SiC Membrane Research Articles

Toward high-quality 3C–SiC membrane on a 3C–SiC pseudo-substrate

The cubic polytype of silicon carbide is an interesting candidate for Micro-Electro-Mechanical-Systems (MEMS) applications due to its tremendous physico-chemical properties. The recent development of multi-stacked Si/SiC heterostructures has demonstrated the possibility to obtain a (110)-oriented 3C–SiC membrane on a 3C–SiC pseudo-substrate, using a silicon layer grown by low pressure chemical vapor deposition as a sacrificial one. However, the (110) orientation of the 3C–SiC membrane led to a facetted and rough surface which could hamper its use for the development of new MEMS devices. Then, in this contribution, an optimized growth process is used to improve the surface quality of the 3C–SiC membrane. The progress relies on the mastering of a (111) orientation for the SiC film, resulting in a smooth surface. Such an optimized structure could be the starting point for the achievement of new MEMS devices in medical or harsh environment applications.

Fabrication and properties of mullite-bonded porous SiC membrane supports using bauxite as aluminum source

Mullite-bonded porous SiC membrane supports were fabricated at 1350–1500°C in air from bauxite and graphite. The phase composition, open porosity, microstructure and mechanical strength as well as pore size distribution of membrane supports were systematically investigated. Due to the enhancement of necks at the contacting points growth by the addition of bauxite (30wt%), a high three-point flexural strength of 85.2MPa was achieved at an open porosity of 31.4%. Moreover, when 20wt% C was added to the above composition, the strength of 36.6MPa was achieved at an open porosity of 47.8%.

Toward a process-based molecular model of SiC membranes: III. Prediction of transport and separation of binary gaseous mixtures based on the atomistic reactive force field

The atomistic model of amorphous silicon-carbide membrane that was developed in Parts I and II of this series is utilized in nonequilibrium molecular dynamics (MD) simulations to study transport and separation of equimolar gaseous mixtures H2/CO2 and H2/CH4 in the membrane at high temperatures. We simulated membranes with up to about 39nm in thickness, containing up to 170,000 atoms, and up to 100ns to obtain reliable statistics. The effect of parameters such as the temperature, the applied pressure drop across the system, and the membrane׳s thickness on the separation properties was studied. The trends in the dependence of the separation factor on the membrane׳s thickness are consistent with experiments, namely the separation factor increases with the thickness up to an optimal value, beyond which it remains constant, or may even decrease. The dependence of the computed separation factor on the membrane׳s thickness is used in conjunction with the experimental values of the separation factor to estimate the thickness of actual membranes. The results are in agreement with the experimental data, demonstrating the value of MD simulations with a reactive force field for characterizing the properties of complex amorphous films, and flow and transport therein.

One-step deposition of ultrafiltration SiC membranes on macroporous SiC supports

We fabricated nearly defect-free SiC membranes for potential ultrafiltration applications by conducting pyrolysis of allylhydrido polycarbosilane in the presence of submicron α-SiC particles. The SiC membranes were developed on commercial macroporous SiC supports by a low-temperature-process in which allylhydrido polycarbosilane acted to bond together crystalline α-SiC particles to form a porous layer. The suspensions of α-SiC powder and allylhydrido polycarbosilane in n-hexane or n-hexane/n-tetradecane were used for membrane fabrication by dip-coating. By using optimized n-hexane suspension with 5% w/w α-SiC powder and mass ratios of allylhydrido polycarbosilane to α-SiC powder of 0.6 and 0.8, we obtained nearly defect-free and uniform mesoporous membranes in a single coating procedure. No defects were found on the surface of these membranes by scanning electron microscopy. Moreover, during filtration tests, these membranes showed a water permeance of 0.05–0.06L (hm2bar)–1 and retention higher than 93% for polyethylene glycol (PEG) with a molecular mass of 100kDa. Retention for PEG with molecular masses of 1kDa, 8kDa and 35kDa was between 25% and 71%.

Dip-coating of nano-sized CeO2 on SiC membrane and its effect on thermal diffusivity.

CeO2-SiC mixed composite membrane was fabricated with porous SiC ceramic and cerium oxide powder synthesized by sol-gel process. This CeO2-SiC membrane and SiC membrane which is made by the purified SiC ceramic were pressed and sintered in Ar atmosphere. And then, the SiC membrane was dip-coated by cerium oxide precursor sol solution and heat-treated in air. The surface morphology, particle size, porosity and structure analysis of the mixing and dip-coating SiC membrane were monitored by FE-SEM and X-ray diffraction analysis. Surface area, pore volume and pore diameter were determined by BET instrument. Thermal diffusivity was measured by laser flash method with increasing temperature. The relation between porosity and thermal diffusivity from different preparation process has been discussed on this study.

Hydrogen Permeation of SiC-CeO2Composite Membrane by Dip-coating Process

A SiC-CeO₂ composite membrane was successfully fabricated using an ally-hydridopolycarbosilane (AHPCS) binder and treated by dip-coating at 60 times with a CeO₂ sol solution. The dip-coated SiC membrane was calcined at 773 K and then sintered at 1173 K under an air atmosphere. The coated membrane was characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and a BET surface analysis. The difference in permeation performance between H₂ and CO gases was measured by varying the temperature. The permeation flux of H₂ on the SiC membrane with layered CeO₂ was obtained as 8.45 × 10 -6 mol/㎡sPa at room temperature. The CO permeation flux was 2.64 × 10 -6 mol/㎡sPa at room temperature. The reaction enthalpy (ΔH°) for the hydrogen permeation process was calculated as .7.82 J/mol by Arrhenius plots.

Preparation and Hydrogen Permeability of SiC-Y2O3Composite Membranes

ABSTRACTSiC-Y 2 O 3 porous composites were fabricated using Y 2 O 3 powders synthesized by sol-gel process to control physical and thermo-chemical properties. Y 2 O 3 powders were mixed with SiC powders by co-pressing with HPCS (hydridopolycarbosilane) binder atmoderate temperature. The properties of membranes were characterized by XRD, FE-SEM, and BET surface analysis. Hydrogenpermeability was performed at various temperatures.Key words : SiC, Y 2 O 3, HPCS, Hydrogen Permeance tests 1. Introduction ydrogen selective SiC membranes have been used forapplication in membrane reactors. 1,2) SiC as a struc-tural material has advantages such as thermal-mechanicalstability and chemical inertness. SiC also has disadvantagein terms of its porosity and mechanical property of hard-ness. Since SiC has the covalent character of Si-C bonds,SiC ceramics can be sintered at high temperature and arelimited in terms of applications. However, very little hasbeen reported on the fabrication of porous SiC ceramics.

Highly permeable porous silicon carbide support tubes for the preparation of nanoporous inorganic membranes

Highly permeable and mechanically strong porous SiC tubular supports, used for the fabrication of nanoporous SiC membranes, are prepared by the pressureless sintering of β-SiC particles. Their transport characteristics are studied via inert gas (He and Ar) permeation tests, while their surface structure is characterized by atomic force microscopy and electron microscopy analysis. In this paper, the effects of the type of starting powders used and their composition, the sintering temperature, and the amount of sintering aids utilized on the transport characteristics and the surface roughness of the sintered SiC porous supports are systematically investigated. These tubular SiC supports exhibit very high fluxes (a He permeance as high as 5.8×10−5molm−2Pa−1s−1) and are mechanically strong to withstand the pressure drops required in their use as membrane supports. They have been used in the preparation of high quality SiC nanoporous membranes.

Hydrogen Permeance of Ce<SUB>0.8</SUB>Y<SUB>0.2</SUB>O<SUB>1.9</SUB>/SiC Membrane

SiC is a promising material for various inorganic membranes because there are properties such as high thermal conductivity, thermal shock resistance and high mechanical strength. Due to these advantages, SiC is also suitable for gas and liquid separation filter material. The SiC used as a support membrane and then added Ce0.8Y0.2O1.9 composite powder to effect catalyst and improve porous. The Ce0.8Y0.2O1.9 composite powder was synthesized with yttrium (III) nitrate hexahydrate and cerium(III) nitrate hexahydrate by sol-gel method. The Ce0.8Y0.2O1.9/SiC membrane was prepared from the purified SiC ceramic and the synthesized Ce0.8Y0.2O1.9 composite powder with aid of hydridopolycarbosilane (HPCS) as a binder, followed by heat-treatments. This membrane was characterized by XRD, FE-SEM, EDXS and BET surface analysis. Thermal diffusivity and hydrogen permeance for this membrane will be discussed.

Fabrication of x-ray diffractive optical elements for laser fusion applications

We review our recent progress on the fabrication of x-ray diffractive optical elements (DOEs) by combining complementary advantages of electron beam, x-ray, and proximity optical lithography. First, an electron beam lithography tool with an accelerating voltage of 100 kV is used to expose initial x-ray mask based on SiC membrane with a low aspect ratio. Second, x-ray lithography is used to replicate x-ray DOEs and amplify the aspect ratio up to 14:1. Third, proximity optical lithography is used to fabricate a large-scale gold mesh as the supporting structures. We demonstrate that this method can achieve high aspect ratio metal nanometer structures without the need of a complicated multilayer resist process. A large number of x-ray DOEs have been fabricated with feature sizes down to 100 nm for the purpose of laser plasma fusion applications. Among them, the ninth-order diffraction peak on the positive side of the zeroth order can be observed for both 3333 and 5000 lines/mm x-ray gold transmission gratings.

Toward a Process-Based Molecular Model of SiC Membranes. 2. Reactive Dynamics Simulation of the Pyrolysis of Polymer Precursor To Form Amorphous SiC

In part 1 of this series we developed the reactive force field ReaxFF, choosing the parameters adjusted to fit quantum mechanics description of prototypical reactions. In the present paper we use ReaxFF for reactive dynamics (RD) simulation of thermal decomposition of a silicon-containing polymer, hydridopolycarbosilane (HPCS) over a wide range of temperature. Many properties of the material during its pyrolysis were computed and were found to be in good agreement with the experimental data. We then simulated pyrolysis of the HPCS under conditions that closely mimic the conditions of the fabrication of nanoporous SiC membranes to produce amorphous SiC material. Again, the computed properties of the SiC ceramic were found to be in good agreement with the experimental data. In particular, the ReaxFF RD simulations generate ceramic SiC that contains mostly Si–C bonds with some Si–Si and C–C bonds, consistent with experimental data. This indicates again the accuracy of the parameters of the ReaxFF determined in Part 1.

Investigation of the properties of SiC membrane on alumina by using polycarbosilane

We investigated the properties of a SiC membrane on alumina, derived from polycarbosilane. The thickness of the deposited SiC layer was approximately 1μm. The topmost layer, having a thickness of approximately 0.6μm, comprised high-purity SiC. The 2.8μm thick layer present below it appeared to consist of SiO2. The contact angles between the SiC membrane and molten alumina at 973K and 1373K were approximately 67% and 86% of that between non-modified alumina and molten alumina.

Properties of Al and Pd Contacts on n-type SiC Membranes

ABSTRACTMembranes with dimensions up to 10 mm x 15 mm have been fabricated in epitaxial 3C-SiC/Si wafers. An array of CTLM metal contacts was deposited onto the upper surface of the n-SiC membrane. Both Al/n-SiC and Pd/n-SiC contacts which were formed on the membrane and on the adjacent substrate have shown an ohmic current/ voltage response. Values of specific contact resistance, ρc, were measured directly on the membranes. These results have shown no consistent difference in ρc of the contacts located either on the membrane or off the membrane. The exposure of SiC surfaces to reactive ion etching in CF4 plasma during the fabrication of a membrane has resulted in ρc which was higher by a factor of 103 than with as-grown and KOH etched silicon surfaces.

Comparison of ceramic and polymeric membrane permeability and fouling using surface water

The trans membrane pressure (TMP) increase at a constant flux of 150 L m −2 h −1 due to membrane fouling by direct filtration with lake water is investigated for four ceramic (Al 2O 3, ZrO 2, TiO 2, SiC) and a PES/PVP polymeric microfiltration membrane(s). The membrane structures and compositions are characterised with permporometry (pore size, porosity) and XPS. The TiO 2 and SiC membrane have a 5 and 24 times larger average pore size than expected based on supplier information. The TMP increase rate due to fouling inversely follows the measured pore size. Reversible fouling decreases in the order of polymeric ≈ Al 2O 3 ≈ ZrO 2 > TiO 2 > SiC, and for irreversible fouling polymeric > ZrO 2 > Al 2O 3 > TiO 2 > SiC. Removal of non purgeable organic carbon (NPOC) is around 30% for the ceramic membranes, and 13–25% for the polymeric membrane. The reversible NPOC load decreases in the following order (Al 2O 3 > (ZrO 2 ≈ TiO 2)) ≈ SiC > polymeric. The higher degree of fouling on the polymeric membrane is partly due to its lower volume/area ratio, compared to ceramic membranes. Mass balance analysis shows that 85 ± 8% of the NPOC is accounted for. Thus, the used soaking method (pH 12 NaOH, >1 h) does not fully remove the NPOC, suggesting that a more aggressive cleaning solution should be used.

Specific contact resistance of ohmic contacts to n-type SiC membranes

ABSTRACTMembranes of epitaxial SiC have been used as a means of eliminating the leakage current into the Si substrate during circular transmission line model (CTLM) measurements. In the n+-3C-SiC/Si wafers, the Si substrate was etched in a patterned window with dimensions up to 10 mm × 15 mm2. An array of CTLM metal contacts was then deposited onto the upper surface of the n+-SiC membrane. The CTLM contacts on the membrane have shown an ohmic current/voltage response while electrodes located on the adjacent substrate were non-ohmic. Values of ρc were measured directly on the membranes. These results have shown a significant increase in the current flow below the metal contacts due to the presence of the Si substrate.

Gas Permeation Property of SiC Membrane Using Curing of Polymer Precursor Film by Electron Beam Irradiation in Helium Atmosphere

SiC membranes were prepared using curing of precursor polymer (polycarbosilane, PCS) film by electron beam irradiation in helium atmosphere. The membrane prepared via curing of PCS film coated using 10mass% PCS solution for dip-coating followed by immersing it for 30 s in PCS solution, showed H2 permeance of 3:1 10 7 mol/m/s/Pa and the selectivity of 51 at 523K. The H2 permeance of the membrane was increased proportional to the temperature by the activated diffusion of H2. It indicates SiC film without pinholes or cracks formed on the support. As the pyrolysis temperature of cured PCS film was increased, the selectivity of the membrane reached the maximum at 923K. [doi:10.2320/matertrans.M2010418]

Robust PECVD SiC membrane made for stencil lithography

Stencil lithography (SL) is a shadow mask technique which allows parallel, resistless, micro- and nano-patterning of material through apertures created in a membrane (stencil) onto a substrate. The stencils are usually made of LPCVD low-stress SiN due to its outstanding physical and chemical stability. However, limitations are found for some specific design cases, where membranes can be distorted because of the deformations induced by stress from the deposited materials. Here we present a recently developed PECVD SiC shadow mask for applications in SL. The SiC has higher Young’s modulus than SiN, which transfers to a better performance for SiC stencil than the shadow mask made of SiN in terms of robustness to the stress-induced deformation. We show direct local etching of SiO2 through SiC stencil, otherwise impossible with SiN masks. SiC stencils with both compressive and tensile stresses were fabricated and compared to the SiN stencils. The higher robustness to deformation and better resistance to etching of the SiC membranes allow for a wider choice of the deposited materials, leading themselves to a more precise pattern duplication and less complicated resistless dry etching applications.

Gas Permeation of SiC Membrane Coated on Multilayer γ-Al2O3 with a Graded Structure for H2 Separation

A promising candidate material for a <TEX>$H_2$</TEX> permeable membrane is SiC due to its many unique properties. A hydrogen-selective SiC membrane was successfully fabricated on the outer surface of an intermediate multilayer <TEX>$\gamma-Al_2O_3$</TEX> with a graded structure. The <TEX>$\gamma-Al_2O_3$</TEX> multilayer was formed on top of a macroporous <TEX>$\alpha-Al_2O_3$</TEX> support by consecutively dipping into a set of successive solutions containing boehmite sols of different particle sizes and then calcining. The boehmite sols were prepared from an aluminum isopropoxide precursor and heated to <TEX>$80^{\circ}C$</TEX> with high speed stirring for 24 hrs to hydrolyze the precursor. Then the solutions were refluxed at <TEX>$92^{\circ}C$</TEX> for 20 hrs to form a boehmite precipitate. The particle size of the boehmite sols was controlled according to various experimental parameters, such as acid types and acid concentrations. The topmost SiC layer was formed on top of the intermediate <TEX>$\gamma-Al_2O_3$</TEX> by pyrolysis of a SiC precursor, polycarbosilane, in an Ar atmosphere. The resulting amorphous SiC-on-<TEX>$Al_2O_3$</TEX> composite membrane pyrolyzed at <TEX>$900^{\circ}C$</TEX> possessed a high <TEX>$H_2$</TEX> permeability of <TEX>$3.61\times10^{-7}$</TEX> <TEX>$mol{\cdot}m^{-2}{\cdot}s^{-1}{\cdot}Pa^{-1}$</TEX> and the <TEX>$H_2/CO_2$</TEX> selectivity was much higher than the theoretical value of 4.69 in all permeation temperature ranges. Gas permeabilities through a SiC membrane are affected by Knudsen diffusion and a surface diffusion mechanism, which are based on the molecular weight of gas species and movement of adsorbed gas molecules on the surface of the pores.

Fabrication and microstructural characterization of porous SiC membrane supports with Al2O3–Y2O3 additives

Porous silicon carbide membrane supports were prepared using alumina and yttria additives (1–4 wt%) with various weight ratios of Al2O3/Y2O3 (23/77–60/40) and sintering at 1500–1800 °C. The relationships between the processing factors and the porosity, pore size and microstructure were examined. Variation of the sintering temperature, the amount of additive used and the ratio of Al2O3/Y2O3 were found to be effective for control of the pore size and microstructure. The pore size and grain size of membrane support with 1 wt% additive increased with the sintering temperature. For SiC membrane supports containing 4 wt% additive, pore size and grain size were dependent on the alumina ratio in additive. Smaller grains and fine pores around 200–300 nm were detected for specimens with higher Al2O3 content (60/40) in the additive. In contrast, for specimens with low Al2O3 content (23/77) in the additive, the grain and pore sizes were enlarged. This difference was attributed to the effect of Al2O3 content on the temperatures of liquid phase formation during sintering.

Amorphous SiC as a structural layer in microbridge-based RF MEMS switches for use in software-defined radio

This paper reports an effort to develop amorphous silicon carbide (a-SiC) films for use in shunt capacitor RF MEMS microbridge-based switches. The films were deposited using methane and silane as the precursor gases. Switches were fabricated using 500 nm and 300 nm-thick a-SiC films to form the microbridges. Switches made from metallized 500 nm-thick SiC films exhibited favorable mechanical performance but poor RF performance. In contrast, switches made from metallized 300 nm-thick SiC films exhibited excellent RF performance but poor mechanical performance. Load-deflection testing of unmetallized and metallized bulk micromachined SiC membranes indicates that the metal layers have a small effect on the Young’s modulus of the 500 nm and 300 nm-thick SiC MEMS. As for residual stress, the metal layers have a modest effect on the 500 nm-thick structures, but a significant affect on the residual stress in the 300 nm-thick structures.