Institute of Silicate Materials

The influence of sol-gel auto combustion synthesis conditions on the phase purity and microstructure of ZnFe 2 O 4 (ZnO Á Fe 2 O 3 ) nanopowders was studied. The phase composition was determined by X-ray diffraction and the particle size measured by scanning electron microscopy. It was found that the external conditions of the auto-combustion reaction affect the phase purity of the synthesized material. Combustion of a xerogel layer on a hot plate forms monophasic spinel zinc ferrite with a particle size $20 nm. Practical use of synthesized ZnFe 2 O 4 nanopowders as gas sensors with reversible response-recovery behavior to ethanol is demonstrated.

Three different methods were used to produce dense ZnFe 2 O 4 ceramics with high electrical resistancespray pyrolysis (SP), conventional sintering (CS) and spark plasma sintering (SPS). The phase purity was determined by X-ray diffraction and the morphology/particle size investigated by scanning electron microscopy. It was found that SP is the most appropriate method to produce a dense ZnFe 2 O 4 ceramics with high electrical resistivity. By using the SP method, about 500 nm thick ZnFe 2 O 4 films were obtained possessing a resistivity with 7 orders of magnitude higher than resistivity of ZnFe 2 O 4 pellet sintered by SPS. Both samples obtained by SP and SPS show density close to the theoretical value. The sample obtained by CS shows a low resistivity and relatively high porosity.

This paper presents an optical characterization of three Bridgman-grown Zn 1−x−y Be x Mg y Se mixed crystals in the near-band-edge interband transitions using temperature-dependent photoluminescence ͑PL͒ in the temperature range of 10-300 K and surface photovoltage spectroscopy ͑SPS͒ at room temperature. PL spectra at low temperatures of the investigated samples consist of an excitonic line, an edge emission due to radiative recombination of shallow donor-acceptor pairs, and a broad band related to recombination through deep level defects. The anomalous S-shape temperature dependence of the exciton emission peak for Zn 1−x−y Be x Mg y Se crystals with high Mg content ͑y = 0.26͒ can be explained as due to localization of excited carriers caused by statistical fluctuations of local composition. The peak positions of the excitonic emission lines in PL spectra correspond quite well to the energies of the fundamental transitions determined from SPS measurements. The parameters that describe the temperature dependence of the transition energy and broadening parameter of the band-edge excitonic emission are evaluated and discussed.

Our objective was to determine the role of cooling rate on gas-sensing properties of annealed nanograined nickel ferrite (NiFe 2 O 4 ). The sol-gel auto combustion method was used for the preparation of NiFe 2 O 4 . To estimate structural and microstructural features, X-ray diffraction, and scanning electron microscopy techniques were used. For gas-sensing measurements different volatile organic compounds (VOCs) were used as testing gases. To identify the contribution of the different sensing layer elements to the conduction, ac impedance spectroscopy (IS) measurements were performed. It was found that the sensors cooled with lower rate exhibited better sensing performance, due to increase of resistance. Overall, this article covers an alternative method for modifying nickel ferrite gas sensor sensitivity.

Magnetic zinc ferrite (ZnFe 2 O 4 ) nanopowders were synthesized using the sol-gel autocombustion method. X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements demonstrated that thermal decomposition of the 1-mm-thick xerogel layers in air formed monophasic spinel ferrite nanopowders with a particle size less than 30 nm. X-ray photoelectron spectroscopy (XPS) showed that the obtained reaction product contained antisite defects, with zinc ions occurring at the octahedral sites of the spinel structure. The concentration of antisite defects or inversion degree decreased when the relatively low annealing temperature was increased from 150 to 500 8C. Overall, the obtained ZnFe 2 O 4 , which is usually antiferromagnetic (exhibits no magnetic moment), demonstrated high saturation magnetization (60.7 emu g À1 ), and coercive force (56 Oe). This was due to the presence of the antisite defects in the structure.

The influence of zinc ion to the NiFe 2 O 4 p-type semiconductor gas response characteristics is demonstrated. For characterization of gas sensor material, synthesized by sol-gel auto combustion method, X-ray diffraction (XRD), scanning electron microscopy (SEM), DC resistance and impedance spectroscopy (IS) measurements were employed. The response change of Zn doped nickel ferrite is related to the interruption of hole hopping between nickel ions. This was improved by change of conductivity type with temperature and gas exposure.

The results presented in this paper were focused on the easy development of cheap, sensitive and integrable spinel zinc ferrite (ZnFe 2 O 4 ) thin film gas sensors for the detection of ethanol. ZnFe 2 O 4 layers of about 450 nm thick were deposited by spray pyrolysis technique. The resulting films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Analysis confirmed the formation of monophasic spinel structured thin films with flat surface and dense microstructure consisting of nanosized grains. The sensor detects ethanol concentrations as low as 1 ppm and shows comparably fast behaviour of response and recovery.

Pure and cobalt-ion-containing nickel ferrite samples Ni 1-x Co x Fe 2 O 4 (x = 0, x = 0.01, x = 0.05 and x = 0.1) were prepared by using the sol-gel auto-combustion method. To form the desired structure after the auto-combustion process, the samples were annealed in air at 800 • C for 1 h, followed by slow cooling in air (2.5 • C/min) or rapid cooling by quenching in water. The influences of cobalt ion addition and the cooling rate on the electronic structure, morphology and electrical properties of Ni-Co ferrite were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The gas-sensing properties of pure and cobalt-ion-containing nickel ferrites were studied by using acetone as a test gas. The gas-response of nickel ferrite samples was strongly influenced by the cooling rate and cobalt addition. The effect was attributed to a change in the Ni 3+ concentration and a restriction of the transduction of the chemical signal into an output electrical signal. It was demonstrated that the gas response of Ni 1-x Co x Fe 2 O 4 tends to increase with decreasing cobalt ion content and with decreasing concentration of the point defects such as cation vacancies and Ni 3+ ions.

Small particles of iron oxides, nominally 7 nm and 13 nm in diameter, have been studied by 57 Fe Mö ssbauer spectroscopy in the temperature range 4 K to room temperature. The Mö ssbauer spectra show, at high temperature, relaxational behaviour in agreement with the superparamagnetism expected. At low temperature the spin motion becomes slower and, finally, blocked. There are also indications that the material is non-stoichiometric. D

Nanocrystalline, homogeneous spinel Ni 1−x Zn x Fe 2 O 4 thin films were deposited on glass substrates by using spray pyrolysis of metal nitrate aqueous solutions. The thickness of deposited films was below 500 nm, but crystallite size was under 30 nm. It has been shown that the DC resistivity, dielectric loss and optical band gap of deposited films are influenced by the zinc content. High DC resistivity and low dielectric losses of thin Ni 1−x Zn x Fe 2 O 4 films are explained by mixed n-p conductivity and nanograin structure of spray pyrolysis deposited coatings which are changing with the ratio of Ni/Zn.

It is demonstrated for the first time that the gas sensing properties of a spinel ferrite complex metal oxide semiconductor can be improved by controlling iron stoichiometry. Conductivity and sensitivity was analyzed for ZnFe 2+z O 4± spinel type ferrite with z from −0.01 to 0.15. By increasing iron content from z = −0.01 to z = 0.1, sensitivity increases up to 3 times but for samples with z > 0.1 sensitivity drops. It was observed from impedance spectroscopy measurements that resistance decreases with an increase of the iron content. Complex impedance spectra reveal two phases with different resistance attributed to depletion layer (R d ) and bulk (R b ). With increasing iron content increases R d /R b ratio due to decrease of depletion layer width. Outer surface of the grain of the sensing material estimated higher ratio (R d(air) /R d(gas) ) with gas exposure in comparison with bulk (R b(air) /R b(gas) ). Overall sensitivity increases due to increase of donor concentration leading down to increase adsorbed oxygen species on grain surface. Sensitivity drops when z > 0.1 due to remarkable decrease of depletion layer width.

High SiO 2 content frit modified with commercial black spinel-type pigment K 6038 of chromium and iron oxide mixed with 40-100 wt% per 100 wt% by weight of frit were deposited on stainless steel alloys by the slip-casting technique and their optical properties were characterised by total reflectance of 300 nm to 20 μm. The enamel coating exhibited moderate spectral selectivity and excellent thermal stability at 600 1C. Solar absorbtance (α) ranged from 0.75 to 0.82 and calculated thermal emittance at 600 1C (ε) ranged from 0.35 to 0.45. Both depended on coating surface and pigment volume in the frit. The highest solar selectivity was measured in enamel with 50% pigment with α and ε (600 1C) of 0.81 and 0.35, respectively.

Sol-gel derived coatings composed of TiO 2 -SiO 2 were deposited on enamelled stainless steel. In order to study the influence of the thickness on the morphological parameters, achieve continuous enamel surfacing and improve the corrosion-resistance and mechanical strength of silicate enamel for stainless steel were used. Solgel coatings with different compositions, heat treatment and numbers of layers were prepared in this work. The structure and morphology of the films were demonstrated by the means of X-ray diffraction analysis (XRD), scanning electron (SEM) and atomic force (AFM) microscopies. Thin triple layer crack-free films with a thickness of 470 nm were obtained. The mechanical strength (HV) of the three-layer coating was 10.4 GPa and was achieved at a temperature of 700°C. Thin SiO 2 -TiO 2 sol-gel coatings were applied in order to enhance the chemical durability of these coatings in both acidic and basic solutions in accordance with ISO 28706 standards. Corrosion tests with the uncoated and sol-gel coated enamel were performed in 3 wt.% boiling oxalic acid at 95-98°C and in 1 N NaOH solution at 80°C. Crack-free 470 nm 3-layer silica-titania coatings significantly improved chemical resistance against the basic medium (up to 80%) and stability against the acid medium (up to 28%) for the S80 sample fired at 700°C.

The results presented in this paper were focused on the synthesis of monophasic spinel cadmium ferrite (CdFe2O4) by sol-gel combustion. The effect of various reducing agents such as citric acid, glycine and urea on the phase purity of the CdFe2O4 was studied. The phase composition was determined by X-ray diffraction and the particle size measured by scanning electron microscopy (SEM). Analysis confirmed the formation of monophasic CdFe2O4 powder with an average particle size of 0.3 ?m after synthesis at 900°C with urea, as a reducing agent. Practical use of synthesized CdFe2O4 powders as gas sensors was demonstrated.

relationship between nickel ion content and structural, colorimetrical, as well as absorbance properties was studied. Structural characterization was performed by using X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared adsorption (IR), CIE-L*a*b colour measurements and diffuse reflectance spectroscopy. Preparation method provides nanometric pigments with monophasic spinel structure. Increasing nickel content, decreases lattice constant and shifts intrinsic stretching vibrations of the metal tetrahedral sites to higher infrared adsorption regions. The optical adsorption bands give evidence for inter-atomic transitions between oxygen atomic 2p-orbitals and iron atomic 3d (t2g)-orbitals. Nickel addition decreases the value of optical band gap. Colour measurements show that H°, C, L* values decrease with nickel ion concentrations, thus changing colour from orange to dark brown.

In this study, the effect of pressure application (0-50 MPa) to fresh concrete right after casting and during the first 24 hours of hardening has been examined. Supplementary cementitious materials in a form of silica fume, nanosilica and ground quartz sand were used. The aim of pressure application was to reduce porosity, thus improving concrete mechanical properties. Considerable reduction in porosity and a subsequent increase in compressive strength reaching the level of Ultra-High Performance Concrete (UHPC) were achieved. Mechanical and physical properties were tested and gas sorption porosimetry, mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), as well as scanning electron microscopy (SEM) were used for material characterization.

Polycrystalline Cd 1 À x Zn x Te thick films with thicknesses of about 30 μm have been deposited on a Mo coated glass substrate by means of close-spaced vacuum sublimation technique. X-ray diffraction measurements have shown that the films obtained have only cubic zinc blende phase. The influence of Zn concentration on the photoluminescence (PL) spectra of Cd 1 À x Zn x Te films was investigated. This let us determine the nature and energy structure of the intrinsic defects and residual impurities in the films. The presence of the most intense acceptor bound exciton A°X-line for x ¼ 0.10 and the lines of localized excitons (x ¼ 0.32 À 0.44) in PL spectra of Cd 1 À x Zn x Te films indicates their fairly good optical quality as well as the p-type conductivity. There were also other intensive broad PL bands, caused by the recombination of donor-acceptor pairs involving complex acceptor centers, extended defects of dislocation type, and microstress in the films. It was also established a correlation between the broadening of exciton lines and the values of microstress in Cd 1 À x Zn x Te thick films. Taking into account the energy position of exciton lines, the concentration dependence of the band gap for the Cd 1 À x Zn x Te thick films is presented.