Pressure-pulsed Chemical Vapor Infiltration Research Articles

Surface-modification of anode carbon for lithium-ion battery using chemical vapor infiltration technique

To reduce the high irreversible capacity of the low crystalline carbon fiber for the anode material of lithium-ion battery, pyrolytic carbon (pyrocarbon) was coated at 950 °C from C 3H 8(30%)–H 2 gas system using pressure-pulsed chemical vapor infiltration. Carbon fiber was coated with the dense pyrocarbon film having the laminar texture and the low surface area of 1.9 m 2 g −1. It was revealed from XRD and Raman spectroscopy that the crystallinity of pyrocarbon is higher than that of the core carbon. Electrochemical properties were measured in ethylene carbonate (EC) and propylene carbonate (PC) base electrolytes. Irreversible capacity was reduced in EC-based electrolyte by coating with 8 mass% pyrocarbon, which would be attributed to the high crystallinity, laminar structure and low surface area of pyrocarbon. Irreversible capacity was also decreased in PC-based electrolyte. The crystallinity of pyrocarbon was not so high as PC-based electrolyte was decomposed in the case of the high crystalline graphite.

Electro-conductive porous ceramics prepared by chemical vapor infiltration of TiN

Using pressure-pulsed chemical vapor infiltration (PCVI) method, TiN was partially infiltrated at 850 °C from gas system of TiCl4 (1%)–N2 (10%)–H2 into the highly porous carbon substrates prepared by the carbonization of cotton-wool, filter paper, and wood at 1,000 °C in Ar for 4 h. After 10,000 pulses of PCVI, electro-conductive porous ceramics having the three-dimensionally continuous current paths were obtained, which had the porosity of 80% and more, the resistivity of 10−5–10−6 Ω m, and the average pore sizes of 10–40 μm. The geometric surface area per unit volume of the sample was higher than that of the conventional foil-type current collector for lithium-ion battery. The surface area showed the highest value for the sample obtained from carbonized wood substrate.

Structure of carbonized paper coated with pyrolytic carbon and its anode property for lithium-ion battery

Using pressure-pulsed chemical vapor infiltration (PCVI) technique, pyrolytic carbon (pyrocarbon) was infiltrated at 950°C from C3H8 (30%)-H2 gas system into the carbonized paper substrate. Carbon fibers in the carbonized paper were coated with dense pyrocarbon films having a laminar texture oriented parallel to the surface of carbonized fibers. It was revealed from XRD and Raman spectroscopy that the crystallinity of pyrocarbon was higher than that of carbonized fiber. BET surface area was decreased from 210 m2g-1 of the original substrate to 1.9 m2 g-1 of the pyrocarbon-coated sample.Reversible capacity of the sample coated with 8 mass % pyrocarbon was 500 mA h g-1, which were nearly close to that of the original carbonized paper. High irreversible capacity of 180 mA h g-1 was observed in the original carbonized paper, reflecting the disordered structure and high surface area. Irreversible capacity was reduced to 100 mA h g-1 by coating with 8 mass % pyrocarbon, which would be attributed to high crystallinity, laminar structure and low surface area of pyrocar-bon. Irreversible capacity was further decreased with increasing the mass fraction of pyrocarbon, however, reversible capacity was also decreased.

Electrochemical performance of LiFePO 4 modified by pressure-pulsed chemical vapor infiltration in lithium-ion batteries

Using the pressure-pulsed chemical vapor infiltration (PCVI) technique, pyrolytic carbon (pyrocarbon) films were deposited on the surface of LiFePO 4 particles for cathode material of lithium-ion batteries. The electrochemical performance of the original LiFePO 4 and PCVIed LiFePO 4 materials was evaluated using a three electrodes cell by galvanostatic charging/discharging at 25, 40 and 55 °C, respectively. Morphology and structure of LiFePO 4 were analyzed by SEM, XRD and Raman. The resulting carbon contents at 500, 1000, 2000, 3000 and 5000 pulses were 2.7, 4.7, 9.5, 15.1 and 19.4%, respectively and these samples were abbreviated as 500P, 1000P, 2000P, 3000P and 5000P, respectively. All the PCVIed samples exhibited excellent rate performance. The tendency was more and more obvious with the increase of the current densities. The specific capacities of 500P, 1000P and 2000P were maintained at 117, 124 and 132 mAh g −1, respectively, which were 120.8, 264.7 and 29.47% larger than those of corresponding original LiFePO 4, respectively, at a 5 C rate at 55 °C. The EIS measurement showed that electrochemical reaction resistance ( R ct) of PCVIed LiFePO 4 were obviously decreased, indicating a fast kinetics compared to the original LiFePO 4. The cycle ability of the 2000P sample was tested at 25 °C and C/2 rate. The cell was cycled for 150 cycles and no obviously capacity fade was observed. Its specific capacity of 115 mAh g −1 at 150th cycle is 1.7 times higher than that of original LiFePO 4.

Low-temperature synthesis of graphite particles by chemical vapor infiltration and their electrochemical properties for lithium-ion battery

Using pressure-pulsed chemical vapor infiltration technique, pyrolytic carbon (pyrocarbon) was deposited at low temperature below 1100°C from the source gases of C6H6 (6%)-H2 into Ni-foam as catalyst. Pyrocarbon could be deposited at 650 °C or higher. The crystallinity was increased with the temperature elevation up to 900°C. It was found by XRD and Raman spectroscopy that high crystalline pyrocarbon particles having mainly graphite phase were deposited at 800-900°C. However, low crystalline carbon was co-deposited under the condition of temperature above 900 °C. For the samples obtained at 900°C or lower, the charge-discharge behaviors were similar to that of high crystalline natural graphite. On the other hand, “sloping region” in charge-discharge curves were observed for the sample obtained at 1000 and 1100 °C, resulting from the co-deposition of low crystalline carbon. The sample obtained at 850 t showed the highest reversible capacity of 352 mAhg-1 at a current density of 30 mAg-1, and 8 % of the capacity was maintained even at 3000 mAg-1.

Pyrocarbon-coating on powdery hard-carbon using chemical vapor infiltration and its electrochemical characteristics

Using the pressure-pulsed chemical vapor infiltration technique, thin pyrocarbon films were coated on the surface of the hard-carbon particles. The crystallinity of pyrocarbon shell was higher than that of the core carbon. The BET surface area and the pores with the diameter of 1.5–5 nm were extremely decreased after coating with 7 wt.% pyrocarbon. These changes in the nano-scaled structure of carbon surface could lead to the reduction of irreversible capacity. The cycleability was improved by pyrocarbon-coating, which would be attributed to the increase of the surface roughness on sub-micron scale by coating with the pebble-like pyrocarbon film to increase the adhesion among the particles by the organic binders.

04/02807 Preparation of negative electrodes for lithium-ionrechargeable battery by pressure-pulsed chemical vapor infiltration of pyrolytic carbon into electro-conductive forms: Ohzawa, Y. et al. Journal of Power Sources, 2003, 122, (2), 153–161

04/02807 Preparation of negative electrodes for lithium-ionrechargeable battery by pressure-pulsed chemical vapor infiltration of pyrolytic carbon into electro-conductive forms: Ohzawa, Y. et al. Journal of Power Sources, 2003, 122, (2), 153–161

Structures and electrochemical properties of pyrolytic carbon films infiltrated from gas phase into electro-conductive substrates derived from wood

Using the pressure-pulsed chemical vapor infiltration technique, pyrolytic carbon (pyrocarbon) films were deposited into two sorts of conductive porous substrates, that is, the carbonized wood (A) and the TiN-coated wood (B). Structures and electrochemical properties were investigated as the negative electrodes of lithium-ion secondary battery. The electrodes had the three-dimensionally continuous current paths in the pyrocarbon-based anodes without the organic binders and the additional conductive fillers. The pyrocarbon films adhered tightly to the carbonized wood or TiN as current collector. These macro-structures of electrodes were effective in improving the high rate property. The sort of substrates affected the nano-structure of pyrocarbon. The pyrocarbon in sample (A) had the relatively high crystallinity, whereas the pyrocarbon in sample (B) was disordered. The capacity of pyrocarbon in sample (B) was higher than that of sample (A), reflecting the disordered microstructure of pyrocarbon film (B). However, sample (A) showed higher Coulombic efficiency at first cycle (i.e. 87%) than that of sample (B), which would result from the high crystallinity, laminar microstructure and low surface area of pyrocarbon in sample (A).

Preparation of negative electrodes for lithium-ion rechargeable battery by pressure-pulsed chemical vapor infiltration of pyrolytic carbon into electro-conductive forms

Abstract The plate-type negative electrodes for lithium-ion rechargeable battery were prepared by pressure-pulsed chemical vapor infiltration of pyrolytic carbon (pyrocarbon) into two sorts of conductive porous forms, that is, the carbonized paper (A) and the TiN-coated paper (B), as the conductive fillers and/or current collectors. The electrodes had the three-dimensionally continuous current paths in the pyrocarbon-based anodes without the organic binders and the additional conductive fillers. The pyrocarbon in sample (A) had the relatively high crystallinity, whereas the pyrocarbon in sample (B) was disordered. Sample (B) possessed higher surface area and larger pore volume with mesopores of 1.5–10 nm, especially below 3 nm, than that of sample (A). The capacity of pyrocarbon in sample (B) was 460 mA h g −1 per mass of pyrocarbon at a current density of 25 mA g −1 , reflecting the disordered microstructure of pyrocarbon film. And 80% of the capacity was maintained even at 1000 mA g −1 . The capacity of pyrocarbon in sample (A) was estimated at ∼300 mA h g −1 , which was lower than that of sample (B). However, sample (A) showed higher Coulombic efficiency at first cycle (i.e. 85%) than that of sample (B), which would result from the high crystallinity, laminar microstructure and low surface area of pyrocarbon in sample (A).

Pressure-Pulsed Chemical Vapor Infiltration of SiC to Three-Dimensional Carbon Fiber/SiC Preform Prepared by Polymer Impregnation and Pyrolysis.

Preforms were prepared by PIP (Polymer Impregnation and Pyrolysis) processing using polycarbosilane to 3-dimensional orthogonal weave fabrics of pitch-based carbon fiber. Porous matrices of the preforms were still more filled with β-SiC by PCVI (Pressure Pulsed Chemical Vapor Infiltration) processing using source gas mixture of SiCl4-CH4-H2 system to obtain dense 3-dimensional carbon fiber reinforced SiC matrix composites (3D-C/SiC). In the PCVI process, influence of the processing conditions on densification of the matrices was investigated in detail. The PCVI filling up to 120000 pulses were carried out under various conditions with varying temperature, residence time and gas flow rate. Then the filling-ratio was evaluated from changes in density and porosity, and crystal-phases of deposits were identified by XRD. The filling-ratio and the phase of deposits varied with the PCVI processing conditions. The PCVI processing under the conditions of 1373K in temperature, 1.5s in residence time, 4mol% each of SiCl4 and CH4 in molar-concentration and 80kPa in total pressure gave the highest filling-ratio. In this case, the number of pulses and operating time necessary for saturation of the filling-ratio were 60000 pulses and 192ks, respectively. The porosity of the densest composite was about 6% from macrosurface to 0.5mm in depth and less than 9% even to 1.5mm in depth. Also, it was found that higher total pressure was effective for increasing filling-speed without increasing the porosity, but disadvantageous in the consumption of the source gases.

Preparation of SiC-based cellular substrate by pressure-pulsed chemical vapor infiltration into honeycomb-shaped paper preforms

Using a pressure-pulsed chemical vapor infiltration technique, SiC was infiltrated from a SiCl4 (4%)–CH4 (4%)–H2 gas phase into carbonized paper preforms at 1100°C. SiC-based cellular substrates with cell wall thicknesses of 25, 50 and 100 μm were obtained by using honeycomb-shaped paper preforms as the templates. The reduction of both wall thickness t and cell pitch d of SiC-based honeycomb substrate successfully led to an increase in geometric surface area per unit volume S, keeping pressure drop ΔP at constant, besides; ΔP decreased without lowering S by the reduction of t and the increase in d. The pressure drop in the prepared honeycomb substrates depended on S 2α−3 value, where α was open frontal area fraction. The compressive strength of the honeycomb substrates with t of 25 μm was about 7 MPa. The strength increased in proportion with 1 − α, which corresponded to the volume fraction of the cell wall in the honeycomb substrate.

Pressure-pulsed chemical vapor infiltration of pyrolytic carbon into fibrous tin prepared from carbonized paper preform

The plate-type negative electrodes for Li-ion rechargeable battery were prepared by pressure-pulsed chemical vapor infiltration of pyrolytic carbon at 950°C from C 3 H 8 (30%)-H 2 into the TiN-based electroconductive forms having the fibrous structure. The electrodes had the three-dimensional current paths in the layers of active materials without the organic binders and the conductive additives. The charge-discharge profiles were similar to that observed in non-graphitizable carbon having disordered structure. Reversible capacity of pyrolytic carbon at current density of 0.2 mA cm −2 and coulombic efficiency at the first cycle were∼500 mA h g −1 and∼72%, respectively. The capacity at current density of 8 mA cm −2 maintained 80% of that at 0.2 mA cm −2 .

Synthesis of highly tailored ceramic matrix composites by pressure-pulsed CVI

Pressure-pulsed chemical vapor infiltration (P-CVI) is a new processing technique to produce ceramic matrix composites (CMCs) (C/C, C/SiC and SiC/SiC) from gaseous precursors with highly tailored interphases and matrices. P-CVI can be used to control the microtexture of pyrocarbon deposited from hydrocarbon in a porous body. It is also used to form (PyC–SiC)n or (BN–SiC)n interphases with elementary layer thickness of a few nanometers. Finally, P-CVI is an efficient way to produce self-healing matrices comprising mechanical fuse layers (PyC or BN) and glass-former layers (B13C2 or/and SiC). The resulting model or real composites display an oxidation resistance, in air at 600–700°C and under load, which is improved by two orders of magnitude.

107 熱プレス発泡ウレタン由来のポーラスカーボンへのパルスCVIによるSiCコーティングとその特性(オーガナイズドセッション : 機械材料・新素材2)

107 熱プレス発泡ウレタン由来のポーラスカーボンへのパルスCVIによるSiCコーティングとその特性(オーガナイズドセッション : 機械材料・新素材2)

Preparation of a fibrous SiC shape using pressure-pulsed chemical vapour infiltration and its properties as a high-temperature filter

Using the pressure-pulsed chemical vapour infiltration (PCVI) process, SiC was partially infiltrated into carbonized cotton–wool preforms having an initial porosity of 0.97. After 70 000 pulses of PCVI, fibrous SiC having a porosity of 0.78, an average pore size of 29 μm and a flexural strength of 15 MPa was obtained. The pressure drop was 2 kPa when air was passed through a sample of 5 mm length at a linear velocity of 1 m s −1. The pressure drop of the present sample was proportional to (1−ε) 2ε −3 D −2 f , where ε and D f were the porosity and the diameter of the fibre in the sample, respectively. The flexural strength after air-oxidation at 1000°C was close to that before oxidation.

Hi‐Nicalon/SiC Minicomposites with (Pyrocarbon/SiC)n Nanoscale Multilayered Interphases

SiC/SiC minicomposites that comprise different pyrocarbon/silicon carbide ((PyC/SiC)n) multilayered interphases and a tow of SiC fibers (Hi‐Nicalon) have been prepared via pressure‐pulsed chemical vapor infiltration. Pyrocarbon and SiC were deposited from propane and a CH3SiCl3/H2 mixture, respectively. The microstructure of the interphases has been investigated using transmission electron microscopy. The mechanical tensile behavior of the minicomposites at room temperature exhibits the classical features of tough composites, regardless of the characteristics of the (PyC/SiC) sequences. The interfacial shear stress has been determined from the width of hysteresis loops upon unloading/reloading and from the crack‐spacing distance at saturation. All the experimental data indicate that the strength of the fiber/interphase interfaces is rather weak (∼50 MPa).

Relation between porosity and pore size or pressure drop of fibrous SiC filter prepared from carbonized cellulose-powder preforms

Abstract Using pressure-pulsed chemical vapor infiltration (PCVI), SiC was partially infiltrated into the carbonized cellulose-powder preforms having the initial porosity of 0.93–0.94, and the relation among porosity ( e ), pore size and pressure drop (Δ P ) was evaluated. After 30 000 pulses of PCVI, fibrous SiC having e of 0.76 and average pore size ( D p ) of 13 μm was obtained. D p was related with e and fiber diameter ( D f ), assuming a model for fiber arrangement at right-angles and regular intervals. The derived relationship showed that D p increased proportionally with D f under the constant e . Δ P followed the relationship based on the Kozeny–Carman equation, i.e. it was proportional to (1− e ) 2 e −3 D f −2 . Δ P was also related with e and D p using the Kozeny–Carman equation and the model for fiber arrangement. From the derived relationship, it was suggested that Δ P of fibrous SiC filter was lower than that of porous carbon or sintered ceramics filter at the same e and D p .

Processing of Ceramic Matrix Composites

The main processing techniques used to fabricate ceramic matrix composites (CMCs) with continuous fibers are reviewed. They involve liquid or gas phase routes, the starting material often being a fiber preform. In the reactive melt infiltration processes, the preform is densified with a liquid (Al or Si) reacting with the preform or the atmosphere (O{sub 2} or N{sub 2}). The polymer impregnation/pyrolysis as well as the slurry impregnation/hot pressing processes are based on techniques previously utilized for polymer matrix composites or/and monolithic ceramics. In the gas phase routes, or chemical vapor infiltration (CVI), the fiber preform is densified with the matrix, which is formed in-situ as the result of chemical reactions involving a gaseous precursor. When performed under isothermal/isobaric conditions (I-CVI) at low temperatures and pressures, the process is slow but highly flexible. The densification rate is significantly improved when a temperature gradient or/and a pressure gradient (F-CVI) are applied to the preform. Finally, pressure-pulsed CVI (P-CVI) is a unique technique for the fabrication of composites with highly engineered interphases or matrices. (orig.) 40 refs.

Fabrication and Characterization of 3D C/SiC Composites via Slurry and PCVI Joint Process

The effect of the pressure-pulsed chemical vapour infiltration on improving the quality of CMC-parts has been investigated. In a fabrication process of three-dimensional C/SiC composites, the PCVI was applied as a final densification step for specimens made by a joint process consisting of slurry infiltration and polycarbosilane impregnation/pyrolysis. PCVI was carried out in the temperature range of 1323-1423 K using a source gas system of SiCl{sub 4}-CH{sub 4}-H{sub 2}. The crystalline phases deposited by PCVI varied with temperature, gas concentration and holding time. At 1373 K, a large amount of SiC deposition into the matrix was observed, and the deposition rate was at least twice higher than for the CH{sub 3}SiCl{sub 3}-H{sub 2} gas system in our previous work. The flexural strength remarkably increased after the PCVI step. (orig.) 6 refs.

Preparation of gas-permeable SiC shape by pressure-pulsed chemical vapour infiltration into carbonized cotton-cloth preforms

Carbonized cotton-fibre preforms were partially infiltrated with SiC by pressure-pulsed chemical vapour infiltration (CVI) from SiCl4–CH4–H2. Fibrous carbon/SiC of 10 mm dia. and 20 mm long having the porosity of 83% was obtained after 15 000 pulses at 1150°C. Pores of the sample after 10 000 pulses distributed below 100 μm (average pore size, 30 μm). Pressure drop of the sample after 15 000 pulses under the axial air-flow was 11 kPa at a face velocity of 1.1 m s-1. On the samples after 15 000 pulses, 15% of preform-carbon were lost within 5 h by air oxidation at 1000°C, and tensile strength along the axis reached 10 MPa, which was close to that before oxidation. In the case of the sample after 5000 pulses, tensile strength lowered significantly after oxidation, however, the strength was recovered by application of 4000 pulses of second CVI. © 1998 Chapman & Hall