Judit Pfeifer

The observation of cubic tungsten trioxide at high-temperature dehydration of tungstic acid hydrate

Abstract The dehydration of tungstic acid hydrate, H2WO4·H2O samples prepared by the methods of Freedman [J. Am. Chem. Soc. 81 (1959) 3834.] ([Na]residual≤10 ppm)) and Zocher and Jacobson [Kolloidchem. Beih. 28–6 (1929) 167.] ([Na]residual=140 ppm) has been studied by in situ high-temperature XRD, thermogravimetry and FTIR at room temperature. The formation of cubic tungsten oxide, c-WO3 has been observed in the course of the dehydration at the loss of the second water molecule: H2WO4→c-WO3. Keeping the temperature constant at 300°C under air, the metastable c-WO3 transforms into the stable orthorhombic, o-WO3 phase. FTIR and TG results have shown that the 300°C dehydration products contained hydrogen in the form of OH groups and bronze-like W–O–H even after the loss of the second water molecule. A H2WO4·H2O sample embedded in silicone grease has also been studied by high-temperature XRD, TG and FTIR. The c-WO3→o-WO3 conversion was blocked in this sample, and FTIR has indicated the presence of CO vibrations in the transmittance spectrum. Lattice parameters of c-WO3 samples produced from H2WO4·H2O (this work), and results from the literature (from other precursors) have been compared with the conclusion, that the formation of closely monophase c-WO3 and the relative stability of this phase are in correlation with the presence of some kind of impurity (originating from silicone grease in this experiment and from various organic or inorganic precursors applied at other laboratories).

Structure and morphology changes caused by wash treatment of tungstic acid precipitates

Abstract Crystalline H 2 WO 4 .H 2 O, (WO 3 .2H 2 O) powder samples have been prepared by acidification of alkali tungstate aqueous solution. In order to decrease the alkali ion content of the precipitate from about 6×10 5 to 10–10 2 ppm, wash treatments have been applied. The influence of the wash treatment on the structure and hydration state of the solid product has been investigated by X-ray powder diffraction (XRD) and infrared absorption spectroscopy (IR). Morphology of the precipitate and the time development of the morphology during washing steps have been studied by scanning electron microscopy (SEM). The formation and transformation of an intermediate, the so-called tungstic acid C phase have been observed in the sequence of the washing steps. Reversible changes in the hydration state: WO 3 .2H 2 O↔WO 3 .H 2 O have also been observed at washing with long time interaction between crystallites and washing liquid with pH≥4.6.

Development of tungsten oxide hydrate phases during precipitation, room temperature ripening and hydrothermal treatment

Abstract Morphology and structure transformation of tungstic acid hydrate, H 2 WO 4 · n H 2 O precipitates, have been studied in aqueous phase with various levels of residual [Na + ] s at room temperature and at 120 °C in autoclave. Spindle-shaped morphology is characteristic for the washing product of the freshly prepared precipitates. Residual sodium is a persistent contaminant in H 2 WO 4 ·H 2 O. Sodium ion content in the solid phase has been found to control reaction route at hydrothermal treatment of tungstic acid hydrates. Morphological changes against [Na + ] s content at 120 °C are significant. Rectangular crystallites of WO 3 ·H 2 O, hexagonal platelets of WO 3 ·1/3H 2 O or their mixture can develop from the precursor spindle-shaped H 2 WO 4 ·H 2 O particles with various [Na + ] s at 120 °C. It is noteworthy that particles that ripened at room temperature display faces and axes seemingly corresponding to those of the crystallites developed in the hydrothermal process.

Synthesis and Sensing Properties to NH3 of Hexagonal WO3 Metastable Nanopowders

WO3 is an important kind of wide-bandgap semiconducting metal oxides, which has a very promising property in gas-detection behavior. It has several polymorphs with triclinic, monoclinic, orthorhombic structures being the stable forms of this oxide. However, by a method known as acid precipitation, new metastable open crystalline forms with hexagonal structure have been successfully synthesized. The nanopowders were characterized by SEM, TEM, and XRD, and their sensing response to reducing gases (NH3) was measured and compared to monoclinic WO3, showing a much better sensing property of the hexagonal WO3.

Tribology Study of Silicon Nitride-Based Nanocomposites with Carbon Additions

Tribology tests were conducted on silicon nitride-based nanocomposites with various carbon additions to explore the effects of microstructure, the type and quantity of carbon additives and the preparation routes on the behavior. The nanocomposites consisted of Si3N4 and C in the proportions of 1 – 10 wt % carbon nanotube (CNT), or carbon black (CB), or graphite, or graphene. Specimens were produced by hot isostatic pressing. X-ray diffraction and scanning electron microscopy were used to reveal phase composition and microstructure. Unlubricated ball-on-disk tribology tests with silicon nitride counter face were carried out at room temperature in ambient atmosphere. Contact profilometer was used to profile the wear tracks. The friction coefficients of the pure Si3N4 and Si3N4 samples with 3% CNT varied between 0,77-0,81. Addition of 10% graphite and 3% CB to Si3N4 resulted in friction coefficients of 0,83 and 0,72 respectively. Si3N4 samples with 3% graphene showed distinctly lower friction levels of 0,52 and smaller scatter of the measured values. The wear track study revealed that high graphene content in the Si3N4 matrix caused relatively big wear particles and an uneven wear track.

The influence of residual sodium on the formation and reductive decomposition of hexagonal tungsten oxide

The metastable hexagonal form of tungsten trioxide (h-WO3) has recently been attracting much interest [1]. The synthesis routes of h-WO3 involve the dehydration of tungstic oxide hydrates, WO3 · nH2O (n= 0.3 [2, 3]; n= 0.9 [4]) or the oxidation of ammonium tungsten bronze [5–7], or ion-exchanging templating ions from hexagonal bronze [8]. Regarding the role and effect of possible residual chemical impurities, considerable uncertainty has remained. In this laboratory, we have prepared the oxide–hydrate precursor, WO3 · 1/3H2O by the route Na2WO4 · 2H2O→ H2WO4 ·H2O → WO3 · 1/3H2O [2, 9, 10]. According to a previous report, the formation of WO3 · 1/3H2O requires the presence of sodium in the reaction system, resulting in sodium incorporation in the WO3 · 1/3H2O phase [11]. This work focuses on the kinetic influence of sodium on the transformation/dehydration of WO3 · 1/3H2O into h-WO3 and on the hydrogen reduction of h-WO3 into elemental tungsten. The dehydration experiments of WO3 · 1/3H2O (of varying sodium contents) into h-WO3 were carried out either at isothermal conditions (300 ◦C, 15, 30 and 90 min, N2–O2 (80–20%) atmosphere) or in thermal gravimetry equipment (Mettler, Registrierender Vakuum-Thermoanalyzer, H2 (5N) ambient) where oxygen loss from the oxide during H2 reduction was also followed. X-ray powder patterns were recorded (at room temperature in a Guinier focusing camera using CuKα radiation, λ = 0.154051 nm) after each isothermal heat treatment and thermal analysis. The positions of the reflection lines on the film were determined by computer-controlled densitometric analysis [12]. Lattice parameters were obtained from the least-squares refinement of the d values of 23–26 reflections in the range 13◦ < 22 < 30◦. The calculated cell parameters for h-WO3 (JCPDS card 33-1387) samples obtained by dehydration of WO3 · 1/3H2O with [Na]= 163– 3420 ppm are collected in Table I. Morphology and grain size of h-WO3 and elemental tungsten were studied by scanning electron microscopy (SEM; Jeol 25). Transparent samples for transmission electron microscopy (TEM; Philips CM 20, operating at 200 kV) were prepared by depositing dispersed crystallites onto a holder, or by ion milling [14, 15]. TEM investigations reveal streaks in the diffraction pattern of h-WO3 (Fig. 1) showing the presence of stacking faults in the lattice. Second phases, precipitates or preferred sites for the allocation of sodium (Na+, Na2O) other than in the hexagonal channel (as suggested in [16]) have not been found; all samples of varying sodium contents, however, have been examined. Sodium concentration of the WO3 · 1/3H2O samples was found to influence the time of heat treatment needed for the dehydration/phase transformation as is demonstrated on XRD spectra and thermogravimetric (TG) plots. Details of the XRD spectra of samples isothermally heated for varying times show the “shift” of the 0 0 4 reflection of the orthorhombic WO3 · 1/3H2O to the position of the 0 0 2 reflection peak of h-WO3 (Fig. 2). This shift, indicating the dehydration of samples, is observed to be faster for the sample with 3400 ppm of sodium than that of samples with 160 and 1050 ppm. In the XRD patterns of the intermediate samples, temporary new peaks also appeared, which cannot be identified either among WO3 · 1/3H2O or h-WO3 peaks. TG curves of the dehydration of WO3 · 1/3H2O samples are plotted in Fig. 3a. In accordance with the observations at isothermal treatment shown in Fig. 2, the influence of the sodium content in the solid phase was also observed on the rate of dehydration. The WO3 · 1/3H2O sample with 160 ppm of sodium dehydrates with the minimal velocity. The dehydration velocity of the samples with 1000 and 3400 ppm of sodium increases with increasing sodium content. Samples with intermediate levels of sodium, 495 and 740 ppm, however, exhibit a greater dehydration velocity than samples with

Long-term behaviour of tungsten oxide hydrate gels in an alkali containing aqueous environment

Abstract The room temperature ageing process of tungsten oxide dihydrate grains (H 2 WO 4 ·H 2 O) prepared by the methods of Zocher and Jacobson (Kolloidchem. Beih. 28 (1929) 167) and Freedman (J. Am. Chem. Soc. 81 (1959) 3834) has been studied. A comparison between these two preparation methods and the ageing characteristics is reported. Zocher and Jacobson-type grains, spindle like in shape, have a strong tendency to morphological conversion depending on the pH value of the surrounding aqueous solution. Under the same conditions the Freedman-type irregularly shaped particles preserve their morphology. Scanning electron microscopy and Fourier transform infrared spectroscopy studies have shown that this structural and morphological stability of Freedman-type grains can be altered by the presence of alkaline (Li + , Na + , K + ) and NH 4 + ions. A limited incorporation of cations into the tungstate structure was observed.

Nanostructured hexagonal tungsten oxides for ammonia sensing

Among the family of wide band-gap semiconducting metal oxides, tungsten trioxide is the most promising oxide for gas sensing. A metastable open-structured hexagonal phase of WO3 was successfully synthesized using acid precipitation method. The oxide was characterized using SEM, TEM and XRD. The sensing property to reducing ammonia gas was measured. The sensitivity is much higher than that of monoclinic tungsten oxides. The oxide polymorph exhibits a p-n type transition when the temperature goes from 100 °C to 300 °C.

Synthesis and Examination of Hexagonal Tungsten Oxide Nanocrystals for Electrochromic and Sensing Applications

Tungsten oxides are among the most used materials in hazardous gas detection. In this work, a soft chemical nanocrystalline processing route was demonstrated for the preparation of hexagonal tungsten oxides. The structure of parent phases was studied by scanning and transmission electron microscopy and by X-ray diffraction.

Electrospinning – A Candidate for Fabrication of Semiconducting Tungsten Oxide Nanofibers

The excellent gas sensing properties of the tungsten oxides have been manifested first of all in nanostructure and 1D, and 2D open structured forms. For optimal performance the sensing layer substrates should be of large specific surface. In this paper we report on electrospinning – a candidate for fabrication of large specific surface tungsten oxide nanofibers. Fibrous tissues doped with tungstic acid hydrate (H2WO4.H2O) and tungsten oxide one third hydrate (WO3.1/3H2O) has been created and characterized by X-ray diffraction, scanning electron microscope and energy dispersive spectroscopy in order to learn about the changes the materials suffer during the process.