Nanocrystalline Silicon Carbide Research Articles

Deposition of boron-doped nanocrystalline silicon carbide thin films using H2-Ar mixed dilution for the application on thin film solar cells

Hydrogen-argon mixed dilution has been applied for the deposition of boron-doped nanocrystalline silicon carbide (nc-SiCx) thin films. The variations of structural, compositional, electrical and optical properties with the varying H2/Ar ratio are systemically investigated through various characterizations. It is shown that by using H2-Ar mixed dilution for deposition, B-doped nc-SiCx thin film possessing both wide optical band gap (∼2.22 eV) and high conductivity (∼1.9 S cm−1) can be obtained at the H2/Ar flow ratio of 360/140. In addition, the B-doped nc-SiCx thin films are fabricated as the window layers of a-Si thin film solar cells, and the highest conversion efficiency (8.13%) is obtained when applying the window layer with the largest optical band gap energy.

The effects of subsonic and supersonic dissociated air flow on the surface of ultra-high-temperature HfB2-30 vol% SiC ceramics obtained using the sol-gel method

By hot pressing (1900 °C, 30 MPa, holding time 15 min) of HfB2-(SiO2-C) composite powder, an ultra-high-temperature ceramic material of HfB2-30 vol% SiC composition with nanocrystalline silicon carbide has been obtained. The effects of subsonic and supersonic dissociated air flow on the surface of produced materials have been studied on a high-frequency induction plasmatron in the geometry of a cylindrical sample with a flat face, fixed in a copper water-cooled holder with a 1 mm protrusion. It has been shown that a sudden rise in the average surface temperature of the samples to ∼2600 °C is characteristic for both modes, which is associated with the occurrence of local sites with a temperature >2000 °C and a subsequent increase in their area. It is a matter of evaporation of the borosilicate glass layer from the surface of the oxidized sample and formation of the ceramic layer of a highly catalytic and low thermal conductive porous HfO2 layer, which is confirmed by the emission spectroscopy data, XRD and elemental analysis of the material surface after the experiments. The features of heating the oxidized surface of the samples under the impact of subsonic and supersonic dissociated air flow have been noted: there are differences in the location of overheated sites, initiating a sharp temperature rise and the rate of growth of their area.

Thermal stability and heat flux investigation of neutron-irradiated nanocrystalline silicon carbide (3C–SiC) using DSC spectroscopy

Nanocrystalline silicon carbide (3C–SiC) particles have been irradiated by neutron flux (2 × 1013 n∙cm−2s−1) up to 5 h at the TRIGA Mark II type research reactor. At the present work, thermal properties of nanocrystalline 3C–SiC are comparatively investigated before and after neutron irradiation at the 300 K < T < 1300 K ranges. Simultaneously, the DSC (Scanning Calorimetry), TGA (Thermogravimetric Analysis) and DTG (Differential Thermogravimetric Analysis) experiments were conducted from 300 K up to 1300 K. Oxidation mechanism of nanocrystalline 3C–SiC particles have been theoretically and experimentally studied before and after neutron irradiation. The kinetics of mass and heat flux were analysed at the heating and cooling processes using DSC spectroscopy.

Thermal parameters investigation of neutron-irradiated nanocrystalline silicon carbide (3C–SiC) using DTA, TGA and DTG methods

Nanocrystalline silicon carbide (3C–SiC) particles were irradiated by neutron flux up to 5 h at the TRIGA Mark II type research reactor. DSC (Differential Scanning Calorimetry), DTA (Differential Thermic Analysis), TGA (Thermogravimetric Analysis) and DTG (Differential Thermogravimetric Analysis) analyzes (in cooling and heating processes) of nanocrystalline silicon carbide (3C–SiC) particles were carried out before and after neutron irradiation. It was revealed that the neutron irradiation causes a change in the numerical value of heat flux. Moreover, it has been shown that the oxidation process is relatively slow due to the influence of the neutrons. It was determine that the numerical value of specific heat capacity is characteristic (500 ÷ 750 J kg−1K−1) for 3C–SiC nanocrystals.

Energy harvesting performance of bifacial and semitransparent amorphous silicon thin-film solar cells with front and rear transparent conducting oxide contacts

Bifacial and semitransparent hydrogenated amorphous silicon (a-Si:H) thin-film solar cells were prepared with front and rear transparent conducting oxide (TCO) contacts using a plasma-enhanced chemical vapor deposition process. Increased power conversion efficiencies (PCEs) and average visible transmissions (AVTs) in the visible wavelength range were realized in the cases of the bifacial and semitransparent solar cells based on the optimization of the device designs, such as front TCO contacts, intrinsic a-Si:H absorbers (i-a-Si:H), p-type hydrogenated nanocrystalline silicon carbide, and n-type hydrogenated nanocrystalline silicon layers. The dependencies of the performance parameters of the solar cells on the illumination direction and intensity were also investigated systematically. As the film thickness of the i-a-Si:H absorber layers increased from 150 nm to 450 nm, the PCEs of the solar cells respectively increased in the ranges of 6.01–7.50% and 5.25–6.25% when front and rear illuminations were used for a standard light intensity of 100 mW/cm2, while the AVTs decreased from 22.76% to 15.67% irrespective of illumination direction. The best PCEs of the bifacial and semitransparent solar cells that were fabricated with the use of optimal conditions were respectively equal to 7.94% and 6.20% when front and rear illumination conditions were used. The corresponding AVT was equal to 14.34%. The performance parameters of bifacial and semitransparent a-Si:H solar cells at varying illumination intensities in the range of 10–100 mW/cm2 were also investigated, and yielded consistent PCE values in the range of 6.39–6.62% with front illumination, and 4.95–5.15% with rear illumination.

Graphite interface mediated grain-boundary sliding leads to enhanced mechanical properties of nanocrystalline silicon carbide

Mechanical properties and microstructural deformation characterization of nanocrystalline silicon carbide (n-SiC) have been studied at room-temperature by using nanoindentation and uniaxial microcompression tests abetted with Raman spectroscopy and transmission electron microscopy (TEM). The enhanced mechanical properties of n-SiC observed from indentation and uniaxial micropillars compression tests is mainly attributed to nano-size effect and soft interface phase at the grain boundaries. TEM reveals that the n-SiC upon deformation undergoes grain boundary sliding with the assistance of graphitic interface phase thereby enhancing the strength, fracture toughness and plasticity. The critical crack length values estimated for n-SiC are two to three orders of magnitude smaller than that usually found for brittle ceramics, which implies that plasticity can be achieved before crack initiation and failure. Our density functional theory molecular dynamics simulations agree with the experimental results and provide an atomistic insight in which the graphite along the grain boundary facilitates the grain boundaries sliding in n-SiC.

Behavior of HfB2–30 vol% SiC UHTC obtained by sol–gel approach in the supersonic airflow

Using the sol–gel process, a composite powder of HfB2–(SiO2–C) composition was synthesized and then utilized for reactive sintering (hot pressing, 30 MPa; 1800 °C, 15 min) of ultra-high-temperature ceramic HfB2–30 vol% SiC composites. It was established that nanocrystalline silicon carbide (36 ± 2 nm) in a cubic modification is formed during hot pressing; HfO2 and HfC impurities were not found. The relative density of the obtained materials was 89.2 ± 2.3%. The long-term (40-min) oxidation resistance of the HfB2–30 vol% SiC sample was studied under the exposure to the supersonic dissociated airflow on a high-frequency induction plasmatron (heat flux changed from 232 to 779 W/cm2, pressure in the plasmatron chamber was 15 hPa), in a configuration that prevented significant heat discharge from the sample to a water-cooled holder (with 1-mm overhang). It was shown that, with a stepwise increase in the heat load in the initial stages, the temperature distribution over the front surface of the sample was relatively uniform. However, at the heat flux of 598 W/cm2, local overheated areas appeared in the central region, which spread over almost the entire surface of the sample within 1–2 min; the average temperature was ~2560 °C. Using emission spectroscopy data from the boundary layer above the sample surface, as well as XRD and SEM of the sample after exposure, it was shown that the sharp increase in temperatures from ~1500–1600 to 2500–2600 °C was associated with a change in the chemical nature of the surface, due to evaporation of borosilicate glass components and the appearance of a porous highly catalytic HfO2 with low thermal conductivity on the surface. It was noted that these processes under the exposure to the supersonic flow started at lower temperature than under the exposure to subsonic dissociated airflows.

The optical properties of silicon carbide thin films prepared by HWCVD from pure silane and methane under various total gas partial pressure

Optical properties of amorphous and nanocrystalline silicon carbide (SiC) thin films prepared by hot wire chemical vapor deposition (HWCVD) from silane (SiH4) and methane (CH4) gases under various total gas partial pressures are investigated. The variation of structure and optical properties of the films as a function of total gas partial pressure are studied using FTIR spectroscopy, Raman scattering spectroscopy, and UV–vis-NIR spectroscopy. Optical constants of the SiC films such as absorption coefficient, optical band gap, and refractive index are determined, and the significant correlation between the optical, structural and chemical composition of the films are discussed. The results indicated that the total gas partial pressure and deposition pressure have a strong influence on the structure of the films and therefore significantly control their optical parameters. Consequently, suitable deposition conditions for obtaining SiC films with desired optical and structural properties could be selected.

Development of Pd-Pt functionalized high performance H2 gas sensor based on silicon carbide coated porous silicon for extreme environment applications

Present work demonstrates the hydrogen gas (H2) sensing characteristics of palladium-platinum (Pd-Pt) functionalized silicon carbide (SiC) thin film grown on porous silicon (PSi) substrate for high temperature applications. Nano-crystalline SiC thin film was deposited by RF magnetron sputtering on anodized PSi substrate. The loading of discrete ultra-thin Pd-Pt bimetallic catalytic layer was carefully controlled by varying the sputtering parameters. The proposed device architecture (Pd-Pt/SiC/PSi) revealed significant advantages, such as stable high sensing response, large tunable detection range (5–500 ppm), fast response/recovery time, excellent reproducibility, high selectivity, wide operating temperature regime (25–500 °C) and good durability. The observed high response may be ascribed to the combined effect of enhanced catalytic activity of bimetallic Pd-Pt layer and increased surface area of the proposed sensor.

Temperature vs. impedance dependencies of neutron-irradiated nanocrystalline silicon carbide (3C-SiC)

At the present work, impedance spectroscopy of nanocrystalline silicon carbide (3C-SiC) has been investigated as a function of temperature. Nanocrystalline 3C-SiC particles irradiated by neutrons (2 × 1013 n cm− 2s− 1) up to 20 h. Impedance of neutron-irradiated nanocrystalline 3C-SiC has been studied at the temperature range of 100–400 K. Impedance spectra of the nanocrystal have been comparatively studied before and after neutron irradiation. The natures of conductivity and the metal—semiconductor transition temperature (TMS = 250 K, 325 K, and 370 K at the various frequencies) have been defined from the complex impedance spectroscopy. Polarization of nanocrystalline 3C-SiC increased corresponding to neutron irradiation duration. The mechanism of all effects observed in the experiments has been given in the work.

Layers of Nanocrystalline SiC as a New Type of Solid-State Hydrogen Storage

This paper presents the results related to the investigation of layers of nanocrystalline silicon carbide (nc-SiC) obtained by direct ion deposition for the purpose of hydrogen accumulation. The parameters of the production process providing the largest amount of accumulated hydrogen (more than 5.5 wt.%) were determined based on the mass spectrometric data on the desorption of atomic and molecular hydrogen from nc-SiC films. Electron microscopic examination revealed the structural features that are responsible for absorption, retention, and desorption of hydrogen at relatively low temperatures and pressures. The study results suggest that the main structural elements acting as the hydrogen traps are the vacant positions of carbon in nc-SiC. The presence of a developed system of intercrystalline boundaries in investigated films promotes the hydrogen desorption at relatively low temperatures.

Modulating temperature dependence of thermal conductivity by nanostructuring

At around room temperature and higher, thermal conductivity of single crystal commonly decreases as temperature increases and shows negative temperature dependence. Crystals with positive temperature dependence, which is often preferred in thermal management, requires high Debye temperature. While this narrows the selection of material candidate, the effective Debye temperature can be enhanced through nanostructuring the single crystals. By investigating heat conduction in nanocrystalline silicon, silicon carbide, and diamond, we here demonstrate that shift of temperature dependence of heat capacity and decrease in magnitude of negative temperature dependence of relaxation time by nanostructuring lead to positive temperature dependence of thermal conductivity.

Impact of a Supersonic Dissociated Air Flow on the Surface of HfB2–30 vol % SiC UHTC Produced by the Sol–Gel Method

A new method to produce ultra-high-temperature ceramic composites under rather mild conditions (1700°C, 30 MPa, treatment time 15 min) was applied to synthesize a relatively dense (ρrel = 84.5%) HfB2–30 vol % SiC material containing nanocrystalline silicon carbide (average crystallite size ∼37 nm). The elemental and phase compositions, microstructure, and some mechanical properties of this material and also its thermal behavior in an air flow within the temperature range 20–1400°C were investigated. Using a high-frequency induction plasmatron, a study was made of the effect of a supersonic dissociated air flow on the surface of the produced ultra-high-temperature ceramic composite shaped as a flat-end cylindrical sample installed into a copper water-cooled holder. On 40-min exposure of the sample to the supersonic dissociated air flow, the sample did not fail, and the weight loss was 0.04%. Although the heat flux was high, the temperature on the surface did not exceed 1400–1590°C, which could be due to the heat transfer from the sample to the water-cooled model. The thickness of the oxidized layer under these conditions was 10–20 μm; no SiC-depleted region formed. Specific features of the microstructure of the oxidized surface layer of the sample were noted.

EPR Spectroscopic Studies of Neutron-Irradiated Nanocrystalline Silicon Carbide (3C-SiC)

Nanocrystalline silicon carbide (3C-SiC) has been irradiated by neutrons (2× 1013n ⋅ cm−2s−1) up to 20 hours. Paramagnetic centers and their nature have been investigated comparatively before and after neutron irradiation. The Electron Paramagnetic Resonance (EPR) measurements were performed over the broad range of magnetic field from 0.05 to 0.55 T (500 to 5500 Gauss) in order to detect different paramagnetic centers. Simultaneously, EPR spectroscopy performed over the range from 0.3270T to 0.3370T which is most paramagnetic centers appears including free radicals. Neutron irradiation effects on the concentration of $V_{Si}^{-}$ and $V_{C}^{+}$ vacancies has been investigated. The number of paramagnetic centers for different values of g-factor has been calculated appropriate to local cases existed around 3300G. After neutron irradiation creation mechanism of paramagnetic centers in the nanocrystalline silicon carbide has investigated.

Study on the conditions and composition of the plasma treatment products of silicon carbide micropowder

The conditions of plasma processing of silicon carbide microgrit, phase and chemical compositions, nanolevel and the mechanism of nanocrystalline silicon carbide formation are investigated.

Assessment of the nanocrystalline silicon carbide effectiveness in electroplating, ceramics, surface modification

Technological advantages and conditions for ensuring a new quality of coatings and products achieved with the use of silicon nanocarbide are established in the processes of composite electrodeposition of coatings, formation of structural ceramics, surface hardening of steels with electric explosive alloying. Nanocarbide of silicon is recommended for use in wear-resistant and corrosion-resistant chromium-carbide electrodepositable coatings operating at elevated temperatures, for hardening tools and rigging, including with especially difficult microrelief of working surfaces. Nanocarbide of silicon in the compositions “silicon carbide – boron – carbon” can be used for the production of ceramic billets by solid-phase sintering in argon with pressure 0.1 MPa and temperature 2273 K. The use of silicon nanocarbide in the technology of surface hardening of tool steels by electric explosive alloying provides a protective layer of high microhardness, wear resistance and heat resistance with depth about 20 μm.

Dielectric loss of neutron-irradiated nanocrystalline silicon carbide (3C-SiC) as a function of frequency and temperature

At the present work, nanocrystalline 3C-SiC has been irradiated by neutron flux (2 × 1013 n·cm−2s−1) up to 20 h in a TRIGA Mark II type research reactor. The dielectric loss of nanocrystalline 3C-SiC was studied comparatively before and after neutron irradiation. The increased dielectric loss was clearly observed after neutron irradiation in both f(tanδ) ∼ f(f) and f(tanδ) ∼ f(T) plots. Furthermore, slope observed on the f(tanδ) ∼ f(f) plots at certain values of the frequency. Dielectric loss increasing and shifted slope explained by the neutron transmutation, dangling bonds, the formation of the defects or additional charge carriers. Moreover, the mechanism of all effects obtained from the experiments was explained by the polarization approach.

Grain size variation in nanocrystalline silicon carbide irradiated at elevated temperatures

AbstractThis study reports on ion irradiation‐induced grain size variations in nanocrystalline SiC films on Si substrates. The SiC grains with average size ranging from ~2 to 20 nm were embedded in amorphous SiC matrices. Irradiation was performed using 5 MeV Xe23+ ions to 1.15 × 1016 ions/cm2 at 700 K. The irradiated films were characterized using X‐ray diffraction, transmission electron microscopy, and Raman spectroscopy. Significant grain growth is observed for smaller grains that tend to saturate at ~8 nm. In contrast, irradiation of larger grains (~20 nm in size) leads to a decrease in the grain size, which could be associated with the production of lattice disorder within the grains. Homonuclear C‐C bonds in the irradiated amorphous SiC matrix are found to be graphitized. This bonding transformation could limit or inhibit grain growth and contribute to the size saturation. The results from this study may suggest nanocrystalline SiC as a promising candidate structural or cladding material for applications in advanced nuclear reactors.

Current-voltage characteristics of neutron irradiated nanocrystalline silicon carbide (3C[sbnd]SiC)

At the present work, nanocrystalline silicon carbide (3CSiC) irradiated by neutrons (2 × 1013 n/cm2s) up to 20 h. The current–voltage (IV) characteristics of the nanocrystalline 3CSiC particles have been investigated in the voltage range of −100 V to + 100 V with a 5 V step. The slope of the line changed at the IV characteristic of the nanomaterial after neutron irradiation. Simultaneously, the resistance of nanocrystalline 3CSiC decreased approximately from 4 MΩ to 1 MΩ after neutron irradiation. Moreover, the Fowler-Nordheim plots showed that the processes occurred in the nanomaterial were directly based on a tunnel effect and no field emission was observed. It is clear from Fowler-Nordheim plots that, thermal activity is dominated at the all experimental intervals.

Neutron Irradiation, Amorphous Transformation and Agglomeration Effects on the Permittivity of Nanocrystalline Silicon Carbide (3C-SiC)

Nanocrystalline 3C-SiC is irradiated by neutron flux ([Formula: see text][Formula: see text]n/cm2s) up to 20[Formula: see text]h in the TRIGA Mark II type research reactor. At the first stage, silicon carbide nanoparticles were analyzed by scanning electron microscope (SEM) and transmission electron microscope (TEM) devices before and after neutron irradiation. Amorphous transformation and agglomeration effects on the permittivity of nanocrystalline silicon carbide (3C-SiC) were comparisons investigated before and after neutron irradiation. Dielectric spectroscopies of nanocrystalline 3C-SiC have been conducted at the frequency ranges of 0.1[Formula: see text]Hz–2.5[Formula: see text]MHz and temperature ranges of 100–400[Formula: see text]K. Real and imaginary parts of the permittivity of the nanomaterial were analyzed for comparison before and after neutron irradiation. Neutron irradiation increased dopant elements concentration in the nanocrystalline 3C-SiC particles, and that directly affects dielectric polarization and increased permittivity. All the mechanisms of the observed effects are given in the work.