Transmission electron microscopy study of the microstructure of carbon/carbon composites reinforced with in situ grown carbon nanofibers

Introduction of carbon nanofibers (CNFs) into carbon/carbon (C/C) composites is an effective method to improve the mechanical properties of C/C composites. In situ grown CNFs reinforced C/C composites as well as conventional C/C composites without CNFs were fabricated by chemical vapor infiltration. Transmission electron microscopy investigations indicate that the entangled CNFs (30–120 nm) formed interlocking networks on the surface of carbon fibers (CFs). Moreover, a thin high-textured (HT) pyrocarbon (PyC) layer (～20 nm) was deposited on the surface of CFs during the growth of CNFs. We find the microstructure of C/C composites depends strongly on the local distribution density (LDD) of CNFs. In regions of low CNF LDD, a triple-layer structure was formed. The inner layer (attached to CF) is HT PyC (～20 nm), the middle layer (150–200 nm) is composed of HT PyC coated CNFs (HT/CNFs) and medium-textured PyC, and the outmost layer (several microns) is composed of HT/CNFs and micropores. In regions of high CNF LDD, a double-layer structure was formed. The inner layer is HT PyC (～20 nm), and the outer layer is composed of HT/CNFs, isotropic PyC and nanopores. However, only medium-textured PyC and micropores were found in the matrix of the conventional C/C composites.     

Electro-oxidation of dimethyl ether on Pt/C and PtMe/C catalysts in sulfuric acid

The electro-oxidation of dimethyl ether (DME) on PtMe/Cs (Me = Ru, Sn, Mo, Cr, Ni, Co, and W) and Pt/C electro-catalysts were investigated in an aqueous half-cell, and compared to the methanol oxidation. The addition of a second metal enhanced the tolerance of Pt to the poisonous species during the DME oxidation reaction (DOR). The PtRu/C electro-catalyst showed the best electro-catalytic activity and the highest tolerance to the poisonous species in the low over-potential range (<0.55 V, 50 °C) among the binary electro-catalysts and the Pt/C, but at the higher potential (>ca. 0.55 V, 50 °C), the Pt/C behaved better than PtRu/C. The apparent activation energy for the DOR decreased in the order: PtRu/C (57 kJ mol⁻¹) > Pt₃Sn/C (48 kJ mol⁻¹) ≈ Pt/C (46 kJ mol⁻¹). On the other hand, the activation energy for the MOR showed a different turn, decreased in the following order: Pt/C (43 kJ mol⁻¹) > Pt₃Sn/C (35 kJ mol⁻¹) ≈ PtRu/C (34 kJ mol⁻¹). The temperature dependence of the DOR was greater than that of the oxidation of methanol (MOR) on the PtRu/C.     

Carbon nanotube reinforced pyrocarbon matrix composites with high coefficient of thermal expansion for self-adapting ultra-high-temperature ceramic coatings

The mismatch in the coefficients of thermal expansion (CTE) of the carbon fiber reinforced pyrocarbon (Cf/C) composites and their thermal barrier coatings (TBCs) has significantly restricted the service life of Cf/C composites in high-temperature environments. Owing to the high CTE of TBCs, it is vital to find a material with similar mechanical properties and higher CTE than Cf/C composites. In this work, carbon nanotube reinforced pyrocarbon (Ct/C) nanocomposites with high CTEs were prepared to self-adapt to the TBCs. Different CTEs (～4.0–6.5 × 10^?6/°C) were obtained by varying the carbon nanotube (CNT) content of the Ct/C composites. Owing to the decreased mismatch in the CTEs, no cracks were formed in the TBCs (SiC and HfB₂-SiC-HfC coatings) deposited on the Ct/C composites. After heat treatment at 2100 °C, several wide cracks were found in the TBCs on the Cf/C composite, whereas the TBCs on the Ct/C composites were intact without cracks. We found that the CTE-tunable Ct/C composites can self-adapt to different TBCs, protecting the composites from oxidation at high temperatures.     

Comparison of different promotion effect of PtRu/C and PtSn/C electrocatalysts for ethanol electro-oxidation

Well dispersed PtSn/C, PtRu/C and Pt/C electrocatalysts were synthesized by a modified polyol process and characterized by X-ray diffraction (XRD), transmission electron microscope (TEM) and inductively coupled plasma-atomic emission spectrometry techniques. XRD patterns show that Ru induces the contraction of Pt lattice parameter while Sn makes the Pt crystal lattice extended. Ethanol oxidation activities on the catalysts were studied via cyclic voltammetry (CV) and chronoamperometry (CA) methods at room temperature. It is found that the electrode potential plays an important role in the electrochemical behavior of ethanol oxidation on PtRu/C and PtSn/C catalysts. In the lower potential region, PtSn/C possesses higher performance for ethanol oxidation, while in the higher potential region PtRu/C is more active. The different promotion effects of PtSn/C and PtRu/C to ethanol oxidation can be explained by the structural effect and modified bi-functional mechanism in different potential region. Single cell test of a direct ethanol fuel cell (DEFC) was also carried out to elucidate the promotion effect of PtRu/C and PtSn/C catalysts on the ethanol oxidation at 90 °C.     

Synthesis, characterization and application of platinum based bi-metallic catalysts for direct glucose alkaline fuel cell

In the context of development of direct glucose fuel cell (DGFC), low metal loading (ca. 15 wt.%) bi-metallic platinum–bismuth (PtBi/C) and platinum–gold (PtAu/C) catalysts are synthesized by immobilizing metal sols on carbon substrate (Vulcan XC 72R). Physical characterization of electro-catalysts, studied using TEM, SEM, EDX and XRD, reveals the formation of nano-sized metal particles on carbon substrate. The cyclic voltammetry and chronoamperometry of the prepared catalysts point out that PtAu/C is more active and stable than PtBi/C and commercial PtRu/C towards glucose electro-oxidation in alkaline medium. The catalysts are tested as anode in batch DGFC using activated charcoal as cathode in different glucose and electrolyte (KOH solution) concentrations at ambient temperature (30 °C). Open-circuit voltage of ∼0.9 V is obtained for PtAu/C and commercial PtRu/C and 0.8 V for PtBi/C anode in 0.2 M glucose and in 1 M KOH. However, the peak power density per unit metal loading or specific peak power density obtained is 1.6 mW cm⁻² mg⁻¹ for PtAu/C followed by PtBi/C (1.25 mW cm⁻² mg⁻¹) and commercial PtRu/C (1.13 mW cm⁻² mg⁻¹). For PtBi/C and PtRu/C, the cell performance increases up to 0.2 M glucose concentration and then decreases. However, for PtAu/C catalyst the cell performance increases up to 0.3 M glucose concentration and then decreases. A prominent transition zone is observed in which current density sharply decreases with the decrease in voltage (increase in overpotential) for PtBi/C and PtRu/C at 0.3 M glucose concentration, which is not observed in the case of PtAu/C. The transition zone for PtAu/C is insignificant and at higher glucose concentration (0.4 M) pointing out that PtAu/C is much stable catalyst than PtBi/C and commercial PtRu/C.     

Effects of iron-tetrasulfophthalocyanine on the catalytic activities of Pt/C,PtRu/C,and Pd/C catalysts in a multi-anode direct formic acid fuel cell

This paper reports a systematic study of the effects of a promoter, iron-tetrasulfophthalocyanine (FeTSPc), on the catalytic activities of carbon supported Pt, PtRu, and Pd catalysts (Pt/C, PtRu/C, and Pd/C) for formic acid oxidation. A multi-anode direct formic acid fuel cell (DFAFC) was used to compare the effects on each catalyst of adding FeTSPc to the fuel stream. The FeTSPc significantly enhanced the activity of the Pt/C catalyst, but had little effect on the PtRu/C catalyst. The activity of the Pd/C catalyst was inhibited by the FeTSPc. A FeTSPc modified Pt/C was also evaluated in a conventional 5 cm² DFAFC.     

Effect of preparation method on the mechanism for oxidation of C/C-BN composites

C/C-BN composites and C_f/BN/PyC composites exhibiting different structures for pyrolytic carbon (PyC) and boron nitride (BN) were studied comparatively to determine their oxidation behavior. This study used five types of samples. Porous C/C composites were modified with silane coupling agents (APS) and then fully impregnated in water-based slurry of hexagonal boron nitride (h-BN); the resulting C/C-BN preforms were densified by depositing PyC by chemical vapor infiltration (CVI), resulting in three types of C/C-BN composites. The other two C_f/BN/PyC composites were obtained by depositing a BN interphase and PyC in carbon fiber preforms by CVI; one was treated with heat, and the other was not. This study was focused on determining how the PyC deposition mechanism, morphology and pore structure were affected by the method of BN introduction. In the 600–900 °C temperature range, the C_f/BN/PyC composites and C/C composites underwent oxidation via a mixed diffusion/reaction mode. The C/C-BN composites had a different pore structure due to the formation of nodules comprising h-BN particles; both interfacial debonding and cracking were reduced, resulting in higher resistance to gas diffusion, lower oxidation rate and larger activation energy (Ea) in the temperature range 600–800 °C. In addition, the mechanism for oxidation of C/C-BN composites gradually exhibited diffusion control at 800–900 °C because the formation of h-BN oxidation products healed the defects. The oxidation mechanism was more dependent on pore structure than on BN structure or content.     

Polyoxometalate impregnated carbon systems for the in situ degradation of sulphur mustard

Kinetics of in situ degradation bis-(2-chloroethyl) sulphide (sulphur mustard, HD) on polyoxometalate impregnated carbon systems such as 11-molybdo-1-vanadophosphoric acid (V₁/C), phosphotungstic acid (PTA/C), sodium phosphotungstic acid (PTANa/C), phosphomolybdic acid (PMoA/C), sodium phosphomolybdic acid (PMoANa/C) and silicotungstic acid (SiTA/C) have been studied. These carbons were characterized for micropore volume and surface area by N₂ Brunauer, Emmett and Teller (BET) equation. For degradation studies the solution of HD in chloroform was prepared and taken for the uniform adsorption on the carbon systems using incipient volume. Degradation kinetics was monitored by gas chromatograph equipped with flame ionization detector (GC/FID) and found to be following the pseudo first order kinetics. The values of kinetic rate constant and half-life were calculated. V₁/C system showed the fastest degradation of HD. Hemimustard, thiodiglycol, 1,4-oxathiane, sulphoxide and vinyl-2-chloroethyl sulphide were found to be the degradation products with V₁/C system which indicated the oxidative, hydrolytic and dehydrohalogenation reactions, responsible for HD degradation. Effect of moisture was also studied on most reactive system, i.e., V₁/C. The study indicated that V₁/C can be used as a promising adsorbent system for the degradation of HD.     

Synthesis of Lead Barium Niobate Powders and Thin Films by the Sol-Gel Method

Crack-free, dense, and transparent Pb_0.6Ba_0.4Nb₂O₆ (PBN60) thin films have been prepared by a sol-gel method with metal alkoxides and metal acetate. A homogeneous and stable precursor solution was obtained from Ba metal, Pb(CH₃COO)₂, and Nb(OEt)₅ in 2-methoxyethanol. PBN60 powder crystallized to the hexagonal phase at 600°C and then completely transformed to the orthorhombic phase of the tungsten bronze structure at 1250°C. The hexagonal phase was formed on SiO₂ glass, MgO(lOO), and sapphire(R) substrate at 600°C, while the orthorhombic phase was only on a sapphire(C) substrate. Orthorhombic PBN60 films with c -axis preferred orientation were successfully synthesized on sapphire(C) substrates at 600°C.     

Ablation behavior of thin-blade co-deposited C/Cx-SiCy composites under the influence of the complex fluid conditions of the oxyacetylene torch

This paper offers a new way of testing the ablation property of material under an oxyacetylene torch using a thin-blade specimen, which costs much less time to reach the maximum temperature and provides a harsh turbulence fluid field that's closer to reality. The thin-blade specimen experiences a higher turbulent intensity than the traditional disk-like specimen, leading to more efficient heat exchange. The fluid field simulation agrees with the testing results. In addition, we manage to synthesize the C/Cx-SiCy composites with the co-deposition chemical vapor infiltration (CVI) method. The C/Cx-SiCy composites exhibit a similar anti-ablation property as C/C composites and consist of enough SiC phase simultaneously, combining the advantages of both C/C composites and C/SiC composites. The thin-blade C/Cx-SiCy composites show a lower linear ablation rate (1.6 μm/s) than C/C composites (4.1 μm/s) and C/SiC composites (19.6 μm/s) during the oxyacetylene test. The glass layer formed on the surface of C/Cx-SiCy could cling to the bulk material instead of peeling off due to the high PyC content in the matrix could protect the SiO₂ from blowing away.     

The influence of zeolite acidity for the coupled hydrogenation and ring opening of 1-methylnaphthalene on Pt/USY catalysts

The influence of framework and extraframework composition of USY zeolite on the catalytic performance of bifunctional Pt/USY (1 wt.% Pt) catalysts for the coupled hydrogenation and ring opening of 1-methylnaphthalene (1-MN) has been studied on a continuous fixed bed high pressure reactor. All Pt/USY catalysts showed very high methylnaphthalene (MN) conversions under the reaction conditions studied (T=300–375 °C, P=4.0 MPa, WHSV=2 h⁻¹, H₂/1-MN=30 mol/mol). Product yields and selectivities were mainly determined by the zeolite composition (i.e. acidity). Selectivity to products with the same number of carbon atoms than the feed (C11) increased, at constant temperature, with decreasing the Brönsted acidity of the USY zeolite, that is, with decreasing the concentration of framework Al (FAL) and increasing extraframework Al (EFAL). Selectivity to high cetane ring opening products (ROP=C11-alkylbenzenes (C11AB) and C11-alkylcycloalkanes) within the C11 fraction was higher for the less acidic catalysts. A maximum yield of ROP of ca. 15 wt.% at a C11 yield of ca. 73 wt.% was obtained at 350 °C (P=4.0 MPa, WHSV=2 h⁻¹, H₂/1-MN=30 mol/mol) for a USY zeolite with an intermediate degree of dealumination (a₀=24.33 Å) and containing all the EFAL (bulk Si/Al ratio of 2.6). For this catalyst, a slight increase in ROP yield (ca. 17 wt.%) at similar C11 yield (ca. 74 wt.%) was obtained by working at lower temperature (300 °C) and lower space velocity. Increasing the reaction pressure above 4.0 MPa had only a marginal influence on product yields and selectivities.     

Preparation of Strontium Barium Niobate by Sol–Gel Method

Crack-free SBN (Sr_xBa_1-xNb₂O₆) thin films have been prepared by a sol–gel method with metal alkoxides. A homogeneous and stable precursor solution was obtained from Sr and Ba metal and Nb(OEt)₅ in ethanol with a key additive of ethoxyethanol. SBN (where x = 0.5) powder crystallized to orthorhombic phase at 700°C, and then transformed completely to tetragonal phase at 1200°C. The formation of tetragonal SBN was observed on sapphire (R) substrates at 700°C, whereas the tetragonal phase began to appear in the powders at 1000°C. SBN films with highly preferred orientation were successfully synthesized on MgO (100) substrates at 670°C.     

Decomposition of NO on Tungsten Carbide and Molybdenum Carbide Surfaces

The decomposition of ¹⁵NO on C/W(111), C/W(110), and on monolayer and bulk C/Mo/W(111) surfaces is compared based on temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES) measurements. Our results indicate that the decomposition of ¹⁵NO occurs readily over all surfaces, and the only ¹⁵N-containing reaction products are ¹⁵N₂ and ¹⁵N₂O under our experimental conditions. Much higher surface reactivity for ¹⁵NO decomposition was observed over the more open-structured C/W(111) surface, with a value of 0.68 ¹⁵NO/W, in contrast to the surface reactivity of 0.24 ¹⁵NO/W over the close-packed C/W(110) surface. The selectivity of these two ¹⁵N-containing reaction products depends on the structure of the substrates as well. The more open-structured C/W(111) surface favors the production of ¹⁵N₂, with a product selectivity of ¹⁵N₂ being approximately 87%. In contrast, the selectivity to ¹⁵N₂ is only about 52% on C/W(110). In addition, we have investigated the decomposition of ¹⁵NO on C/Mo surfaces that were epitaxially grown on W(111). The selectivity of ¹⁵N₂ on C/Mo/W(111) surfaces is 88%, which is very similar to that observed on C/W(111). Finally, the general similarity between the DeNOx chemistry on carbides and on Pt-group metals will also be discussed.     

Adhesion control of interface between cellulose and polypropylene by vapor‐phase assisted surface copolymerization

In order to achieve the effective interface bonding between biomass microfiller and commodity plastics, consecutive copolymerization of hydrophilic acrylic acid (AA) and hydrophobic butyl acrylate (BA) using vapor‐phase assisted surface polymerization (VASP) technology was applied to prepare microcomposites consisting of cellulose microcrystal (CμC) and polypropylene (PP). After the copolymerization by VASP, CμC surfaces were covered by accumulated polymers: P(AA‐co‐BA) including block‐type copolymer and homopolymers of 6.2–25.3 wt % versus CμC. Although structures of the products were unspecified, it was expected to be mixtures of block copolymers and homopolymers. Subsequently prepared P(AA‐co‐BA) on CμC/PP (5/95 wt/wt) composites expressed a superior mechanical toughness, which had increased threefold when compared to intact CμC/PP composite. This increase in toughness was mainly based on an increase in elongation rate, reflecting improvement of the adhesion strength at the interface between CμC surface and PP. The trace amounts: 0.31 wt % of accumulated P(AA‐co‐BA) on CμC surface must function as an effective adhesive/compatibilizer at the interface. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45647.     

High Temperature Microhardness of Single Crystal Mullite

The microhardness ( H ) of single crystal, orthorhombic mullite was measured on (010) and (001) faces from room temperature up to 1400°C. The microhardness versus temperature curves display sigmoidal shapes. The mean microhardness at room temperature is ∼16 Gpa and it decreases with temperature to H is ∼ 13 GPa at 300°C. At >300°C, the microhardness is only slightly reduced with temperature to H is ∼10 GPa at 1000°C. Above this temperature limit, it markedly decreases again, to a mean value of ≈6 GPa at 1400°C. Up to ∼1000°C the (010) and (001) microhardnesses are very similar. Above ∼1000°C, however, the hardness gradually becomes anisotropic, with H ₍₀₁₀₎ being twice as high as H ₍₀₀₁₎ at 1400°C ( H ₍₀₁₀₎∼ 8 GPa, H ₍₀₀₁₎∼ 4 GPa).     

Fiber/matrix interfacial shear strength of C/C composites with PyC–TaC–PyC and PyC–SiC–TaC–PyC multi-interlayers

The fiber/matrix (F/M) interfacial shear strength (IFSS) of carbon/carbon (C/C) composites with PyC–TaC–PyC and PyC–SiC–TaC–PyC multi-interlayers was investigated. To obtain C/C composites with PyC–TaC–PyC and PyC–SiC–TaC–PyC multi-interlayers, a thin layer of PyC was deposited on carbon fibers. After this, TaC and SiC–TaC layer(s) were uniformly deposited on the PyC coated carbon fibers. As an outer-layer, a PyC layer was deposited on these TaC and/or SiC–TaC coated carbon fibers by isothermal chemical vapour infiltration (CVI) and then densified with resin carbon by impregnation and carbonization. Finally, C/C composites with PyC–TaC–PyC and PyC–SiC–TaC–PyC multi-interlayers were obtained. The effects of PyC–TaC–PyC and PyC–SiC–TaC–PyC multi-interlayers on interfacial shear strength (IFSS) of C/C composites were investigated. Single fiber push-out tests were conducted on the fibers aligned perpendicularly on the thin slices specimen surface using nano-indentation. Results showed that the IFSS of C/C composites decreased with the introduction of PyC–TaC–PyC and PyC–SiC–TaC–PyC multi-interlayers. After heat treatment (at temperatures ranging from 1400 to 2500 °C) of C/C composites with PyC–TaC–PyC multi-interlayers, it was found that the IFSS decreased with the increase in temperature. This decrease in IFSS is explained by taking into account the microstructural variations on heat treatment.     

Structure–protein adsorption relationships of polyurethanes

A series of hydroxyl‐terminated polybutadiene (HTPB) and 4,4′‐dicyclohexylmethane diisocyanate (H₁₂MDI)‐based polyurethanes (PUs) with different molecular weight, hard‐segment content, or 4‐vinyl pyridine content (4‐VP content) were synthesized by solution polymerization. Protein adsorption ratio of fibrinogen to albumin (F/A molar ratio), which was adopted as the indicator of blood compatibility, was measured. The F/A molar ratio on the film's surface was affected by surface composition. The surface composition was quantified by carbonyl group to butadiene group (C=O/C=C) adsorption ratio on FTIR‐ATR spectra and oxygen to carbon atom (O/C) ratio, which was determined by ESCA. PUs with more hard‐segment content on the surface (i.e., high C=O/C=C ratio) possess more fibrinogen adsorption and less albumin deposition (i.e., high F/A molar ratio). The C=O/C=C ratio, hydrogen‐bonding index (HBI value), frequency shift and difference (Δν), glass transition temperature of soft segment (T_gs) as a measure of homogeneity, average strength of interpolymer hydrogen bonds, and interpenetrating networks (IPNs) were utilized to study the surface composition, intermolecular attraction, and IPN formation of the prepared PUs. The effect of hard‐segment content, molecular weight or 4‐VP content on the F/A molar ratio were investigated. The results of FTIR and ESCA explain well the surface composition, and hence, the F/A molar ratio as well. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 297–305, 1999     

Simultaneous enhancement of mechanical and ablation properties of C/C composites modified by (Hf-Ta-Zr)C solid solution ceramics

In order to improve the mechanical and ablative resistance of C/C composites, (Hf-Ta-Zr)C single-phase solid solution ceramics were introduced into C/C composites by polymer infiltration and pyrolysis (PIP) to fabricate (Hf-Ta-Zr)C modified C/C composites (HTZ). Their mechanical property and ablation resistance were studied. The results showed that HTZ achieved simultaneous enhancement of mechanical property and ablative resistance. Their flexural strength and modulus could reach 219.34 MPa and 24.82 GPa, respectively. In addition, the mass and linear ablation rate of HTZ were 0.379 mg/s and 0.667 µm/s, respectively after the 90 s oxyacetylene ablation. A dense Hf-Ta-Zr-O multiphase oxide layer was formed on the surface of the HTZ during ablation process, which protected the interior modified C/C composites from ablation. Our work expands a rational design of modified C/C composites and broaden the application of solid solution ceramic in the field of ultra-high temperature ablation resistance for carbon or ceramic-based composites.     

Synergistic Effect in the Dehydrogenation of Cyclohexene on C/W(111) Surfaces Modified by Submonolayer Coverage Pt

The dehydrogenation and decomposition of cyclohexene on the Pt-modified C/W(111) surfaces have been studied by temperature-programmed desorption (TPD), Auger electron spectroscopy (AES) and high-resolution electron energy loss spectroscopy (HREELS). The objective of the current study is to investigate how the surface reactivity of tungsten carbide is modified by the presence of submonolayer Pt. Similar to that observed on Pt(111), Pt(100) and C/W(111) surfaces, the characteristic reaction pathway on Pt/C/W(111) is the selective dehydrogenation of cyclohexene to benzene. At a Pt coverage of 0.52 monolayer, the selectivity to the gas-phase benzene product is 86±7%, which is slightly higher than that on Pt(111) (75%) and on C/W(111) (67±7%). More importantly, the desorption of benzene on Pt/C/W(111) is a reaction-limited process that occurs at 290 K, which is much lower than the benzene desorption temperature of 400 K from Pt(111).     

Screening of PdM and PtM catalysts in a multi-anode direct formic acid fuel cell

Direct formic acid fuel cells (DFAFC) currently employ either Pt-based or Pd-based anode catalysts for oxidation of formic acid. However, improvements are needed in either the activity of Pt-based catalysts or the stability of Pd-based catalysts. In this study, a number of carbon-supported Pt-based and Pd-based catalysts, were prepared by co-depositing PdM (M = Bi, Mo, or V) on Vulcan^® XC-72 carbon black, or depositing another metal (Pb or Sn) on a Pt/C catalyst. These catalysts were systematically evaluated and compared with commercial Pd/C, PtRu/C, and Pt/C catalysts in a multi-anode DFAFC. The PtPb/C and PtSn/C catalysts were found to show significantly higher activities than the commercial Pt/C catalyst, while the PdBi/C provided higher stability than the commercial Pd/C catalyst.