Continuous-feed nanocasting process for the synthesis of bismuth nanowire composites

We present a novel, continuous-feed nanocasting procedure for the synthesis of bismuth nanowire structures embedded in the pores of a mesoporous silica template. The immobilization of a bismuth salt inside the silica template from a diluted metal salt solution yields a sufficiently high loading to obtain electrically conducting bulk nanowire composite samples after reduction and sintering the nanocomposite powders. Electrical resistivity measurements of sintered bismuth nanowires embedded in the silica template reveal size-quantization effects

ZrO2-ZrW2O8 Composites with tailor-made thermal expansion

Most of the materials expand upon heating. There are a few families of materials which exhibit negative thermal expansion (NTE). ZrW2O8 is an example which gained a lot of interest in international literature recently. This cubic material has an exceptionally large and isotropic negative thermal expansion over its entire stability range (0.5 to 1050 K). At 430 K a phase transition occurs from a-ZrW2O8 (a = -9.1 x 10-6 K-1) to b-ZrW2O8 (b = -5.4 x 10-6 K-1). At high pressures an orthorhombic phase is formed, g-ZrW2O8, which possesses a small negative expansion coefficient. A broad range of applications have been suggested for these NTE materials. In composites, their thermal expansion coefficient can be tailor-made by combining a NTE material with a positive expansion material. Adjusting the volume fraction of the different phases results in a positive, negative or even zero thermal expansion. The ZrW2O8 - ZrO2 - composites studied in this paper were prepared in two ways. The first synthesis method applied, started from off-stoichiometry mixtures of the pure oxide powders of ZrO2 and WO3. This novel in situ process included a heating step up to 1450 K which combines the formation and sintering of ZrW2O8. In the conventional synthesis the starting materials were ZrO2 and ZrW2O8. ZrW2O8 was first obtained using an optimised spray drying technique. Obviously, our "in situ" method does not require such an additional step. The crystal structure, morphology, thermal expansion behaviour and mechanical properties of these composites were tested and compared

Pair Distribution Function Analysis of ZrO2 Nanocrystals and Insights in the Formation of ZrO2-YBa2Cu3O7 Nanocomposites

The formation of superconducting nanocomposites from preformed nanocrystals is still not well understood. Here, we examine the case of ZrO2 nanocrystals in a YBa2Cu3O7-x matrix. First we analyzed the preformed ZrO2 nanocrystals via atomic pair distribution function analysis and found that the nanocrystals have a distorted tetragonal crystal structure. Second, we investigated the influence of various surface ligands attached to the ZrO2 nanocrystals on the distribution of metal ions in the pyrolyzed matrix via secondary ion mass spectroscopy technique. The choice of stabilizing ligand is crucial in order to obtain good superconducting nanocomposite films with vortex pinning. Short, carboxylate based ligands lead to poor superconducting properties due to the inhomogeneity of metal content in the pyrolyzed matrix. Counter-intuitively, a phosphonate ligand with long chains does not disturb the growth of YBa2Cu3O7-x. Even more surprisingly, bisphosphonate polymeric ligands provide good colloidal stability in solution but do not prevent coagulation in the final film, resulting in poor pinning. These results thus shed light on the various stages of the superconducting nanocomposite formation

Systematic and Controllable Negative, Zero, and Positive Thermal Expansion in Cubic Zr1–xSnxMo2O8

We describe the synthesis and characterization of a family of materials, Zr1–xSnxMo2O8 (0 < x < 1), whose isotropic thermal expansion coefficient can be systematically varied from negative to zero to positive values. These materials allow tunable expansion in a single phase as opposed to using a composite system. Linear thermal expansion coefficients, αl, ranging from −7.9(2) × 10–6 to +5.9(2) × 10–6 K–1 (12–500 K) can be achieved across the series; contraction and expansion limits are of the same order of magnitude as the expansion of typical ceramics. We also report the various structures and thermal expansion of “cubic” SnMo2O8, and we use time- and temperature-dependent diffraction studies to describe a series of phase transitions between different ordered and disordered states of this material

Calibrated quantitative thermogravimetric analysis for the determination of portlandite and calcite content in hydrated cementitious systems

Portlandite and calcite are compounds of interest regarding different processes related to microstructural and durability issues of cementitious materials, such as carbonation, pozzolanic action, and hydration degree. The quantification of their contents in cementitious systems is thus frequently required. Thermogravimetry (TG) measures the change in mass of a material (as a function of time) over a temperature range using a predetermined heating rate. The TG method can be applied with certain success to estimate portlandite and calcite contents in the hydrated cement system, considering the temperature range at which each compound decomposes and releases water or carbon dioxide, respectively. However, a mature hydrated cement paste contains hydrated calcium silicate, portlandite and calcite phases. The quantification procedure is therefore complicated because of the concurrent interference among them. The tangential method over the TG signal or the integral method over the derivative TG curve is usually used to discount water loss from hydrated calcium silicates over the temperature range at which portlandite and calcite decompose. However, by the use of TG consistent underestimation of portlandite content in hydrated cementitious systems is still described in the literature. Potential causes for this underestimation are analysed in this paper, and a calibration procedure by means of an internal standard is proposed. Cement paste and aggregate samples are analysed. Differences between TG quantifications of these pure samples and those diluted with low contents of internal standards are compared with the added amounts of internal standard. In that way, a calibration method of the device is applied for correction of the actual portlandite and calcite contents in the samples. For this purpose, both analytic quality calcite and freshly prepared portlandite were used as internal standards. The results show that improved quantifications can be achieved with this calibration. Also, calcite seems to be more suitable as internal standard than portlandite as the best outcome was obtained for the first case.Fil: Villagrán Zaccardi, Yury Andrés. Provincia de Buenos Aires. Gobernacion. Comision de Investigaciones Cientificas. Laboratorio de Entrenamiento Multidisciplinario para la Investigación Tecnológica; Argentina. University of Ghent; BélgicaFil: Egüez Alava, H.. Escuela Superior Politécnica del Litoral; Ecuador. University of Ghent; BélgicaFil: De Buysser, K.. University of Ghent; BélgicaFil: Gruyaert, E.. University of Ghent; BélgicaFil: de Belie, Nele. University of Ghent; Bélgic

Structure and phase transition of Sn-substituted Zr(1-x)SnxW2O8

A conventional solid state reaction between ZrO2, SnO2 and WO3 was used to prepare the negative thermal expansion material Zr(1-x)SnxW2O8. The strong negative thermal expansion over a broad temperature range, which is well known for the pure zirconium tungstate compound, is also demonstrated in this substituted material. However, the order-disorder phase transition of the cubic materials was shown to shift towards lower temperatures, dependent on the degree of Sn4+-substitution, by dilatometry and temperature variable X-ray diffraction. This is attributed to the lower bond strength of the Sn-O bond in comparison to the Zr-O bond. The unit cell parameters of the material are significantly smaller due to the insertion of smaller Sn4+-cations on the Zr4+-position in the structure. For one composition (x = 0.3), the structure of Zr(1-x)SnxW2O8 was studied by neutron diffraction at two temperatures, 293 K and 473 K, corresponding to respectively the low temperature alpha-, and high temperature beta-polymorph of Zr(1-x)SnxW2O8. The refined structures were found to be similar to that of ZrW2O8 at the same temperatures. Variable temperature X-ray diffraction of the same sample was used to establish the phase transition temperature, by refining the fractional occupancy of the possible tungstate orientations with temperature