On the behaviour of minor active elements during oxidation of selected Ni-base high-temperature alloys

We have examined the distribution of active minor elements in the oxide scales developed by selected Ni-base alloys with commercial grades. Emphasis is placed upon Mn, La and Si in a chromia-forming alloy and Y in an alumina-forming alloy. Initially, La and Y have been segregated at free surfaces and then become constituents of the oxides in contact with the substrates. A continuous layer of MnCr₂O₄ is formed above La- and Si-modified inner chromia layer. Silicon has been homogenously distributed throughout the grain structure, however, some La is present as LaCr₂O₃ particles and most of the remainder has been segregated at grain boundaries. The results indicate that the collective effect of Mn, Si and La is to extend protection by chromia to temperatures in excess of 1000 °C. Yttrium in the alumina- forming alloy is found to predominantly segregate at grain boundaries of nanostructured oxide with improved mechanical strength.     

Gold catalysts for the abatement of environmentally harmful materials: Modeling the structure dependency

FeO_x, TiO₂ and CeO_x layers were deposited by pulsed laser deposition (PLD) technique onto Au films or Au nanoparticles supported on SiO₂/Si(100). The samples were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS) and their reactivity was studied in catalytic CO oxidation. Comparison was made with reference samples of FeO_x/SiO₂/Si(100), TiO₂/SiO₂/Si(100), CeO_x/SiO₂/Si(100) and Au/SiO₂/Si(100) layers. The catalytic activity of the metal-oxide/Au/SiO₂/Si(100) samples must be attributed to active sites located on the metal-oxides overlayer modified by gold underneath, since no Au was exposed to the surface according to the XPS and SIMS. We found a promoting effect of gold on the catalytic activity of the FeO_x overlayer and an inhibiting effect of gold on the TiO₂ and CeO_x overlayers. These findings are discussed in terms of electronic interactions at the Au/metal oxide interface.     

Growth of carbon nanotubes over Fe-Co and Ni-Co catalysts supported on different phases of TiO2 substrate by thermal CVD

The aim of this work is to examine the properties of CNTs formed on Fe-Co and Ni-Co bimetallic catalysts supported on different phases of TiO₂ (anatase and rutile) by wet impregnation method. The CNTs are grown from decomposition of acetylene via Thermal CVD at 700°C using the prepared catalysts. The nanomaterials were characterized by XRD, Xmap, BET, FESEM, TEM, and Raman spectroscopy. It was found that the catalyst samples supported on rutile TiO₂ have higher specific surface area, smaller catalytic nanoparticles with denser distribution and very more activity compared to anatase ones. Consequently, the CNTs nucleated from nanoparticles supported on rutile TiO₂ possess higher density, smaller average diameters and narrower diameter distribution compared to grown CNTs on anatase samples. Moreover, it was observed that the Fe-Co bimetallic catalysts regardless of TiO₂ support phase, possesses more catalytic activity and higher average growth rate of CNTs in compare with Ni-Co catalysts.     

碳纳米管修饰LaFeO3/TiO2复合材料的光催化性能与机理

采用超声辅助溶胶凝胶法制备了LaFeO₃颗粒,进一步以碳纳米管(CNTs)为基底和钛酸丁酯为前体,通过一步水热法煅烧合成CNTs/TiO₂/LaFeO₃(CTF)三元异质结光催化复合材料。通过扫描电子显微镜(SEM)、X射线衍射分析(XRD)、氮气吸附-解吸等温线(BET)、紫外-可见分光光度计(UV-Vis)、光致发光光谱(PL)等表征手段对材料的形貌与特征结构、比表面积和孔径结构以及光学特征进行了分析,并在紫外光下通过降解活性黑五(RB5)测试样品的光催化性能。结果表明,以CNTs作为载体,能够有效提升LaFeO₃/TiO₂复合材料的光催化性能。当CNTs在复合材料中的质量占比为5%时,150 W汞灯照射下RB5的50 min去除率可达99.5%。CNTs一方面通过增加复合材料的比较面积为催化反应的进行提供了更多的活性位点,更为重要的是,CNTs作为光生载流子传输的通道加快了电荷分离效率,提升了复合材料的降解能力和催化反应动力学进程。     

Use of oxygen isotope to study the transport mechanism during high temperature oxide scale growth

Abstract

The examination of high temperature (HT) oxide scale growth mechanisms was performed using secondary ion mass spectrometry (SIMS) and secondary neutral mass spectrometry (SNMS), in conjunction with ¹⁶O₂/¹⁸O₂ HT oxidation experiments. Cr₂O₃, NiO, ZrO₂ and Al₂O₃ were studied because they constitute excellent representative thermally grown oxide scales: they grow by cationic diffusion (Cr₂O₃, NiO), anionic diffusion (ZrO₂) or mixed anionic-cationic diffusion (Al₂O₃). The oxidation tests were performed first in ¹⁶O₂ and subsequently in ¹⁸O₂ at several temperatures (600–1000°C for NiO, 600°C for ZrO₂, 1000°C for Cr₂O₃ and 1100°C for Al₂O₃). The oxygen isotope distribution observed by SIMS and SNMS profiles are discussed and related with the HT oxidation mechanisms proposed in the literature.     

Effects of nano-Fe2O3 powder on thermal emission property of microarc oxidation coating on titanium alloy

Abstract

A ceramic coating was formed on the titanium alloy by microarc oxidation in an electrolyte containing nano-Fe₂O₃, emulsifier OP-10 and sodium phosphate. The composition, surface and cross-sectional morphology and the element compositions of the coatings were characterised by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis system. The spectral emissivity of the coatings was measured by a Fourier transform spectrometer apparatus. The bonding strength between the coating and the titanium alloy was studied by tensile strength test. The thermal shock resistance of the coatings was also evaluated. The results showed that nano-Fe₂O₃ was incorporated into the coating, and the coating had high emission at the wavelength range of 3–20 μm. The bonding strength was 33·2 MPa, and after being subjected to severe thermal shocking for 50 cycles, little peeling-off of the coating occurred.     

CuO+ZnO/CeO2/Al2O3水煤气变换活性及其表面形貌网络特征分析

以φ3mm Al₂O₃作为主载体,采用浸渍与焙烧工艺,制备水煤气低温变换催化剂：CuO+ZnO/CeO₂/Al₂O₃.利用X射线衍射（XRD）、扫描电子显微镜（SEM）、Raman散射光谱分别对催化剂的化学组成、表面形貌以及表面元素键合状态进行表征;对催化剂的水煤气变换反应(WGSR)活性进行测试.在对催化剂表面形貌进行数据挖掘的基础上,利用复杂网络方法对催化剂的表面形貌进行网络建模,并对其网络拓扑参数和同步性进行了计算.计算结果表明,CuO+ZnO/CeO₂/Al₂O₃表面形貌网络度分布具有幂律分布特征;在催化WGSR以后,催化剂表面形貌网络同步性有所增强.     

3D navicular ceria micro/nanocomposite structure with multi-layered arrangement and its application in CO oxidation

In this work, novel three-dimensional (3D) navicular ceria micro/nanocomposite architecture with multi-layered structure was synthesized for the first time via solution reaction followed by a simple hydrothermal treatment in the presence of lysozyme. During the synthesis procedure, a 3D navicular ceria precursor (Ce₂O(CO₃)₂·H₂O) with multi-layered structure was obtained after hydrothermal treatment at 100 °C for 10 h. Ceria with the same morphology can be obtained after thermal decomposition of this Ce₂O(CO₃)₂·H₂O precursor. Structural properties of the products were characterized by XRD, TG-DTA, FT-IR, SEM, TEM, and N₂-sorption techniques. Then a possible layer-by-layer electrostatic self-assembled growth mechanism was proposed for the formation of this 3D navicular architecture based on the experimental results and detail analysis of the evolution process of ceria precursors. Furthermore, a mold reaction of the catalytic oxidation of CO was conducted on the as-obtained CeO₂ and gold supported catalyst, both showed excellent activity and durability for CO conversion.     

制备参数对花状Au/CeO2催化剂上CO低温氧化性能的影响

采用水热合成法、沉积沉淀法分别制备花球状CeO_2和负载型Au/CeO_2。考察了反应液pH、金的负载量和煅烧温度对Au/CeO_2催化氧化CO活性的影响,确定最佳制备参数,并对优化的Au/CeO_2进行稳定性、储存性和再生性测试。结果表明:反应液的最适宜pH为8.5~9,最适宜的负载量和焙烧温度分别是2%(质量分数)和300℃。优化的Au/CeO_2催化剂,室温下将1%CO催化氧化至1.8×10-6,连续反应67h活性开始下降,当温度升至55℃时,连续反应700h,CO浓度仍然保持在8×10-6以下。此外,该催化剂还表现出良好的储存性和再生性。     

Synthesis of ordered mesoporous CuO/CeO2 composite via co-nanocasting replication method and its improved reactivity towards hydrogen

Ordered mesoporous CuO/CeO₂ composites with cubic Ia3d or 2-D hexagonal p6mm structure have been synthesized by a co-nanocasting-replication method using mesoporous silica KIT-6 or SBA-15 as hard template. XRD, TEM, N₂-adsorption and H₂-TPR techniques were used for the structural analysis and catalytic activity characterization. The results indicated that the high surface area mesoporous CuO/CeO₂ composites with different amounts of CuO addition were well-crystallized and exhibited much improved reactivity towards hydrogen than pure mesoporous CeO₂, and the CuO/CeO₂ composite with 20 wt.% CuO addition possessed the highest reactivity towards hydrogen. The enhanced H₂-TPR property could be attributed to the synergetic catalytic effects between copper oxide and cerium oxide.     

Cyclic oxidation of FeCrAlY/Al2O3 composites

Abstract

Three-ply composites consisting of a FeCrAlY matrix and continuous single crystal Al₂O₃ (sapphire) fibers were cyclically oxidized at 1,000° and 1,100°C for up to 1,000 1-h cycles. FeCrAlY matrix only samples were also fabricated and tested for comparison. Fiber ends were exposed at the ends of the composite samples. Following cyclic oxidation, cracks running parallel to and perpendicular to the fibers were observed on the large surface of the composite. In addition, there was evidence of increased scale damage and spallation around the exposed fiber ends, particularly around the middle ply fibers. This damage was more pronounced at the higher temperature. The exposed fiber ends showed cracking between fibers in the outer plies, occasionally with Fe and Cr-rich oxides growing out of the cracks. Large gaps developed at the fiber–matrix interface around many of the fibers, especially those in the outer plies. Oxygen penetrated many of these gaps resulting in significant oxide formation at the fiber–matrix interface far within the composite sample. Around several fibers, the matrix was also internally oxidized showing Al₂O₃ precipitates in a radial band around the fibers. The results show that these composites have poor cyclic oxidation resistance due to the CTE mismatch and inadequate fiber–matrix bond strength at temperatures of 1,000°C and above.     

A novel method to prepare Cr2O3 nanoparticles

By the reaction system of CrO₃ and HCHO in aqueous solution, Cr₂O₃ nanoparticles were first prepared via hydrothermal synthesis. The process can be easily scaled up. The reaction time was only 1 h and the reaction temperature was 170 °C. The products were loosely agglomerated Cr₂O₃ particles of 50-70 nm in average particle size calculated from the Scherrer's formula, whose microstructure and that of the precursor were investigated by SEM. And IR, TG and BET were other characterization methods to study the process. The findings showed that the higher calcination temperature and the higher total concentration were factors to result in the larger average particle size.     

Effects of Si,Mn, and water vapour on the microstructure of protective scales grown on Fe–20Cr in CO2 gas

Abstract

Model alloys Fe–20Cr–0.5Si and Fe–20Cr–2Mn (wt-%) were exposed to Ar–20CO₂ and Ar–20CO₂–20H₂O at either 818 or 650°C. In dry gas, protective scales on Fe–20Cr–0.5Si consisted of an outer Cr₂O₃ layer and an inner SiO₂ layer. In wet gas, additional chromia whiskers were formed on top of the duplex scale. Chromia grains formed in wet gas were much smaller than those in dry gas. A TEM analysis revealed that phase constitutions of the protective scale on Fe–20Cr–2Mn were not uniform: Mn₃O₄ and MnCr₂O₄ above alloy grain boundaries and Mn₃O₄, Cr₂O₃ and MnCr₂O₄ on alloy grains. Formation of different oxides and morphologies are discussed in terms of changes in diffusion paths and thermodynamics caused by the presence of carbon and hydrogen.     

UV Raman spectroscopic study on the surface phase of ZrO2 modified with Nd2O3

The Nd₂O₃ modified ZrO₂ was synthesized using two methods of co-precipitation (Nd-ZrO₂) and wet impregnation (Nd/ZrO₂). The surface and bulk crystalline phases of Nd₂O₃ modified ZrO₂ were investigated by using UV Raman spectroscopy, visible Raman spectroscopy, and X-ray diffraction (XRD). It is observed that the tetragonal phase in the surface region of Nd-ZrO₂ was not effectively stabilized by Nd₂O₃, as Nd₂O₃ is mainly present in the bulk of Nd-ZrO₂. However, in Nd/ZrO₂, it is found that with the impregnation of 0.5 mol% Nd₂O₃ on ZrO₂, the surface tetragonal phase of Nd/ZrO₂ can be stabilized even after calcination at 700 °C. The UV Raman results indicate that a disordered structure, or intermediate structure, which is involved in the transition from the tetragonal to the cubic phase, is formed at the surface region of Nd/ZrO₂. The formation of the aforementioned intermediate structure inhibits the phase transition from tetragonal to monoclinic in the surface region of Nd/ZrO₂. Furthermore, it is observed that the mixed tetragonal and monoclinic phases in the surface region of ZrO₂ which has been impregnated with Nd₂O₃ can also be stabilized after calcination at 700 °C. This work provides a simple method for controlling the surface phase of ZrO₂ at high temperatures.     

YBa2Cu3O7 superconductor as a mild,deep oxidation catalyst

Several Y-Ba-Cu-O compositions including the superconducting YBa₂Cu₃O₇ are scanned for their oxygen uptake using low temperature oxygen chemisorption (LTOC) technique as the principal probe at 195 K. BET surface area and pore size distribution were also determined. The results indicate their potential for use as mild, deep oxidation catalysts even under ambient and sub-ambient conditions.     

Hybrid sonochemic urea-nitrate combustion preparation of CuO/ZnO/Al2O3 nanocatalyst used in fuel cell-grade hydrogen production from methanol: Effect of sonication and fuel/nitrate ratio

Fuel cell-grade hydrogen production has been studied via steam reforming of methanol (SRM) over a series of CuO/ZnO/Al₂O₃ nanocatalysts fabricated by the combustion method. The effect of sonication and urea/nitrate ratio on the characteristics and catalytic properties of the prepared catalysts has been investigated. The synthesized catalysts were characterized by x-ray diffraction (XRD), field emission scanning electron microscope (FESEM), Particle Size Distribution (PSD), energy dispersive x-ray (EDX), Brunauer-Emmett-Teller (BET) and FTIR analyses XRD patterns showed positive influence of urea/nitrate ratio on CuO and ZnO crystallite sizes. The ultrasonic mixing of primary gel compared with conventional mixing led to lower crystallite size. FESEM images showed that the sample mixed by sonication with a urea/nitrate ratio of 1 had more homogeneous morphology with narrow particle size distribution. EDX results proved the presence of all metals on the surface of the nanocatalysts and better consistence between the gel and surface composition of elements in samples prepared by sonication. Catalytic performance showed that sonication during the mixing of primary gel dramatically increased the methanol conversion. It was also proved that increasing the amount of urea led to lower catalytic activity. The ultrasound-treated nanocatalyst with urea/nitrate?=?1 was the best sample in terms of activity and selectivity. It was stable in the SRM for 1200?min without considerable change in methanol conversion and product selectivity.     

Synthesis, structure and exchange bias in Cr2O3/CrO2/Cr2O5 particles

We report on the synthesis, structure and magnetic properties of a novel exchange bias system with Cr₂O₃/CrO₂/Cr₂O₅ interfaces. Chromium oxide particles with mixed chromium valences were prepared by sintering CrO₃ in air. X-ray diffraction patterns show that CrO₃ lost its oxygen gradually with increasing temperature and time through Cr₃O₈, Cr₂O₅, CrO₂, and finally Cr₂O₃ at temperatures above 760 K. X-ray photoelectron spectra indicate a low CrO₂ content and a binding energy of 579.3 eV for Cr 2p_3/2 photoelectrons in Cr₂O₅. Chromium dioxide was found to stably coexist with Cr₂O₃ and Cr₂O₅ in the particles. Magnetic measurements show hysteresis loop shifts in the sample, indicating an exchange bias induced by antiferromagnetic Cr₂O₃/Cr₂O₅ in ferromagnetic CrO₂. An exchange bias of 9 mT at 5 K and a coercivity of 26.3 mT were observed in the chromium oxide particles containing CrO₂.     

Simple method for synthesis of carbon nanotubes over Ni-Mo/Al2O3 catalyst via pyrolysis of polyethylene waste using a two-stage process

Synthesis of valuable multi-walled carbon nanotubes (MWCNTs) by thermal pyrolysis of low-density polyethylene (LDPE) waste was investigated via a two-stage process. The first stage was the thermal pyrolysis of LDPE to gaseous hydrocarbons, and the second stage was the catalytic decomposition of the pyrolysis gases over Ni-Mo/Al₂O₃ catalysts. Two catalysts with the compositions of 5.2%Ni-10.96%Mo/Al₂O₃ and 10%Ni-9.5%Mo/Al₂O₃ were tested for carbon nanotubes (CNTs) formation. The catalyst containing 10%Ni showed better activity in terms of CNTs production. Accordingly, the impact of either pyrolysis or decomposition temperatures was investigated using the 10%Ni-9.5%Mo/Al₂O₃ catalyst. TEM, XRD, Raman spectroscopy, TGA, TPR, and BET analysis tools were used to characterize the fresh catalysts as well as the obtained carbon nanomaterials. TEM images proved that MWCNTs with various morphological structures were obtained at all pyrolysis and decomposition temperatures. Moreover, cup-stacked carbon nanotubes (CS-CNTs) were observed at the decomposition temperature of 600°C. MWCNTs with the best quality were produced at decomposition temperature of 750°C. The optimum pyrolysis and decomposition temperatures in terms of CNTs production were at 700 and 650°C, respectively.     

Effect of CuO addition to Nd(Zn1/2Ti1/2)O3 ceramics on sintering behavior and microwave dielectric properties

The microwave dielectric properties and microstructures of CuO-doped Nd(Zn_1/2Ti_1/2)O₃ ceramics prepared by the conventional solid-state route were investigated. The prepared Nd(Zn_1/2Ti_1/2)O₃ exhibits a mixture of Zn and Ti showing 1:1 order in the B-site. As an appropriate sintering aid, not only did CuO lower the sintering temperature, it could effectively hold back the evaporation of Zn in the Nd(Zn_1/2Ti_1/2)O₃. Moreover, CuO only resided in boundaries, which was confirmed by EDX analysis. The measured lattice parameters of CuO-doped Nd(Zn_1/2Ti_1/2)O₃ (a = 5.4652 ± 0.0005 ?, b = 5.6399 ± 0.0007 ?, c = 7.7797 ± 0.0008 ? and β = 90.01 ± 0.01°) retained identical to that of the pure Nd(Zn_1/2Ti_1/2)O₃ in all cases. In comparison with the pure Nd(Zn_1/2Ti_1/2)O₃ ceramics, specimen with 1 wt.% CuO addition possesses a compatible combination of dielectric properties with a ε_r of 30.68, a Q × f of 158,000 GHz (at 8 GHz) and a τ_f of − 45 ppm/°C at 1270 °C. It also indicated a 60 °C lowering in the sintering temperature. The proposed dielectrics can be a very promising candidate material for microwave or millimeter wave applications requiring extremely low dielectric loss.     

碳纤维/TiO2改性树脂复合材料的制备及力学性能

针对环氧树脂脆性大、与碳纤维形成的界面性能较差等问题,本文选用纳米TiO₂对5284环氧树脂进行改性,并以角联锁机织物为增强体制备了碳纤维/环氧树脂复合材料。使用FT-IR、旋转流变仪、表面张力仪等设备对TiO₂/环氧树脂进行表征,并研究了树脂改性对复合材料压缩与层间剪切性能的影响。研究表明：TiO₂的羟基与环氧树脂的环氧基和羟基发生了反应;经1wt.%TiO₂改性的树脂复数黏度为0.066 Pa·s,纤维与树脂间接触角为28.85°,浸润效果较好;相较于未改性复合材料,树脂改性的复合材料纵向压缩强度与模量分别提高了7.46%和11.03%,横向压缩强度与模量分别提高了6.99%和4.96%,纵向、横向的剪切强度分别提高了6.88%和4.65%。TiO₂改性环氧树脂提高了复合材料的承载能力,改善了界面结合强度。