Microfibrous entrapment of small catalyst particulates for high contacting efficiency removal of trace CO from practical reformates for PEM H2-O2 fuel cells

Preferential oxidation (PROX) of CO in H₂ is the most efficient way to remove CO from a practical reformate stream for PEM H₂-O₂ fuel cells. Pt/Al₂O₃ has long been known as a suitable catalyst for this purpose. Over the conventional Pt/Al₂O₃ catalyst, however, PROX of CO in H₂ has been known to occur at temperatures above 150°C, and the maximum CO conversion usually takes place at about 200°C. In this study, the promotion of Pt/Al₂O₃ with a transition metal results in significantly enhanced catalytic performance in the temperature range of 25 to 150°C. The active reaction temperature window is enlarged to 25 to 200°C compared with a narrow window at about 200°C over the conventional Pt/Al₂O₃. A high void and a tailorable sintered microfibrous carrier consisting of 5 vol.% of 4 and 8 μm diameter Ni fibers is used to entrap 15 vol.% 150 to 250 μm diameter Al₂O₃ particulates. The alumina support particulates are uniformly entrapped into a sinter-locked, three-dimensional network of 4 and 8 μm Ni fibers. Promoter and Pt are then dispersed onto the microfibrous entrapped alumina support particles by the incipient-wetness impregnation method. The composite catalysts possess 80 vol.% voidage. At equivalent bed volumes, microfibrous entrapped catalysts achieve complete CO reduction (GC detection limit ∼40 ppm CO) at five times the higher gas hourly space velocity value compared with packed beds of 1 to 2 mm catalyst particles demonstrating ultrahigh contacting efficiency provided by the microfibrous entrapped catalysts. This paper was presented at the ASM Materials Solutions Conference & Show held October 18–21, 2004 in Columbus, OH.     

Performance of an iron aluminide metal filter in a reducing environment

A laboratory-scale apparatus has been used for unattended, long duration, continuous, flow-through testing of an iron aluminide sintered metal filter under reducing conditions. Two candle specimens were exposed for 1009 to 2251 h to 600 °C gas containing 5% CO, 11% H₂, 12% CO₂, 12% H₂O, 60% N₂, 0.5% H₂S, 2.4 ppmv NaCl, 4.7 ppmv KCl, 26.1 ppmv HCl, and 5530 ppmw ash from a transport reactor operated in gasification mode. A database was established on pressure drop of the as-received and exposed filter as a function of face velocity and temperature. Tests were conducted to investigate the effects of back pulse parameters on filter regenerability. Results are reported on the critical reservoir pressure and pulse duration for maintaining a stable saw-tooth profile of pressure drop across the filter element. Data are obtained to characterize the effect of chemical and thermal aging on tensile strength, fast fracture strength, and microstructure.     

High-temperature sulfidation behavior of iron-based alloys in hydrogen sulfide-hydrogen gas mixtures

Sulfidation of Fe-Cr binary alloys, Fe-Cr-Al ternary alloys, and commercial stainless steels has been carried out at 540°C in a H₂S-H₂ gas mixture of 0.7 vol.% H₂S and a total pressure of 1 atm. Sample exposure time was from 3 to 5 days. Gravimetric, metallographic, electron microprobe, and x-ray studies were made on a few selected alloys. Sulfidation rates of Fe-Cr-Al alloys were generally slower than corresponding Fe-Cr binary alloys and in many cases were orders of magnitude slower. Scales were extensively dual-layered and showed Al and Cr enrichment at the alloy-scale interface. Tests on the commercial stainless steels confirmed the results obtained with the laboratoryprepared Fe-Cr-Al alloys.Supported by The American Petroleum Institute.     

High transmittance and superhydrophilicity of porous TiO2/SiO2 bi-layer films without UV irradiation

In order to obtain the TiO₂ films with high transmittance and superhydrophilicity without UV irradiation, porous TiO₂/SiO₂ bi-layer films were prepared by spin coated SiO₂ sol and TiO₂ sol including polyethylene glycol 2000 (PEG 2000) onto glass and subsequent calcination at 550 °C. Meanwhile, factors that affect the TiO₂/SiO₂ bi-layer films transmittance and superhydrophilicity were investigated in details by observing their surface morphologies and measuring their water contact angles (WCAs), spreading time and transmittances. The results indicated that the as-prepared TiO₂/SiO₂ bi-layer film showed superhydrophilicity without UV irradiation when 0.5 wt.% PEG 2000 was added in TiO₂ sol. At the same time, its maximum transmittance was as high as 92.3%. The spreading time was only about 0.16 s. More importantly, the resultant film had an excellent stability of the superhydrophilic property.     

The oxidation of cobalt based alloys containing partially dissolved particles of Si3N4 at 1000°C

The isothermal oxidation behavior of Co-Cr and Co-Cr-Si alloys with and without 5, 10, and 15 vol.% dispersions of unstable Si₃N₄ particles was studied in 1 atm of oxygen at 1000°C. The dispersion of Si₃N₄ which dissolved partially in the matrix, greatly reduced the oxidation rate of Co-Cr alloys. Silicon nitride was found to promote the formation of continuous Cr₂O₃ layers at low chromium concentrations. Furthermore, the unstable Si₃N₄ was more effective in reducing the oxidation rate than an alloy containing an equivalent amount of silicon. Additions of 15vol.% Si₃N₄ tended to increase the oxidation rate by forming nonprotective SiO₂ particles which disrupted the protective Cr₂O₃ scale. The mechanism of oxidation was altered due to the Si₃N₄ additions. Marker experiments indicated oxygen diffusion inward via the CoO lattice, rather than a combination of both oxygen and metal ion motion which is encountered in Co-Cr alloys.     

ZrB2-玻璃陶瓷复合材料氧化分析

对ZrB₂-玻璃陶瓷复合材料氧化行为进行热力学分析,对氧化形成的氧化层进行物相分析和显微结构分析。结果表明：在1000°C-1400°C的反应温度范围内,ZrB₂氧化生成ZrO₂,B₂O₃玻璃相,氧化产物ZrO₂与SiO₂反应生成ZrSiO₄,当温度低于1177°C(1450K)时,氧化层主要包括ZrO₂,B₂O₃玻璃相,ZrSiO₄。当氧化温度超过1177°C(1450K)时,B₂O₃玻璃相蒸发,此时SiO₂玻璃相具有良好的流动性,氧化层主要包括ZrO₂,SiO₂玻璃相,ZrSiO₄。氧化过程中的反应产物B₂O₃玻璃相,ZrSiO₄和流动性良好的SiO₂玻璃相,均对氧气向基体的扩散均起到了良好的阻碍作用。     

The preparation and gas sensitivity study of polypyrrole/zinc oxide

Polypyrrole (PPy) was prepared by chemical oxidation polymerization, analyzed by FT-IR, elemental analysis and HRTEM, and studied for gas sensitivity. It suggested that PPy had sensitivity to NH₃, H₂S and NO_x, and showed irreversibility to NO_x gas. The organic–inorganic hybrid materials PPy/ZnO with different PPy weight percents were prepared by mechanical mixing, and the sensitivity study of the materials to toxic gases NH₃, H₂S, NO_x was carried out at different operating temperatures 30, 60, and 90 °C. It was found that PPy/ZnO materials (PPy(1%)/ZnO, PPy(3%)/ZnO, PPy(5%)/ZnO, PPy(10%)/ZnO, PPy(20%)/ZnO) had better selectivity and reversibility to NO_x than pure PPy, and much lower working temperature than the reported working temperature of ZnO (about 350–450 °C). Their sensitivity increased with the increasing concentration of NO_x at particular working temperature, and among them PPy(10%)/ZnO had the maximum sensitivity to NO_x in the same condition. They showed no response to 1000, 1500, 2000 ppm NH₃ or H₂S. The response mechanism of PPy/ZnO materials to NO_x was discussed.     

Corrosion of Three Commercial Steels Under ZnCl2 –KCl Deposits in a Reducing Atmosphere Containing HCl and H2S at 400–500°C

The corrosion of three commercial steels in a reducing atmosphere containing HCl plus H₂S in the presence of ZnCl₂–KCl deposits has been investigated at 400–500°C and compared with the corrosion of the same materials in a similar gas mixture free from H₂S. The presence of H₂S in the gas accelerated the corrosion of the three commercial steels beneath ZnCl₂–KCl deposits. All materials suffered from severe corrosion with partial detachment and spalling of the external scales. Degradation of the steels resulted from the penetration of chlorine-containing species through the scale formed initially down to the metal matrix, because chorine-rich species were detected close to the alloy/scale interface. Although the corrosion resistance generally increased with increasing Cr content, even the high-Cr stainless steel SS304 is still unable to provide good corrosion resistance against the ZnCl₂–KCl deposits in the presence of H₂S due to the bad adherence of the scales to the alloy. The mechanisms of attack are discussed on the basis of thermodynamic considerations and of the active-oxidation model.     

Microstructure of a plasma-sprayed Mo-Si-B alloy

Powders of Mo₅₂Si₃₈B₁₀ were plasma sprayed under inert conditions onto stainless steel substrates to determine if high density free standing forms could be synthesized by this process. Thermal spray conditions were varied to minimize porosity and oxygen impurities while minimizing evaporative metal losses. The assprayed and sintered microstructures were characterized using scanning and transmission electron microscopy and quantitative x-ray diffraction (XRD). The as-sprayed microstructure consisted of elongated splats tens of microns in length and only one to three microns in thickness. The splats contained submicrometer grains of primarily MoB and Mo₅Si₃B _x (T1) and minor amounts of MoSi₂ and a glassy grain boundary phase. The interior of the splats typically consisted of a fine eutectic of MoB and T1. Small pieces were cut out of the cross section of the sample and pressureless sintered for 2, 6, and 10 h at 1800 °C in flowing Ar. After sintering for 2 h at 1800 °C, the samples exhibited a coarser but equiaxed microstructure (1 to 5 μm grain size) containing 78 vol.% T1, 16 vol.% MoB, and 6 vol.% MoSi₂ as determined by XRD. Approximately 8 at.% of the Si formed silica. The high-temperature anneal removed all vestiges of the layered structure observed in the as-sprayed samples.     

The effect of preoxidation on the sulfidation of Ni-20Cr (2–5)Al Alloys

Ni-20Cr alloys with 2, 3.5, and 5 wt.% Al have been preoxidized up to 100 hr at 1000°C in dry H₂, in H₂/23% H₂O and in air and subsequently exposed to an H₂/5% H₂S atmosphere at 750° C. During the preoxidation treatment different types of oxide scales were formed which affect the sulfidation protection in different ways. Optimum results were obtained for alloys with 3.5 and 5 wt.% Al after 20 hr exposure to dry H₂ at 1000°C. A thin Al₂O₃ scale is formed which decreases the sulfur attack by more than one order of magnitude. Preoxidation conditions for Ni-20Cr-2Al alloys in H₂ and for Ni-20Cr-2Al and Ni-20Cr-3.5Al in H₂/H₂O were observed to be less effective. No improvement was found for preoxidation in air or for Ni-20Cr-5Al alloys preoxidized in H₂/H₂O.     

High field superconductivity in (sio2)100-xPbx sputtered films

Sputter-quenched SiO₂ films containing Pb, which is insoluble to SiO₂, have been found to exhibit a remarkably enhanced upper critical field (H_c2), which is about 90 times higher than that of pure Pb The sputtered films consisted of an amorphous SiO₂ matrix and fcc Pb particles, dispersed homogeneously in the amorphous matrix. The particle sizes and interparticle distances were about 5 to 350 nm and 5 to 50 nm, respectively The superconducting transition temperature (T_c), upper critical magnetic field (H_c2) at 4.2 K, and residual resistivity (p₁₀) at 10 K were 7.05 K, 4.37 T and 1.4x10² μΩm for (SiO₂)₂₉Pb₇₁(vol %), respectively Since thel _eff evaluated from linearized Ginzburg-Landau equation was 0.524 nm for x=71%, while theof pure Pb at 4.2 K was valued at 2.3xl0⁵ nm, it can be concluded that thel _eff of these SiO₂-Pb films was drastically reduced owing to the refinement of Pb particles and internal defects (stress) induced by vapor quenching. Therefore the high field superconductivity can be interpreted mainly due to the significant decrease of coherence length, resulting from the large reduction of the effective mean free path length (l _eff) of electrons The dual phase microstructure also had influence on the increase of the fluxoid pinning force and behaved like a type-II superconductor     

Influence of Preoxidation on High-Temperature Corrosion of a FeCrAl Alloy Under Conditions Relevant to Biomass Firing

Preoxidation of a commercial FeCrAl alloy (Kanthal APM) was evaluated as a surface modification approach to reduce alkali chloride-induced corrosion during biomass firing in power plants. Samples of the alloy preoxidized at 900 °C in O₂ or O₂ + 10 vol% H₂O, and at 1100 °C in O₂, were coated with KCl and exposed at 560 °C to a gas mixture comprising of 12 vol% CO₂, 6 vol% O₂, 3 vol% H₂O, 400 ppmv HCl and 60 ppmv SO₂. The oxide formed at 1100 °C showed no reactivity with the corrosive species. By contrast, all samples preoxidized at 900 °C suffered severe attack, resulting in formation of Fe-, Cr- and Al-containing corrosion products in a heterogeneous morphology, similar to non-preoxidized samples. The observed differences with respect to the degree of corrosion attack on the preoxidized samples are discussed in terms of the composition and thickness of the different types of Al₂O₃ layers obtained by the preoxidation treatment.     

Controlled crystallite orientation in ZnO nanorods prepared by chemical bath deposition: Effect of H2O2

ZnO nanorods with controlled crystallite orientation are grown on glass substrate by chemical bath deposition (CBD) method via hydrogen peroxide (H₂O₂) route. The crystallite orientation in the film is successfully controlled by varying content of H₂O₂ in the bath solution. The crystallites became increasingly oriented as content of H₂O₂ in the bath solution increased, resulting in the formation of vertically aligned ZnO nanorods. The possible growth mechanism for the vertically aligned ZnO nanorods is proposed. The influence of content of H₂O₂ in the bath solution on structural, surface morphological, electrical and optical properties is studied and reported.     

Structural and optical properties of electrodeposited ZnO thin films on conductive RuO2 oxides

ZnO thin films were grown on the 150 nm-thick RuO₂-coated SiO₂/Si substrates by electrochemical deposition in zinc nitrate aqueous solution with various electrolyte concentrations and deposition currents. Crystal orientation and surface structure of the electrodeposited ZnO thin films were characterized by X-ray diffraction (XRD) and scanning electron microscopy, respectively. The XRD results show the as-electrodeposited ZnO thin films on the RuO₂/SiO₂/Si substrates have mixed crystallographic orientations. The higher electrolyte concentration results in the ZnO thin films with a higher degree of c-axis orientation. Moreover, the use of an ultra-thin 5 nm-thick ZnO buffer layer on the RuO₂/SiO₂/Si substrate markedly improves the degree of preferential c-axis orientation of the electrodeposited ZnO crystalline. The subsequent annealing in vacuum at a low temperature of 300 °C reduces the possible hydrate species in the electrodeposited films. The electrodeposited ZnO thin films on the 5 nm-thick ZnO buffered RuO₂/SiO₂/Si substrates grown in 0.02 M electrolyte at −1.5 mA with a subsequent annealing in vacuum at 300 °C had the best structural and optical properties. The UV to visible emission intensity ratio of the film can reach 7.62.     

SiO_2含量对激光熔覆含La_2O_3生物陶瓷涂层性能的影响

采用激光熔覆技术,利用CaHPO_4·2H_2O,CaCO_3,La_2O_3,SiO_2粉末和Ti粉在医用钛合金Ti-6Al-4V表面制备掺杂的质量分数分别为0%,5%,10%,15%SiO2的含La2O3梯度生物陶瓷涂层,并研究了SiO_2含量对涂层性能的影响。利用金相显微镜、扫描电镜、能谱仪、X射线分析仪分别对涂层进行显微结构、表面元素、物相组成分析,利用MTT细胞活性检测法对涂层上种植的小鼠破骨前体细胞(RAW264.7)进行细胞活性分析。结果显示:掺杂10%~15%的SiO_2后,涂层的裂纹明显减少,且厚度变均匀。涂层在模拟体液(SBF)中浸泡2周后,羟基磷灰石(HA)在掺杂10%SiO_2涂层表面的沉积量最多,涂层表面的金属元素含量稀少,这表明含10%SiO_2涂层在SBF中具备更加优良的沉积HA的能力,同时有效的阻碍了基体中重金属离子的析出。MTT结果表明,掺杂SiO_2的涂层对破骨前体细胞的活性有更加明显的抑制作用。     

Effect of Water Vapor on the Oxidation Behavior of Fe–1.5Si in Air at 1073 and 1273 K

The oxidation behavior of Fe–1.5Si was investigated at 1073 and 1273 K in air, air–H₂O, Ar–H₂O, O₂–H₂O, and O₂ atmospheres. The extent of corrosion in atmospheres containing H₂O increased rapidly after an incubation period of slow oxidation, the incubation period becoming shorter in the order, O₂–H₂O, air–H₂O, and Ar–H₂O. With increasing H₂O contents in air–H₂O, the incubation time decreased. During the incubation period, oxidation was slow, because of the formation of an inner Si-rich oxide layer and a Pt marker was located between the external Fe₂O₃ (Fe₃O₄ included) and an inner Si-rich oxide layer. During the rapid oxidation, the inner FeO+Fe₂SiO₄ layer thickened and a Pt maker was at the interface between an external Fe-oxide and an inner FeO+Fe₂SiO₄ layer. Observations of scale cross sections indicated that voids made channels along the boundaries of columnar FeO crystals, suggesting transport of water molecules. The Si-rich oxide layer changed into an FeO+Fe₂SiO₄ mixture due to penetration of water molecules. A combined process of perforating dissociation and transport of water molecules is suggested to be the cause of the rapid growth of the inner FeO+Fe₂SiO₄ layer.     

Oxidation of molten Al-Mg alloy in air,air-SO2, and air-H2S atmospheres

Rates of oxidation of 8 g samples of molten Al-Mg alloys in air, air-SO₂, and air-H₂S atmospheres were determined at 750°C by gravimetry. Weight gains in air containing 10 or more volume percent SO₂ averaged 0.2 % in up to 100 hr, whereas samples heated in air alone gained over 5% weight in 20 hr and 10% in 70 hr. Heating for only 1 hr in SO₂ concentrations of 10% or greater prevented extensive oxidation during additional heating in air alone. H₂S was more effective than SO₂ heating for as little as 1/2hr in 0.25 vol. % H₂S prevented extensive oxidation during subsequent heating for at least 90 hr in air alone. The inhibiting effect of either SO₂ or H₂S probably involves oxidation to SO₃. This reacts with portions of the initial protective amorphous MgO film to form MgSO₄, which has a high volume quotient and maintains protection as nonprotective crystalline MgO forms at the end of an induction period.     

Low temperature sintering and properties of CaO–B2O3–SiO2 system glass ceramics for LTCC applications

The P₂O₅ + ZnO, ZrO₂ + TiO₂, B₂O₃ and a low-melting-point CaO–B₂O₃–SiO₂ glass (LG) are selected as the sintering additives, and the effect of their additions on the microwave dielectric properties, mechanical properties and microstructures of CaO–B₂O₃–SiO₂ system glass ceramics is investigated. It is found that the sintering temperature of pure CBS glass is higher than 950 °C and the sintering range is about 10 °C. With the above additions, the glass ceramics can be sintered between 820 °C and 900 °C. The dielectric properties of the samples are dependent on the additions, densification and microstructures of sintered bodies. The major phases of this material are CaSiO₃, CaB₂O₄ and SiO₂. With 10 wt% B₂O₃ and LG glass additions, the CBS glass ceramics have better mechanical properties, but worse dielectric properties. The _r values of 6.51 and 7.07, the tan δ values of 0.0029 and 0.0019 at 10 GHz, are obtained for the CBS glass ceramics sintered at 860 °C with 2 wt% P₂O₅ + 2 wt% ZnO and 2 wt% ZrO₂ + 2 wt% TiO₂ additions, respectively. This material is suitable to be used as the LTCC material for the application in wireless communications.     

Interfacial reaction in ASZ/A384 Al composite and its influence on mechanical properties

In an ASZ/A384 Al composite, the interfacial reaction was observed to take place between the SiO₂ binder layer and Mg within the matrix to form MgAl₂O₄ at the interface. Formation of MgAl₂O₄ at the interface between ASZ short fibers and the Al matrix alloy is believed to enhance the interfacial bonding strength, resulting in improved composite strength. However, the interfacial reaction in the ASZ/A384 Al proceeds at the expense of Mg in the matrix, resulting in a composite devoid of Mg bearing precipitates such as Al₂CuMg and Mg₂Si.     

Simultaneous synthesis and densification of MoSi2-20vol.%Nb composite and MoSi2-20vol.%ZrO2 composite by field-activated and pressure-assisted combustion

A method to simultaneously synthesize and consolidate MoSi₂-20vol.%Nb composite and MoSi₂-20vol.% ZrO₂ composite from powders of Mo, Si, Nb and ZrO₂ was investigated. Combustion synthesis was carded out under combined electric field and mechanical pressure. Highly dense MoSi₂-20vol.%Nb and MoSi₂-20vol.%ZrO₂ composites up to 96% of theoretical density were produced from powders of Mo, Si, Nb and ZrO₂ with 60MPa of pressure and 3000A of current on the reactant.