Heat capacity of Bio-SiC and SiC/Si ecoceramics prepared from white eucalyptus, beech, and sapele tree wood

This paper reports on measurement of the heat capacity at constant pressure C _p of silicon bio-carbide prepared within the 5–300 K temperature interval from beech tree wood (bio-SiC(BE)), and within 80–300 K, from tree wood of sapele (bio-SiC(SA)), as well as SiC/Si ecoceramics of beech, sapele, and white eucalyptus wood. It has been shown that in bio-SiC(BE) the measured heat capacity contains a significant contribution of surface heat capacity, whose magnitude decreases with increasing temperature. Of the ecoceramics, only SiC/Si(SA) characterized by a high enough porosity has revealed a small contribution to the heat capacity coming from its surface component. The experimental results obtained are discussed.     

Mechanism for crystalline Si surface passivation by the combination of SiO2 tunnel oxide and µc‐SiC:H thin film

This work demonstrates that the combination of a wet‐chemically grown SiO₂ tunnel oxide with a highly‐doped microcrystalline silicon carbide layer grown by hot‐wire chemical vapor deposition yields an excellent surface passivation for phosphorous‐doped crystalline silicon (c‐Si) wafers. We find effective minority carrier lifetimes of well above 6 ms by introducing this stack. We investigated its c‐Si surface passivation mechanism in a systematic study combined with the comparison to a phosphorous‐doped polycrystalline‐Si (pc‐Si)/SiO₂ stack. In both cases, field effect passivation by the n‐doping of either the µc‐SiC:H or the pc‐Si is effective. Hydrogen passivation during µc‐SiC:H growth plays an important role for the µc‐SiC:H/SiO₂ combination, whereas phosphorous in‐diffusion into the SiO₂ and the c‐Si is operative for the surface passivation via the Pc‐Si/SiO₂ stack. The high transparency and conductivity of the µc‐SiC:H layer, a low thermal budget and number of processes needed to form the stack, and the excellent c‐Si surface passivation quality are advantageous features of µc‐SiC:H/SiO₂ that can be beneficial for c‐Si solar cells. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Epitaxial TiC/SiC multilayers

Epitaxial TiC/SiC multilayers were grown by magnetron sputtering at a substrate temperature of 550 °C, where SiC is normally amorphous. The epitaxial TiC template induced growth of cubic SiC up to a thickness of ～2 nm. Thicker SiC layers result in a direct transition to growth of the metastable amorphous SiC followed by renucleation of nanocrystalline TiC layers.

     

Chloride‐based CVD of 3C‐SiC epitaxial layers on 6H(0001) SiC

The heteroepitaxial growth of 3C‐SiC on 6H‐SiC(0001) on‐axis substrates is demonstrated in this study. A hot‐wall CVD reactor working at a reduced pressure was used to perform growth experiments at temperatures between 1300 °C and 1500 °C. The addition of hydrogen chloride to standard precursors allowed a wide window of operating parameters, which resulted in the growth of very high quality and purity 3C‐SiC layers, with a morphology characterized largely by single‐domains, especially when nitrogen was intentionally added. Growth rate of 10 µm/h and n‐type background doping in the low 10¹⁵ cm^–3 range were achieved. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth protocols and characterization of epitaxial graphene on SiC elaborated in a graphite enclosure

The epitaxial growth of graphene by the sublimation of Si-terminated silicon carbide (SiC) is studied inside a graphite enclosure in a radio-frequency furnace by comparing different in situ processes involving hydrogen etching or not and different growth conditions. For the growth under vacuum, even with the surface preparation of hydrogen etching, the morphology of the synthesized graphene is found full of voids and defects in the form of a multilayer graphene film. For the growth under Ar, the hydrogen etching plays a vital role to improve the graphene quality in terms of surface roughness, the number of graphene layers and the domain size. For the graphene samples grown with the proposed protocol, the original combination of micro-probe Raman spectroscopy and simultaneous optical transmission and reflection measurements reveals a detailed spatially resolved image of the graphene domains with monolayer domain size of ~5×5 µm² on about 2/3 of the total sample surface. The magnetotransport data yield charge-carrier mobilities up to 2900 cm²/Vs as found for high quality graphene on the Si-face of SiC. The observed magnetoquantum oscillations in the magnetoresistance confirm the expected behavior of single-layer graphene.     

Very high crystalline quality of thick 4H‐SiC epilayers grown from methyltrichlorosilane (MTS)

200 µm thick 4H‐SiC epilayers have been grown by chloride‐based chemical‐vapor deposition using methyltrichlorosilane (MTS) as single precursor. The very high crystalline quality of the grown epilayer is demonstrated by high resolution X‐Ray Diffraction rocking curve with a full‐width‐half‐maximum value of only 9 arcsec. The high quality of the epilayer is further shown by low temperature photoluminescence showing strong free exciton and nitrogen bound exciton lines. The very high crystalline quality achieved for the thick epilayer grown in just two hours at 1600 °C suggests that MTS is a suitable precursor molecule for SiC bulk growth. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Linear expansion coefficient of the SiC/Si biomorphic composite

In the temperature range 100–650 K, the linear expansion coefficient β was measured for the SiC/Si biomorphic composite, a new cellular ecoceramic fabricated from a porous cellular carbon matrix prepared through pyrolysis of wood (white eucalyptus) in an argon ambient with subsequent infiltration of molten Si into the channels of the matrix and the formation of 3C-SiC. The SiC/Si samples studied had an “excess” ～30% volume concentration of Si and a porosity of ～13–15%. The measurements were conducted on samples cut along (β_∥) and across (β_⊥) the tree growth direction. The measured values of β(T) of SiC/Si are compared with literature data available for the linear expansion coefficients of Si and 3C-SiC.     

From Si nanowire to SiC nanotube

Si nanowires (NWs), with diameters of about 800 nm and lengths of about 10 ??m, previously synthesized by the VLS method with gold catalyst, were carburized at 1,100 °C under methane for conversion into SiC nanostructures. These experiments have shown that Si NWs have been transformed into SiC nanotubes (NTs) with approximately the same sizes. Nanotubes?? sidewall thickness varies from 20 to 150 nm depending on the NTs?? height. These SiC nanotubes are hexagonal in shape and polycrystalline. A model of growth based on the out-diffusion of Si through the SiC layer was proposed to explain the transformation from Si nanowires to SiC nanotubes. This model was completed with thermodynamic calculations on the Si?CH₂?CCH₄?CO₂ system and with results from complementary experiment using propane precursor. Routes for obtaining crystalline SiC NTs using this reaction are proposed.     

The structure and photoluminescence properties of Cr-doped SiC films

Cr-doped SiC films are prepared by the RF-magnetron sputtering technique on Si substrates with a composite target of a single-crystalline SiC containing several Cr pieces on the surface. The as-deposited films are annealed in the temperature of 1000 °C under nitrogen ambient. The structure of the samples has been characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and Raman scattering measurement. The results show that the SiC crystal is formed and that majority of Cr doped in the SiC resulted in the formation of the C clusters. Then the photoluminescence (PL) spectra of the samples are observed in the visible range at room temperature. The optical properties of the samples have also been discussed briefly. We attribute the origin of the 412-nm PL band to a kind of C cluster center.     

Comparisons of interface shear stress degradation rate between C/SiC and SiC/SiC ceramic-matrix composites under cyclic fatigue loading at room and elevated temperatures

The interface shear stress in C/SiC and SiC/SiC ceramic-matrix composites with different fiber preforms, i.e. unidirectional, cross-ply, 2D woven, 2.5D woven, and 3D braided, under cyclic fatigue loading at room and elevated temperatures have been estimated. An effective coefficient of the fiber volume fraction along the loading direction was introduced to describe the fiber preforms. Based on fiber slipping mechanisms, the hysteresis loops models considering different interface slip cases have been developed. Using the experimental fatigue hysteresis dissipated energy, the interface shear stress degradation rates of C/SiC and SiC/SiC composites with different fiber preforms at room and elevated temperatures have been obtained and compared. It was found that the interface shear stress degradation rate is the highest for 3D braided SiC/SiC at 1300 °C in air, and the lowest for 2D woven C/SiC at room temperature under cyclic fatigue loading.     

Biomorphous SiC ceramics prepared from cork oak as precursor

Porous ceramic materials of SiC were synthesized from carbon matrices obtained via pyrolysis of natural cork as precursor. We propose a method for the fabrication of complex-shaped porous ceramic hardware consisting of separate parts prepared from natural cork. It is demonstrated that the thickness of the carbon-matrix walls can be increased through their impregnation with Bakelite phenolic glue solution followed by pyrolysis. This decreases the material’s porosity and can be used as a way to modify its mechanical and thermal characteristics. Both the carbon matrices (resulted from the pyrolysis step) and the resultant SiC ceramics are shown to be pseudomorphous to the structure of initial cork. Depending on the synthesis temperature, 3C-SiC, 6H-SiC, or a mixture of these polytypes, could be obtained. By varying the mass ratio of initial carbon and silicon components, stoichiometric SiC or SiC:С:Si, SiC:С, and SiC:Si ceramics could be produced. The structure, as well as chemical and phase composition of the prepared materials were studied by means of Raman spectroscopy and scanning electron microscopy.     

Electrical properties of the SiC/Si composite and the biomorphic SiC ceramic fabricated from spanish beech wood

A study has been made of the dependences of the electrical resistivity and the Hall coefficient on the temperature and magnetic field for the SiC/Si composite fabricated from spanish beech wood and bio-SiC, a high-porosity material formed by chemical extraction of silicon from this composite. The main charge transport parameters of these materials have been determined and analyzed. It has been shown that electric transport in bio-SiC is effected by n-type carriers with a high concentration of ～10¹⁹ cm^?3 and a low mobility of ～1 cm² V^?1 s^?1. The relations obtained have been analyzed by invoking the theory of quantum corrections to conductivity.     

Deposition of BN interphase coatings from B-trichloroborazine and its effects on the mechanical properties of SiC/SiC composites

Boron nitride thin films were deposited on silicon carbide fibers by chemical vapor deposition at atmospheric pressure from the single source precursor B-trichloroborazine (Cl₃B₃N₃H₃, TCB). The film growth and structure, as a function of deposition temperature, hydrogen gas flow rate, and deposition time, were discussed. The deposition rate reaches a maximum at 1000 °C, then decreases with the increasing of temperature, and the apparent activation energy of the reaction is 127 kJ/mol. Above 1000 °C, gas-phase nucleation determines the deposition process. The deposited BN films were characterized by Raman spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effect of BN interphase on the mechanical properties of the unidirectional SiC fiber-reinforced SiC matrix (SiC/SiC) composites was also investigated. The results show that the flexural strength of SiC/SiC composites with and without coating is 276 MPa and 70 MPa, respectively, which indicates that BN interphase coating deposited from B-trichloroborazine precursor can effectively adjust the fiber/matrix interface, thus causing a dramatic increase in the mechanical properties of the composites.     

Large-scale synthesis and characterization of SiC nanowires by high-frequency induction heating

SiC nanowires were synthesized in large quantity by heating SiO and activated carbon fibers without metal catalysts. The morphologies of product were studied by SEM and TEM and it indicated that the SiC nanowires obtained at the bottom of the activated carbon fibers surfaces were coated by an amorphous silicon oxide layer and the core of SiC nanowires were measured, typically as about 8-20 nm in diameter, while in situ formed nanotree-bud network, Y-junction and the “S” shape nanostructure were observed in those SiC nanowires obtained at the up of the activated carbon fibers surfaces. These SiC/SiO₂ heterostructure, particularly novel 2-D heterostructure, would be expected as important blocks in building nanodevices and as reinforcement in advanced composites.     

Understanding compressive deformation in porous titanium

The aim of this study was to understand and compare the compression deformation behavior of porous metals with random and designed porosity. Direct observation, analysis and quantification of porosity parameters using microcomputed tomography (µCT) enabled the determination of relationship between porosity characteristics and compressive deformation of porous titanium. Porosity and pore size variations before and after deformation showed relatively uniform deformation in the sample with random porosity compared to designed porosity. Strong, continuous and regular arrangement of load-bearing sections in the designed porosity sample imparted higher Young's modulus and 0.2% proof strength than for the random porosity sample. The experimental results clearly showed the dependence of deformation behavior and mechanical properties on pore distribution and continuity of load-bearing cross-section.     

Local breaking stresses in loaded microporous SiC ceramics

The ballistic strength and hardness have been measured for a series of SiC ceramic samples with variable (from 1 to 20%) porosity. It is shown that interpore bridges are stress concentrators, and the dependence of the factor of stress concentration on porosity is determined. The conclusions based on the experimental results are confirmed by the finite-element calculations of stress fields in porous systems.     

The structure and optical properties of SiC film on Si (111) substrate with a ZnO buffer layer by RF-magnetron sputtering technique

We have fabricated a multiply layer SiC/ZnO on Si substrates using the RF-magnetron sputtering technique with the targets of a single crystalline SiC and a polycrystalline ZnO. The as-deposited films were annealed in the temperature range of 600–1000 °C under nitrogen ambient. We have observed a strong ultraviolet (UV) emission (370 nm) from the as-deposited SiC/ZnO film and an intense violet emission (412 nm) from the film annealed at high temperature (1000 °C) under nitrogen ambient. The SiC film quality and the PL intensities are considered to be strongly dependent on the crystalline quality of the ZnO buffer layer. With the increase of the annealing temperature, the crystalline quality of the ZnO buffer layer is improved, resulting in the improvement of the SiC film quality and the increase of the PL intensities. The thin films have been characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) to provide the evidences of photoluminescence (PL). We suggest that the UV emission could be attributed to the nanocrystal silicon particles, that the 395 nm band is related to ZnO buffer layer and has a great relation to the crystalline quality of the ZnO film, and that the violet emission is associated with the emission luminescence from 6H-SiC, which bears on the SiC film quality. The obtained results are expected to have important applications in modern optoelectronic devices.     

Fast etching and metallization of SiC ceramics with copper-vapor-laser radiation

The etching of polycrystalline SiC is studied with the help of radiation of a copper-vapor laser either in air or under the layer of a liquid (H₂O, DMSO). The etching rate in air is as high as 0.24 m/pulse, in DMSO 0.07 gm/pulse at an energy density of 16 J/cm². The etched surface is characterized with Scanning Electron Microscopy (SEM) and X-ray diffractometry. Etching of SiC ceramics in air revealed the partial amorphization of SiC and the formation of microcrystals of elementary Si with an average size of 300 Å. The etched surface of SiC ceramics takes on the ability to reduce Cu from a corresponding electroless plating solution. The adherence of the deposit is as high as 30 N/mm² and is a function of the scanning velocity of the laser beam.     

SiC based Si/SiC heterojunction and its rectifying characteristics

The Si on SiC heterojunction is still poorly understood, although it has a number of potential applications in electronic and optoelectronic devices, for example, light-activated SiC power switches where Si may play the role of an light absorbing layer. This paper reports on Si films heteroepitaxially grown on the Si face of (0001) n-type 6H-SiC substrates and the use of B₂H_6 as a dopant for p-Si grown at temperatures in a range of 700--950~\du. X-ray diffraction (XRD) analysis and transmission electron microscopy (TEM) tests have demonstrated that the samples prepared at the temperatures ranged from 850~℃ to 900~℃ are characterized as monocrystalline silicon. The rocking XRD curves show a well symmetry with FWHM of 0.4339° Omega. Twin crystals and stacking faults observed in the epitaxial layers might be responsible for widening of the rocking curves. Dependence of the crystal structure and surface topography on growth temperature is discussed based on the experimental results. The energy band structure and rectifying characteristics of the Si/SiC heterojunctions are also preliminarily tested.     

不同退火方式对Ni/SiC接触界面性质的影响

采用快速热退火(rapid thermal annealing, RTA)法和脉冲激光辐照退火(laser spark annealing, LSA)法, 在n型4H-SiC的Si面制备出Ni电极欧姆接触. 经传输线法测得RTA样品与LSA样品的比接触电阻分别为5.2×10^-4 Ω·cm², 1.8× 10^-4 Ω·cm². 使用扫描电子显微镜、原子力显微镜、透射电子显微镜、拉曼光谱等表征手段, 比较了两种退火方式对电极表面形貌、电极/衬底截面形貌和元素成分分布、SiC衬底近表层碳团簇微结构的影响. 结果表明, 相比于RTA, LSA法制备出的欧姆接触在电极表面形貌、界面形貌、电极层组分均匀性等方面都具有明显优势, 有望使LSA成为一种非常有潜力的制备欧姆接触的退火处理方法.