C. Leach

NO2 sensitivity of a heterojunction sensor based on WO3 and doped SnO2

Abstract The NO 2 sensing properties of a heterojunction gas sensor formed between WO 3 and 3 wt.% Nd 2 O 3 doped SnO 2 were evaluated using DC and AC measurements at 300xa0°C and compared with the responses obtained from single component tungsten oxide and doped tin oxide sensors. The heterojunction sensor showed rectifying behaviour with a forward bias direction defined as SnO 2 − /WO 3 + . The heterojunction sensor and the component materials were all found to be sensitive to NO 2 , showing almost linear resistance increases with concentration when exposed to 0–5 ppm NO 2 . However, the sensitivity of the heterojunction sensor was almost an order of magnitude greater than either of the component sensors. Under reverse bias the effective sensor resistance did not change with NO 2 concentration. AC impedance spectroscopy demonstrated that the improved NO 2 response of the heterojunction sensor occurred primarily as a result of increases in the forward bias resistance of the heterojunction interface. The long-term stability of the heterojunction sensor was characterised over 53 days using continuous DC resistance measurements, under an applied forward bias voltage of 0.4 V. The sensor showed good stability, with low drift and a reproducible response to NO 2 in the concentration range 0–10 ppm throughout the test period.

Ceramic varistors based on ZnO-SnO2

Abstract Model ceramic varistor formulations based on 98% ZnO–SnO2 (plus 2% oxides of Bi, Co and Mn) were prepared by conventional powder processing routes; specimens were sintered at 1150–1275xa0°C. The product density increased with SnO2 content, but decreased with increase in sintering temperature. The microstructures contained ZnO, Zn2SnO4 spinel and very small bismuth rich phases; with increasing Sn content the spinel became the dominant phase. From I–V measurements, the non-linear coefficients were found to be in the range 24–32. For all compositions, the α values decreased with increasing sintering temperature; the maximum α value was obtained with samples containing 20 mol% SnO2 sintered at 1200xa0°C. Breakdown fields were in the range 3000–11000 V/cm and increased with increasing tin content. Leakage currents were in the range 1–11 μA. Tin substitution for Zn appears to cause a strong donor effect.

Problems associated with imaging resistive barriers in BaTiO3 PTC ceramics using the SEM conductive mode

Abstract The micro-electrical properties of BaTiO3 have been studied using the remote electron beam induced current technique. Difficulties associated with applying this technique to low resistance, inhomogeneous ceramics are discussed and illustrated with the example of PTCR materials. At temperatures above the Curie temperature, contrast steps corresponding to resistive interfaces are observed with differing sensitivities according to the imaging conditions used. The image noise decreases and the current sensitivity increases with the sample resistance. Remote electron beam induced current contrast in BaTiO3, showing the high resistivity transition does not occur uniformly as a function of temperature across all grain boundaries.

Structural study of Ca0.7Nd0.3Ti0.7Al0.3O3 dielectric ceramics using synchrotron X-ray diffraction

Abstract The crystal structure of Ca 0.7 Nd 0.3 Ti 0.7 Al 0.3 O 3 has been determined using synchrotron X-ray diffraction. Data were collected from a polycrystalline sample for Rietveld analysis. At room temperature, the structure exhibits an octahedral tilted framework (perovskite-type) with an orthorhombic lattice ( a =5.3803 A, b =5.4003 A and c =7.6140 A). The positions of the atoms in the unit cell and their isotropic temperature factors have been determined, together with the occupations of sites by Ca/Nd and Al/Ti atoms. The structure was determined to feature an a − a − c + orthorhombic tilt system in association with displacement of the A-site cations.

Grain boundary SEM conductive mode contrast effects in additive free zinc oxide ceramics

Conductive mode imaging of sintered zinc oxide has been carried out in the SEM using the remote electron beam induced current mode. The use of this technique enables the electrical effects of built-in fields and resistive barriers in the material to be imaged in the microstructure. Two contrast types were observed at grain boundaries where such fields or barriers were active; parallel bright and dark lines, or single bright/dark lines. Possible structures giving rise to these contrast effects are discussed.

Microwave dielectric ceramics in the system BaO–Li2O–Nd2O3–TiO2

Abstract Ceramics in the system BaO-Li 2 O–Nd 2 O 3 –TiO 2 (BNT–LNT) were prepared by the mixed oxide route. Powders were mixed, milled, calcined and sintered at 1475°C for 4xa0h. Fired densities decreased steadily along the series from BNT to LNT. The microstructures of samples rich in BNT were dominated by small needle-like grains; the LNT samples comprised larger (6xa0μm) cubic grains. X-ray diffraction showed that there was a transition from orthorhombic BNT to cubic LNT; small amounts of LNT could be accommodated in BNT, but between 10–20% LNT there was the development of the second phase. Small additions of LNT led to a small increase in relative permittivity, but decreased the dielectric Q -value (from the maximum of 1819 at 4 GHz). As BNT and LNT exhibit negative and positive temperature dependencies of permittivity respectively, the addition of 10–20% LNT to BNT should yield samples with zero temperature dependence of e r Impedance spectroscopy showed that data could only be acquired at elevated temperatures for BNT rich samples (above 500°C), but at modest temperatures (less than 100°C) for the more conductive LNT.

A time resolved study of the response of a WO3 gas sensor to NO2 using AC impedance spectroscopy

Abstract The transient response of a tungsten oxide (WO 3 ) based gas sensor to 1.5xa0ppm nitrogen dioxide (NO 2 ) was studied using AC impedance spectroscopy. It was found that the time taken for the sensor to re-equilibrate fully at 300xa0°C after the introduction of the target gas was 5xa0h. Changes in the material’s resistance, capacitance and grain boundary barrier height in response to the introduction of the NO 2 , were characterised by analysis of AC impedance spectra using a combination of Nyquist and complex modulus plots. The grain boundary resistivity was found to increase substantially on introduction of the NO 2 , together with a small, associated increase in the bulk resistivity. The grain boundary capacitance remained constant, within experimental error, throughout the equilibration period. The grain boundary barrier height was found to increase from 212 in air to a stable final value of 291xa0meV after the gas was introduced.

The effect of sintering temperature variations on the development of electrically active interfaces in zinc oxide based varistors

Abstract A series of zinc oxide based varistors containing 0.5 wt.% Bi 2 O 3 and 0.5 wt.% Mn 2 O 3 were prepared by a conventional mixed oxide route and sintered at temperatures between 950 and 1300°C. All samples showed the varistor effect, although as the sintering temperature was increased above 1000°C, the non-linear coefficient decreased from 22 to 3 at 1300°C. Local grain boundary property measurements were carried out using remote electron beam induced current (REBIC) configuration conductive mode scanning electron microscopy. The proportions of electrically active interfaces and those showing strong resistive contrast were found to increase with sintering temperature.

Direct observation of resistive barriers in a BaTiO3 based thermistor

The remote electron beam induced current technique has been used to form resistive contrast images of the resistive barriers developed in 80 °C thermistor materials. Linescans extracted from the images have been used to calculate the temperature variation of resistivity of selected interfaces within the material. It is seen that the major resistive changes are associated with only a few of the grain boundaries in this material.

Synchrotron x-ray diffraction study of Ba4.5Nd9Ti18O54 microwave dielectric ceramics at 10-295 k

The structure of ceramic Ba 4.5 Nd 9 Ti 18 O 54 was investigated by synchrotron x-ray powder diffraction from 10 to 295 K. Reitveld refinement and Le Bail profile analysis were applied to the data. Based on an orthorhombic structure, unit cell parameters of a = 22.3479(3) A, b = 7.6955(1) A, and c = 12.2021(2) A were obtained at room temperature and a = 22.3367(5) A, b = 7.6738(1) A, and c = 12.1842(3) A at 10 K. No evidence was found for any major structural change from 10 to 295 K. Within the tungsten bronze framework the two pentagonal channels were fully occupied by Ba; the remaining Ba atoms shared the rhombic channels with Nd. Thermal expansion of the unit cell was found to be anisotropic. The largest expansion occurs along the b cell edge, and the least along the a cell edge. It is proposed that the anisotropy is due to enhanced bending of the TiO 6 polyhedra chains along the b direction.