Zinc oxide containing either Ni or Co dopants in various proportions has been investigated. Only zincite phase was found by XRD analysis. Incorporation of Ni or Co into the ZnO structure resulted in a marked shift in the XRD peaks. Remarkable microstructural uniformity was obtained by the addition of 1 mol-% of either Ni or Co oxides. Densification was achieved through atomic diffusion along grain boundaries. All samples showed non-linear I-V dependence. Ni concentration had a non-monotonic effect on breakdown voltage and the non-linear coefficient decreased with increasing Ni content. In contrast, breakdown voltage and non-linear coefficient increased with increasing CoO content.
The oxygen permeability of oxide composite membranes containing similar volume fractions of the components, including (La0.9Sr0.1)(0.98)Ga0.8Mg0.2O3-delta (LSGM)-La0.8Sr0.2Fe0.8Co0.2O3-delta (LSFC), LSGM-La2Ni0.8Cu0.2O4+delta (LNC), SrCoO3-delta-Sr2Fe3O6.5+/-delta, Ce0.8Gd0.2O2-delta (CGO)-LSFC and CGO-La0.7Sr0.3MnO3-delta (LSM), was studied at 973-1223 K. In most cases, oxygen transport is substantially affected by component interaction, decreasing ionic conductivity due to cation interdiffusion, and formation of intermediate phases and/or blocking layers at grain boundaries. This interaction is maximised in systems where the phase components have similar structure and thus may form continuous solid solutions, for example LSGM-LSFC, or intermediate compounds such as Roddlesden-Popper phases in LSGM-LNC composites. The results show that, in addition to knowledge of the transport properties and volume fractions of percolating phases, analysis of ionic conduction in oxide composite materials requires assessment of phase interaction and grain boundary processes.
The addition of small amounts (0-25 wt-%) of calcium aluminate cement (CA C) to water greatly accelerates the hydration of Al powder. This behaviour is attributable to the increased basicity of the solution owing to the CAC leading to dissolution of the native Al2O3 film and early hydration products, as well as additional heat released by the accelerated hydration of A I and CA C. The significance of this for the use of Al powder in CAC bonded refractory castables is discussed.
Safety linings of steel ladles are made of high content alumina castable. In situ observations confirm that during drying a network of macrocracks appears in the monolithic castable lining, owing to both dehydration of the cement paste and temperature. Crack formation kinetics have been characterised over the life of the lining. To simulate the behaviour of the lining, the thermomechanical behaviour of the castable was characterised in compression and three point bend tests, thermal dilatation tests and TGA analysis in the 20-600degreesC temperature range. The change in properties is shown to be due mainly to transformation of hydrates in the castable. Numerical simulations of the lining drying process were also performed using a finite element code to verify the causes of cracking. A parametric study was carried out to examine the possibility of reducing the stresses induced by thermal loading. Improved thermal insulation and better control of heating reduce the risk of failure in the lining.
The microstructures of silicate layers on Si3N4 ceramics formed by oxidation or by spray coating with SiO2 and subsequent annealing were compared in order to clarify the effects of oxidation on pore formation and crystallisation in the different surface layers. In samples containing Al2O3/Y2O3, intensive separation and dissolution of sialon grains from the bulk into the oxidised layer was observed, and this is likely to be one of the reasons for the lower oxidation resistance of Si3N4 sintered with Y2O3/Al2O3 than with MgO/Y2O3. Oxygen supplied from the atmosphere plays an important role in bubble formation and in the crystallisation of cristobalite in the silicate layer. The silicate coating layer inhibits the oxidation of Si3N4 effectively up to 1200°C. At 1300°C, uncoated samples gained 0·75 by weight after oxidation for 500 min, whereas the weight gain of spray coated Si3N4 was 0·4% under the same conditions.
Whiskers of beta-SiC have been produced on the surfaces of polycrystalline SiC fibres by carbothermal synthesis. The growth of whiskers occurred simultaneously with siliconisation of the carbon fibres. The chemical composition, structure and morphology of the whiskers were studied using X-ray diffraction (XRD), Raman microspectroscopy, electron energy loss spectroscopy (EELS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that the presence of silica on the surface of initially carbonised fibres was essential for whiskerisation. Nucleation and growth of the whiskers followed the vapour-liquid-solid (VLS) mechanism. The diameter of the whiskers produced ranged from 100 to 300 nm and the average aspect ratio was about 300. Owing to the small size of the whiskers, they are expected to provide more efficient reinforcement in composites than conventional whiskers of larger diameter.
A drilling procedure for obtaining superconducting rings from top seeded melt grown samples is reported. The method described is based on the use of a column drilling machine with a copper–diamond drill bit. Modifications have been made in order to avoid structural damage to the superconductor during drilling. The success of the technique was assessed by measuring the critical current of samples before and after machining, with similar values obtained in both cases.
Pyroelectricity was probably first observed by the Greeks more than 24 centuries ago. The philosopher Theophrastus wrote that lyngourion (most likely the mineral tourmaline) hod the property of attracting straws and bits of wood. For two millennia the peculiar properties of tourmaline were more a part of mythology than of science. In the eighteenth century pyroeleetric studies made a major contribution to the development of our understanding of electrostatics. In the nineteenth, research on pyroelectricity added to our knowledge of mineralogy, thermodynamics and crystal physics. Pyroelectricity gave birth to piezoelectricity in 1880, and ferroelectricity in 1920. The field of pyroelectricity flourished in the twentieth century with man-ill applications, particularly in infrared detection and thermal imaging. Pyroelectric sensors have been carried on many space missions and have contributed significantly to our astronomical knowledge.
One of the residues or byproducts generated in coal burning power plants that consume pulverised solid fuels is fly ash. Owing to its morphological characteristics, physicochemical properties and pozzolanic activity, this residue has potential for use in the production of refractory insulating bricks in combination with clays, a binder (sodium silicate) and a foaming agent (hydrogen peroxide 50%). The bricks thus obtained present the appropriate characteristics of mechanical resistance, porosity and thermal conductivity.
Zinc oxide samples with different contents of Sb2O3 with or without Cr2O3 and CoO have been prepared by conventional ceramic processing for use as varistors. Reaction sintering was performed at 1200 and 1300degreesC X-ray studies indicate that the mixtures contained, mainly zincite phase together with two different chemical formulas of spinel-like structure, [beta-Zn7Sb2O12 and Zn2.33Sb0.67O4, at 1200 and 1300degreesC respectively. ZnCr2O4 and CoCr2O4 were also formed as spinel-like structures besides Zn2.33Sb0.67O4 and zincite phase in the mixtures doped with Cr2O3 and CoO at 1200 and 1300degreesC Variation in current-voltage (I-V) characteristics is discussed in terms of microstructural and crystallographic changes in the varistors. The addition Of Cr2O3 and CoO to the ZnO varistors containing Sb2O3 has been confirmed to be effective in enhancing non-linear behaviour.
Semiprocessed kaolin clay was sieved through sieves of various particle size ranges. Particle size distribution was determined using a Malvern laser particle sizer and kaolinite content in each particle size range was determined by X-ray diffractometry. It was found that kaolinite content increased with decreasing particle size range.
The present study was carried out on the matrix composition of spinel forming high alumina castable to determine the effects of silica, magnesia particle size and temperature on spinel formation, the alumina content of the spinel formed, and associated physical characteristics at high temperature. It was found that an increase in silica content and use of finer magnesia helps to increase spinel formation. However to increase the alumina content of the spinel, increased temperature and magnesia particle size were more effective than an increased amount of silica. Furthermore the permanent linear change and porosity of the matrix are adversely affected when the amount of silica is increased or when magnesia is added in the form of fines. These findings suggest that medium and fine magnesia particles and an optimum amount of silica in the matrix composition of a spinel forming castable are necessary to obtain a matrix with increased alumina rich spinel content with controlled permanent linear change and porosity when exposed to high temperature during service.
There has been a surge of interest in polymer-clay nanocomposites over the past decade. This review surveys these new materials with emphasis on the debate surrounding interaction mechanisms and the behaviour of intercalated polymer in clay galleries. Swelling properties, high cation exchange capacities, high aspect ratio and large surface area give smectite clays the new role of high performance filler for thermoplastic or thermosetting polymers for the creation of intercalated or exfoliated nanocomposites. These nanocomposites can be prepared by three routes: in situ polymerisation, solution methods, or melt processing. Modification of either clay or polymer can change the type of polymer-clay composite. X-ray diffraction and transmission electron microscopy are often employed as the main characterisation techniques to establish the state of the clay. A very low volume fraction of clay significantly improves the mechanical and barrier properties of the pristine polymer, which makes these nanocomposites very promising materials.
Aluminium powders with different average particle sizes were synthesised into aluminium nitride in nitrogen atmosphere using simultaneous thermal analysis (TG/DTA). Experiments showed that reaction of Al powders started below the melting point of aluminium (660degreesC) in the solid-gas state, however most of the reaction took place above the melting point, in the liquid gas state. Further conversion of Al to AlN took place up to 1200degreesC. The influence of particle size on the nitridation process was demonstrated by the use of different Al powders. Microstructural characterisation showed that two kinds of whisker were formed in the original Al powder bed, one with wool-like morphology and the other with needlelike morphology and sharp changes of direction.
parameter in the free energy expansion, phenomenological theory can also describe inhomogeneous structures appearPhenomenological theories for ferroelectrics are based ing in the ferroelectric phase transition, such as twins, on L andau–Devonshire type models, which can be conantiphase structures and domain walls. Here two examples structed based on thermodynamic principles and symwill be used to show the procedure and the power of metry relationships between parent and product phases. phenomenological theories in multidimensions for describPhenomenological theories can be directly linked to ing domain walls in ferroelectric systems and antiferroelecmeasurable macroscopic quantities and at the same tric phase transitions. time preserve microscopic information. T he competition between gradient energy and non-linear energy in Ginzburg–L andau type models provides an interpretFREE ENERGY BASED ON CRYSTAL ation of inhomogeneous structures in ferroelectrics, for SYMMETRY example twins and domain walls. Phenomenological In phenomenological theories, the key quantity is the order models can be extended to antiferroelectric systems parameter. A proper ferroelectric phase transition is driven using a multicomponent order parameter related to by a zone centre transverse optical soft mode, which is microscopic local dipoles, rather than the average related to the amplitude of the polarisation. Thus a ferropolarisation vector. Such a model can provide a more electric system is a textbook example of the Landau theory rigorous description of the antiferroelectric phase transof phase transitions.3–5 Following Landau’s ideas for a ition. BCT /704 second order phase transition,6,7 the free energy density is written in terms of a polynomial expansion of the polaris
Alloys of PbZrO3-PbTiO3 (PZT) exhibit a complex series of ferroic lattice instabilities that depend on the Zr/Ti ratio and give rise to exceptional pyroelectric and piezoelectric properties. An approach based on energetics and crystal chemical systematics is discussed that relates structure and bonding in PZT to its subsolidus phase diagram. Heats of formation, heat contents and contributions to lattice energies across the phase diagram tire correlated with the perovskite tolerance factor and with displacements of Pb and (Zr, Ti) cations from their high symmetry positions. It is shown that at 973 K the cubic phase contains precursors of the lattice instabilities and the differing displacements of Ti and Zr inferred for the lower symmetry phases at lower temperatures. Modulations in local structure, revealed in the ratio of Pb to (Zr, Ti) cation displacements, are related to the transition temperature front the paraelectric cubic phase to the ferroic phases.
For the past 40 years, Eric Cross has been the worldwide leader of the international ferroelectrics community, organising meetings, advising students and colleagues, and building up an outstanding research programme tit Penn State University. This paper describes the beginnings of the Penn State programme and some of the people involved, including George Brindley, Wilhelm Buessem, Ray Pepinsky and Rustum Roy. Three important events in its history were the work on multilayer capacitors with Erie Technological Products and Sprague Electric leading to the formation of the NSF Center for Dielectric Studies; the longstanding programme on piezoelectric transducers sponsored by the ONR which led to new families of composite ferroelectrics, to phased array ultrasound, and to near perfect electromechanical coupling factors: and the electrostriction measurements on relaxor ferroelectrics which played a key role in the development of reliable active optics systems. These were exciting times, and I am proud to have been a member of the Penn State ferroelectrics group and to express in ' v gratitude to Eric Cross for his advice, leadership and friendship. He has a great mind and a big heart.
The aim of this work was to study the effect of Nd3+ doping on the structural, dielectric and electrical properties of Pb(SnTi)O-3 ceramics, to give materials with the general formula (Pb1-xNdx)(Sn0.45Ti0.55)(1-x/4)O-3 (x = 0, 0.05, 0.07, 0.1). These materials were synthesised using a conventional high temperature solid state reaction technique. X-ray diffraction studies at room temperature revealed orthorhombic symmetry, and uniform grain size distribution throughout the surface of the samples for x = 0.05-0.1 was observed by SEM. Study of the dielectric behaviour of compounds as a function of temperature shows that with increasing concentration of Nd3+, the dielectric constant maximum decreases; transition temperature T-c shifts to lower temperatures for x = 0-0.07 but increases again at x = 0.1. It is observed that at each concentration of Nd3+ the dielectric constant epsilon' reaches a maximum at the same Curie temperature for all three test frequencies (1, 10, 100 kHz), which reveals the non-relaxor ferroelectric behaviour of these materials.
Growth of ZnO thin films with c axis (002) orientation has been demonstrated at room temperature on Si(100) SiO2, and amorphous ZnO substrates by the if magnetron sputtering method. The structural properties of the ZnO thin films were investigated with varying if power. X-ray diffraction analysis revealed that increasing if power helped to increase the c axis lattice constant and grain size, regardless of substrate material. Scanning electron microscopy indicated that the structural morphology of the ZnO films was not dependent on the substrate material.
Low cost ceramic moulding composites (CMCs) are composed of inorganic metal silicates, fillers and different types of chopped fibre reinforcements such as glass fibres (AR50/1 or AR62/2). The impact properties of CMCs were investigated by instrumented falling weight impact testing. Variations in impact energy absorption were examined as a function of fibre length and volume fraction. Processing and moulding conditions were also studied, revealing differences in interfacial shear strength between matrix and rein for different moulding temperatures were cement when used This in turn induced differences in energy absorption during impact testing.
