Synthesis of Mn-doped CeO<Subscript>2</Subscript> nanorods and their application as humidity sensors

Mn-doped CeO₂ nanorods have been prepared from CeO₂ particles through a facile composite-hydroxide-mediated (CHM) approach. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The analysis from the X-ray photoelectron spectroscopy indicates that the manganese doped in CeO₂ exists as Mn^4 +. The responses to humidity for static and dynamic testing proved doping Mn into CeO₂ can improve the humidity sensitivity. For the sample with Mn% about 1·22, the resistance changes from 375·3 to 2·7MΩ as the relative humidity (RH) increases from 25 to 90%, indicating promising applications of the Mn-doped CeO₂ nanorods in environmental monitoring.     

Growth and physicochemical characterization of CuAlTe2 films obtained by reaction, induced by annealing, between Cu/Al/Te/Al/Cu... Al/Cu/Al/Te layers sequentially deposited

CuAlTe₂ thin films have been synthesized by annealing under an argon flow a multilayer structure of thin Cu, Al and Te layers sequentially deposited by evaporation under vacuum. The films have been characterized by X-ray diffraction, microprobe analysis, photoelectron spectroscopy and Raman scattering. At the end of the process, the XRD spectra demonstrate that textured CuAlTe₂ films have been obtained with preferential orientation of the crystallites along the (112) direction. The Raman patterns are in good accordance with the reference. The XPS spectra show that the binding energies of the elements are in good agreement with bonds of CuAlTe₂. Even after a decrease of the oxygen contamination by improvement of the depositing process the oxygen present in the films is found to be about 12 at %. This revised version was published online in September 2006 with corrections to the Cover Date.     

The effect of shock-loading on the aging behavior of an Al-Mg-Si alloy

The impact of pre-shock loading on the precipitation reactions in an Al-Mg-Si alloy (AA6022) was studied by means of differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and hardness measurements. The samples were solutionized and quenched in water prior to subsequent shock-loading and aging treatment. The TEM and DSC results show that, while shock-loading prior to aging facilitated the precipitation of Q ′ and β, no significant effect on β′′ precipitates was observed. The hardness studies indicate that pre-shock loading strengthens the material by forming a high concentration of microstructural defects, however the resultant mechanical properties of the shocked sample are comparable to those without shock processing at the peak of aging. It was found that the rate of overaging is higher in shocked samples, which is in agreement with the DSC and TEM results.     

Combustion synthesis and structural characterization of Li–Ti mixed nanoferrites

Polycrystalline samples of the mixed nanoferrites, Li_{0·5 + 0·5x} Ti _x Fe_{2·5 − 1·5x} O₄ (0·02 ≤ x ≤ 0·1), were prepared by combustion method at lower temperatures compared to the conventional high temperature sintering for the first time at low temperatures, using PEG which acts as a new fuel and oxidant. XRD patterns reveal a single-phase cubic spinel structure. The as synthesized Li–Ti ferrites are in nanocrystalline phase. The crystallite size was found to be in the range 16–27 nm. SEM images reveal rod-like morphology in all the samples with a discontinuous grain growth. The B–H loops have been traced using VSM technique, for all the compositions, at room temperature and the hysteresis parameters are calculated. Saturation magnetization decreases with increase in Ti content due to the fact that the Ti^4 + ion, which is a non-magnetic ion, replaces a magnetic Fe^3 + ion. The hysteresis loops show clear saturation at an applied field of ±10 kOe and the loops are highly symmetric in nature. The cation distribution is known indirectly by using saturation magnetization values.     

The Effect of Sintering Temperature Variation on the Superconducting Properties of ErBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript> Superconductor Prepared via Coprecipitation Method

The effect of heat treatment on the superconducting properties of ErBa₂Cu₃O_7−δ (ErBCO) ceramic materials has been studied. The nano-metal oxalate precursor was prepared using coprecipitation (COP) method. The prepared materials were subjected to calcination process at 900 °C for 12 h and then sintered under oxygen environment for 15 h at 920 °C, 930 °C, 940 °C, and 950 °C, respectively. All samples showed a metallic behavior and single-step transition in the R–T curves. The best zero critical current, T _C(R=0)=91.4 K, was for the sample sintered at 920 °C. XRD data showed single phase of an orthorhombic structure. As the sintering temperature increases, the formation of nonsuperconducting phases (impurities) was observed when the samples sintered above 920 °C. The formation of nano-oxalate powders via COP method is a very efficient procedure to produce high-quality superconductors with less processing temperature required.     

Oxidation of gold metal particles supported on TiO<Subscript>2</Subscript>: an FTIR study by means of low-temperature CO adsorption

Two Au/TiO₂ samples with different gold loadings (0.7 and 4.0 wt.% Au) were prepared by deposition-precipitation with urea and calcined at 673 K. TEM revealed gold particles of 3.2 and 3.9 nm for the 0.7 and 4.0 wt.% samples, respectively. The samples were subjected to different red-ox treatments and then the state of gold was determined by the FTIR spectra of CO adsorbed at low temperature. Several kinds of gold carbonyl species were detected during the experiments: (i) Au⁰–CO at around 2107 cm⁻¹; (ii) Au⁺–CO at ca. 2175 cm⁻¹; (iii) Au^δ+–CO in the region of 2140–2137 cm⁻¹ and (iv) Au^δ′+–CO (δ^′ > δ) at around 2155 cm⁻¹. The 4.0 wt.% sample contained mainly metallic gold after evacuation at 673 K. Subsequent interaction with oxygen at 373 K leads to oxidation of a fraction of the surface metallic gold sites to Au^δ+ sites. These sites were considered as cations located on the surface of the metal particles with a partially positive charge δ+ (0 < δ < 1) because of electron transfer from the gold bulk. Evacuation at 673 K leads to back reduction of the Au^δ+ sites to metallic gold. The oxidation of gold particles was more efficient when performed with a NO + O₂ mixture. It resulted in creation of Au^δ′+ sites with a higher positive charge than that of the Au^δ+ sites. In this case the oxidation involved a higher number of Au⁰ sites. A similar treatment of the 0.7 wt.% Au sample, however, resulted in formation of “isolated” Au⁺ species. The results indicate that small metal particles are more easily oxidized by a NO + O₂ mixture. A model of the formation of the different sites, explaining well the experimental results, is proposed.     

Facile Synthesis of Co<Subscript>3</Subscript>O<Subscript>4</Subscript>/CoO Nanoparticles by Thermal Treatment of Ball-Milled Precursors

Co₃O₄/CoO nanoparticles have been synthesized by a simple method which is based on the ball-milling and calcination of cobalt acetate and citric acid. The samples were characterized using X-ray diffraction, transmission electron microscope, and Fourier transform infrared spectroscopy. The results show that Co₃O₄ nanoparticles with an average particle size of ∼40 nm can be obtained by calcination of ball-milled precursors at relatively low temperature (350 °C) for 3 hours. It should be noted that it is possible to control the size of Co₃O₄ particles by calcination temperature, calcination time and also by ball-milling duration using this method. Meanwhile, the pure CoO nanoparticles were obtained successfully by thermal decomposition of Co₃O₄ at 950 °C and quickly quenching to liquid nitrogen.     

Phase Formation and Transition Temperature Variation of TlSr2(Ca1−x R x )Cu2O7 (R = Dy, Ho and Tb) Superconductor

The TlSr₂(Ca_1−x R _x )Cu₂O₇ (Tl-1212) phase with R = Dy, Tb and Ho for x=0.1–0.6 has been studied by powder x-ray diffraction method and electrical resistance measurement. All the samples showed a metallic normal state behavior. No correlation between ionic radius and T _c onset is observed. The three series show the highest volume fraction (>97%) of Tl-1212 phase for x=0.5. The highest T _c onset does not correspond to the highest volume fraction. The above results are explained as due to the mixed valence state of Dy and Tb in the Tl-1212 phase.      

Analysis on insulator–metal transition in yttrium doped LSMO from electron density distribution

Yttrium doped LSMO (La_1 − x Sr _x MnO₃) was prepared using sol–gel technique and analysed for the insulator–metal transition from charge density variation in the unit cell with respect to different stoichiometric inclusion of yttrium. X-ray powder diffraction profiles of the samples were obtained and the well known Rietveld method and a versatile tool called maximum entropy method (MEM) were used for structural and profile refinement. The charge density in the unit cell was constructed using refined structure factors and was analysed. The charge ordering taking place in the insulator–metal transition was investigated and quantified. The insulator–metal transition was found to occur when 20% of La/Sr atoms were replaced by yttrium. The changes in the charge environment have also been analysed.     

Formation of new materials based on cast aluminum alloy

We show the dynamic formation of composite materials using the polycrystalline cast alloy Al + 12% Si as the example. The volume fraction of the synthesized framework (“influence zones”) of the composite material has been estimated to be at a level of 11%. Upon the introduction of lead a material practically insoluble under the experimental conditions was synthesized in the framework. Upon the introduction into the Al + 12% Si cast alloy of silicon carbide particles, we obtained a material with a specific corrosion exceeding by a factor of 5.6 the given index of the source matrix material.[1–7]     

Preparation and characterization of ZrB 2 -SiC ultra-high temperature ceramics by microwave sintering

ZrB₂-SiC ultra-high temperature ceramic composites reinforced by nano-SiC whiskers and SiC particles were prepared by microwave sintering at 1850°C. XRD and SEM techniques were used to characterize the sintered samples. It was found that microwave sintering can promote the densification of the composites at lower temperatures. The addition of SiC also improved the densification of ZrB₂-SiC composites and almost fully dense ZrB₂-SiC composites were obtained when the amount of SiC increased up to 30vol.%. Flexural strength and fracture toughness of the ZrB₂-SiC composites were also enhanced; the maximum strength and toughness reached 625 MPa and 7.18 MPa·m^1/2, respectively.     

Pressureless sintering of dense hydroxyapatite–zirconia composites

Hydroxyapatite (HA)–TZP (2.5 mol% Y₂O₃) containing 2, 5, 7.5 and 10 wt% TZP were prepared using calcium nitrate, diammonium hydrogen orthophosphate, zirconium oxychloride and yttrium nitrate. The composite powder was prepared by a reverse strike precipitation method at a pH of 10.5. The precipitates after aging and washing were calcined at 850°C to yield fine crystallites of HA and TZP. TEM study of the calcined powder revealed that while HA particles had both spherical and cuboidal morphology (∼50–100 nm) the TZP particles were only of spherical nature (∼50 nm). X-ray analysis showed that the calcined powder of all the four composition had only HA and t-ZrO₂. Uniaxially compacted samples were sintered in air in the temperature range 1,150–1,250°C. High sintered density (>95% of theoretical) was obtained for composites containing 2 and 5 wt% TZP, while it was 92% for 7.5 wt% and 90% for 10 wt% TZP compositions. X-ray analysis of sintered samples shows that with 2 wt% TZP, the retained phases were only HA and t-ZrO₂. However, for 5, 7.5 and 10 wt% TZP addition both TCP and CaZrO₃ were also observed along with HA and t-ZrO₂. Bending strength was measured by three point bending as well by diametral compression test. While in three point bending, the highest strength was 72 MPa, it was 35.5 MPa for diametral compression. The strength shows a decreasing trend at higher ZrO₂ content. SEM pictures show near uniform distribution of ZrO₂ in HA matrix. The reduction in sintered density at higher ZrO₂ content could be related to difference in the sintering behaviour of HA and ZrO₂.     

Effect of low level substitution of Sr–Ba on transport and magnetic behaviour of La<Subscript>0·67</Subscript>Ca<Subscript>0·33</Subscript>MnO<Subscript>3</Subscript>

In this paper we report the investigation of transition metal oxide compound, La_0·67Ca_0·25Sr_0·04Ba_0·04MnO₃ (LCSBMO), along with La_0·67Ca_0·33MnO₃ (LCMO), synthesized by sol–gel route under identical conditions. The effect of simultaneous low level substitution of large size ions such as Sr²⁺ and Ba^2 + for Ca^2 + ions on the electronic transport and magnetic susceptibility properties are analysed and compared apart from microstructure and lattice parameters. The temperature dependent electrical transport of the polycrystalline pellets of LCSBMO and LCMO when obeying the well studied law, r = r₀ + r₂   T²\rho = \rho_{0} + \rho_{2} \;T^{2} for T < T _MI , is observed to differ by more than 50% from the values of ρ ₀ and ρ ₂, with the former compound showing enhanced electrical conductivity than the latter. Similarly in fitting the adiabatic small polaron model for resistivity data of both the samples for T > T _MI , the polaron activation energy is found to differ by about 11%. In addition, the temperature dependent a.c. magnetic susceptibility study of the compounds shows a shift of about 6% in the paramagnetic to ferromagnetic transition temperature (285 K for LCSBMO and 270 K for LCMO).     

Structural and optical characterization of thick and thin polycrystalline diamond films deposited by microwave plasma activated CVD

Preliminary results of growth of thin diamond film in a recently installed 3 kW capacity microwave plasma activated CVD (MW-PACVD) system are being reported. The films were deposited on Si (100) substrate at 850°C using methane and hydrogen mixture at 1·5 kW MW power. The grown polycrystalline films were characterized by micro-Raman, transmission electron microscope (TEM), spectrophotometer and atomic force microscope (AFM). The results were compared with that of a thicker diamond film grown elsewhere in a same make MW-PACVD system at relatively higher power densities. The presence of a sharp Raman peak at 1332 cm^− 1 confirmed the growth of diamond, and transmission spectra showed typical diamond film characteristics in both the samples. Typical twin bands and also a quintuplet twinned crystal were observed in TEM, further it was found that the twinned region in thin sample composed of very fine platelet like structure.     

Structure sensitivity of methanol decomposition on Ni/SiO<Subscript>2</Subscript> catalysts

Three different silica-supported nickel samples were prepared by successive adsorption, reduction, and passivation (SARP) of nickel. The materials obtained were characterized by various techniques (TEM, XRD, H₂ chemisorption, FTIR spectroscopy of adsorbed CO, FMR). Metal nickel particles were uniformly distributed by size with all samples. With increasing the number SARP cycles (1, 3, and 5, respectively) the metal concentration (3.6, 7.6, and 12.6 wt%, respectively) and the mean particle size (4–5, ca. 6 and ca. 7 nm, respectively) also increased without substantial increase of the number of metal particles. The samples were tested as catalysts in methanol decomposition to CO and H₂. It was found that this reaction was structure sensitive and the turn-over frequency decreased with the particle size increase. In contrast, the secondary interaction between the reaction products, i.e., CO methanation (occurring above 515 K) appears to be structure insensitive.     

Preparation of anatase F doped TiO<Subscript>2</Subscript> sol and its performance for photodegradation of formaldehyde

Anatase fluoride doped TiO₂ sol (F-TiO₂) catalyst was prepared by a modified sol-gel hydrothermal method, using tetra butyl titanate as a precursor. The influences of F doping, temperature of hydrothermal, values of medium pH on the morphology and crystallization were studied. The microstructure and morphology of sol sample were characterized by XRD, TEM, FTIR, UV–Vis–DRS, particle size distribution (PSD) and XPS. The results showed that F-TiO₂ particles in sol were spherical and partly crystallized to anatase structure, and dispersed in the aqueous medium homogeneously and that the average particle size was ca. 10.5 nm calculated from XRD and TEM results. It was also found that the addition of fluorine could improve the crystallization and adsorption of particles significantly, the photocatalytic activity for decomposition of formaldehyde were enhanced remarkably with the doping of fluorine. Possible mechanism of anatase F-TiO₂ formed under hydrothermal conditions was discussed.     

Synthesis of CNTs via ethanol decomposition over ball-milled Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> coated copper sheets

Carbon nanotubes (CNTs) were synthesized on ball-milled Fe₂O₃ coated copper sheets by the catalytic decomposition of ethanol vapor at 650°C. TEM, SEM, and EDX revealed the presence of 30–50 nm diameter multiwalled carbon nanotubes with catalytic particles at their tips. CNTs, α-Fe, and Fe₃C were detected by XRD. Raman and TG analyses show that the product is CNTs with less than 10 wt % residues. The carbon yield was the maximum at 354 wt %. The text was submitted by the authors in English.     

Influence of Heat Treatment on Structure and Superconducting Properties of YBCO Film by Chemical Solution Deposition Method

YBCO films have been fabricated on a (00l) LaAlO₃ single-crystal substrate via self-developed fluorine-free polymer-assisted metal organic deposition (PA-MOD) method. The influence of heat treatment on texture, microstructure and superconducting properties of YBCO films has been investigated. After a pyrolysis process ranging from 145 °C to 500 °C with different heating rates, the samples were fired at 760–780 °C in Ar and O₂ mixture gas followed by annealing at 450 °C in pure O₂. The results indicate the film fired at 770 °C after decomposition at the rate of 0.5 °C/min showed the highest T _c of 90.4 K and J _c (77 K, 0 T) over 2 MA/cm². According to the XRD patterns, phi-scan and omega-scan curves as well as SEM images, the good properties may be attributed to better biaxial texture and purer YBCO phase as well as better grain connectivity.     

The Transition from Paramagnetic to Ferromagnetic States as Influenced by Evolving Microstructure of Ni0.5Zn0.5Fe2O4

In this paper the effect of sintering temperature on Ni_0.5Zn_0.5Fe₂O₄ is examined closely. The evolution of toward magnetically ordered materials was to be tracked with the parallel evolving microstructure subjected to sintering temperatures in an ascending order. The starting powder of Ni_0.5Zn_0.5Fe₂O₄ was prepared via mechanical alloying and later molded into toroidal samples. After each sintering, we observed the resulting changes in the materials. The XRD data showed a single phase being formed as early as 600 °C and the peak intensity was increasing with the sintering temperature indicating an increase in the degree of crystallinity. The BH hysteresis loops showed the evolution from paramagnetism to moderate ferromagnetism to strong ferromagnetism with microstructural changes. For lower sintering temperatures, the samples showed paramagnetic behavior dominating the samples. As sintering temperature increased, paramagnetic states decreased and, at 900 °C, a moderately ferromagnetic state appeared. Sintering at 1000 °C produced a strongly ferromagnetic state giving a well-formed sigmoid-shape hysteresis loop.     

Preparation and characterization of solid-state sintered aluminum-doped zinc oxide with different alumina contents

Aluminum-doped zinc oxide (AZO) ceramics with 0− 2 ·5 wt.% alumina (Al₂O₃) content were prepared using a solid-state reaction technique. It was found that AZO grains became finer in size and more irregular in shape than undoped ZnO as the Al₂O₃ content increased. Addition of Al₂O₃ dopant caused the formation of phase transformation stacking faults in ZnO grains. The second phase, ZnAl₂O₄ spinel, was observed at the grain boundaries and triple junctions, and inside the grains. In this study, a 3-inch circular Al₂O₃ (2 wt.%)-doped ZnO ceramic target sintered at 1500°C for 6 h has a relative density of 99·8% with a resistivity of 1·8 × 10^− 3 Ω-cm. The AZO film exhibits optical transparency of 90·3% in the visible region and shows an electrical resistivity of 2·5 × 10^− 3 Ω-cm.