings (8YSZ CAS_c) show that the deposition efficiency and porosity are respectively 78.5% and 21.8%, the bond strength is 4.6 MPa, the cycle number of thermal shock resistance is 37 times, those parameters prove that the fabricated 8YSZ CAS_c are promising abradable seal coatings.     

Synthesis and properties of bulk graphene nanoplatelets consolidated by spark plasma sintering

Graphene nanoplatelets (GNPs) are consolidated as a bulk structure by spark plasma sintering (SPS) to study the feasibility of its structure retention at extreme processing conditions. Structural characterization of the sintered GNP pellet is performed using Raman spectroscopy and scanning electron microscopy. Mechanical and tribological properties are evaluated through nanoindentation and ball-on-disk tribometer. GNPs survived the SPS processing at an extreme temperature of 1850 °C and a pressure of 80 MPa with minimal damage to the structure. Energy dissipation mechanisms are observed in the form of multiple bending at sharp angles and sliding of the platelets, which could provide effective toughening mechanisms to ceramic matrices. Tribological studies suggest that GNP shear off and weld at higher load and provides a lubricating effect. Our study shows the potential for GNPs to be successfully used as a reinforcing and lubricating phase in ceramic matrix composites synthesized by SPS.     

Hard polycrystalline eutectic composite prepared by spark plasma sintering

A polycrystalline eutectic B₄C–TiB₂ composite was prepared by spark plasma sintering. The starting eutectic powder was obtained by mechanical grinding of the directionally solidified eutectic B₄C–TiB₂ alloy. The microstructure of the polycrystalline composite exhibited randomly oriented eutectic grains with an average size of about 50–100 μm. Eutectic grains consisted of boron carbide matrix reinforced by titanium diboride inclusions. The secondary eutectic structure in the grain boundary is formed at sintering temperature higher than 1700 °C. XRD analysis revealed that the eutectic B₄C–TiB₂ composite consist mainly of B₄C and TiB₂ phases. The measured Vickers hardness was in the range of 32.35–54.18 GPa and the average fracture toughness of the samples was as high as 4.81 MPa m^1/2. The bending strengths of the composite evaluated at room temperature and at 1600 °C were 230 and 190 MPa, respectively.     

Hydrogen plasma etching of diamond films deposited on graphite

Poly- and nanocrystalline diamond films have been deposited using microwave plasma enhanced CVD with gas mixtures of x%CH₄/15%H₂/Ar (x = 0.5, 1, 3, and 5). After deposition the resulting films were exposed to a hydrogen plasma etching for 30 min. The hydrogen plasma produced preferential etching of non-diamond carbon on the surface of the samples and the development of steps and pits. Raman spectroscopy and X-ray photoelectron spectroscopy analyses on the etched films showed increased sp³/sp² ratio and decreased surface oxygen. The etch mechanism proposed is regression of pre-existing steps and step flow.     

BiFeO3 ceramics synthesized by spark plasma sintering

Phase-pure BiFeO₃ particles were synthesized by an improved solid state technique. High density BiFeO₃ ceramics were prepared using these particles by spark plasma sintering (SPS). The dielectric permittivity and loss of SPS samples were measured as functions of sintering temperature, frequency, and annealing conditions. Dielectric spectra of the ceramics annealed at 650 °C were characterized in a broad range of temperature (300–725 K) and frequency (100 Hz to 20 MHz). Two kinds of dielectric relaxation following the Arrhenius law were detected in low and high temperature ranges, respectively. The low temperature dielectric relaxation could be almost completely removed by annealing in vacuum and it should be assigned to be a valence fluctuation of Fe ions, while the high temperature dielectric relaxation was proposed to stem from the short-range motion of oxygen vacancies.     

Molecular dynamics simulations of nanocarbons at high pressure and temperature

A molecular dynamics study of carbon nanoparticles (980 and 10,034 atoms) under high temperature (1000–7000 K) and high pressure (2–45 GPa) has been made using the reactive LCBOPII potential. The most stable structure of the small cluster is onion-like (encapsulated fullerenic) on the whole pressure range, whereas a transition from onion-like to nanodiamond is observed for the big cluster as pressure increases from 2 to 45 GPa. The melting mechanism depends on the structure, initiated in the core in the case of an onion cluster and at the surface for the nanodiamond. A schematic phase diagram is proposed, that takes into account the finite size effects.     

D and D/He plasma interactions with diamond: Surface modification and D retention

Chemical vapour deposition (CVD) diamond is of interest as a plasma facing material in magnetically confined nuclear fusion devices. CVD diamond samples were exposed to ion fluences between 1.5 × 10²² ions m^− 2 and 1 × 10²⁴ ions m^− 2 in D, D/He and He plasmas. A significant surface coverage of 10 nm–100 nm sized hemispherical and conical surface features resulted from chemical erosion and subsequent redeposition on samples exposed to ≥ 10²³ ions m^− 2 when exposed to D containing plasmas. Near edge X-ray absorption fine structure (NEXAFS) spectra showed strong diamond features following plasma exposure, indicating that the diamond crystal structure on the surface was largely retained. A small increase in the sp² fraction (from 1.2% to 4–7%), was observed suggesting some amorphous carbon formation. Elastic recoil detection analysis (ERDA) revealed that D retention saturates at ~ 5.4% averaged over the depth resolution of 20 nm from the surface, for an ion fluence of ~ 10²³ ions m^− 2. The addition of He to the plasma resulted in a slower uptake of D, which was attributed to He ion induced desorption of D.     

Microstructural evolution of ZnO via hybrid cold sintering/spark plasma sintering

In this work, we demonstrate a hybrid cold sintering/spark plasma sintering (CSP-SPS) process to densify ZnO ceramic with controlled grain growth. The densification of ZnO is initially activated at 85 °C, and high densities (>98%) are achieved at 200–300 °C in only 5 min with a low assisted pressure of 3.8–50 MPa. The microstructure of ZnO grains experiences a mild coarsening from ~205–680 nm during the CSP-SPS. In comparison, a much higher temperature (>770 °C) is required to sinter ZnO ceramic via SPS, and the grain size exhibits an obvious overgrowth to ~10 µm. The calculated apparent activation energy of grain growth using CSP-SPS is 69.3 ± 6 kJ/mol, which is much lower than that of SPS samples with 296.8 ± 59 kJ/mol. In addition, the conduction mechanism of the CSP-SPS and SPS samples is investigated using impedance spectroscopy. Overall, CSP-SPS is promising for the fabrication of fine ceramics with mild sintering conditions.