Browsing by Author "Shaw, Christopher P."

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    Characterisation of lead barium zirconate thin films for utilisation of the electrocaloric effect
    (Elsevier, 2016-05-09) Shaw, Christopher P.; Zhang, Qi; Correia, T. M.; Weaver, P. M.
    Characterisation of a composition of Lead Barium Zirconate thin films deposited by a spin coating sol-gel process onto platinised silicon substrates has been undertaken to investigate a potential application utilising the reported electrocaloric properties of the material. Xray diffraction studies revealed that above 700 °C the presence of pyrochlore phase was replaced by ferroelectric phase but the identification of antiferroelectric phases previously reported could not be identified. Wet etching of the films produced an insoluble residue containing only evidence of zirconium out of the original three metals, which resembled a skeletal structure similar to the original rosette morphology of the annealed films. Elemental analysis of the annealed films using EDX indicated a non-target Pb:Ba:Zr ratio, which suggested a reduced Zr content of the films. “Solution-like” rather than “sol-like” nature of the spin coated sol and phase boundary proximity are proposed for reported observations, and a mechanism is proposed to account for the unusual observed trend in dielectric constant.
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    The decrease of depolarization temperature and the improvement of pyroelectric properties by doping Ta in lead-free 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 ceramics
    (Elsevier, 2016-12-02) Balakt, Ahmed M.; Shaw, Christopher P.; Zhang, Qi
    Ta-doped lead-free 0.94NBT-0.06BT-xTa (x=0.0–1.0%) ceramics were synthesized by a conventional solid-state route. XRD shows that the compositions are at a morphotropic phase boundary where rhombohedral and tetragonal phases coexist. The depolarization temperature (Td) shifted to lower temperature with the increase of Ta content. The pyroelectric coefficient (p) of doped ceramics greatly enhanced compared with undoped material and reached a maximum of 7.14×10−4 C m−2 °C−1 at room temperature (RT) and 146.1×10−4 C m−2 °C−1 at Td at x=0.2%. The figure of merits, Fi and Fv, also showed a great improvement from 1.12×10−10 m v−1 and 0.021 m2 C−1 at x=0.0 to 2.55×10−10 m v−1 and 0.033 m2 C−1 at x=0.2% at RT. Furthermore, Fi and Fv show the huge improvement to 52.2×10−10 m v−1 and 0.48×10−10 m v−1 respectively at Td at x=0.2%. FC shows a value between 2.26 and 2.42 ×10−9 C cm−2 °C−1 at RT at x=0.2%. The improved pyroelectric properties make NBT-0.06BT-0.002Ta ceramics a promising infrared detector material.
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    Design of experiment for the optimisation of deep reactive ion etching of silicon inserts for micro-fabrication
    (SPIE, 2012-12-31) Wallis, K.; Shaw, Christopher P.; Alcock, Jeffrey R.
    The following paper describes a design of experiments investigation of the deep reactive of pillar structures on a silicon wafer. The etched wafers would subsequently be used as masters for the fabrication of nickel mould inserts for microinjection moulding. Undercuts occur when the pillar base has a smaller cross-section than the apex of the pillar. They therefore affect tolerances of the subsequent nickel mould, its strength and its de-mouldability from the silicon form. The response measured in these experiments was the degree of undercut of micro-scale (10 μm x 10 μm x 40 μm, 5 μm x 5 μm x 40 μm and 2 μm x 2 μm x 40 μm) The literature suggests that gas pressure, platen power, gas flow rate, phase switching times and mask size can all affect the degree of undercut. After examination of this literature, and of manufacturers guidelines, three parameters were selected for experimental testing: platen power, C 4F 8 gas flow rate during the passivation phase and switching times. Switching times was found to be the only statistically significant parameter for both 10x10 μm and 5x5 μm pillars. The 2x2 μm pillars were not successfully replicated and could therefore not undergo statistical evaluation.
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    Development aspects of an integrated pyroelectric array incorporating a thin PZT film and radiation collectors
    (Taylor and Francis, 2004) Shaw, Christopher P.; Landi, Spartaco; Whatmore, Roger W.; Kirby, Paul B.
    Abstract Work is being undertaken to produce an uncooled pyroelectric infra-red detector array which incorporates within its structure, an array of radiation collectors made by the bulk micro-machining of silicon. Development aspects of the processing route are presented here.
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    The effects of Ba2+ content on depolarization temperature and pyroelectric properties of lead-free 0.94Na0.5Bi0.5TiO3-0.06Ba1+xTiO3 ceramics
    (Springer, 2016-08-11) Balakt, Ahmed M.; Shaw, Christopher P.; Zhang, Qi
    Lead-free 0.94Na0.5Bi0.5TiO3–0.06Ba1+xTiO3 (NBT–0.06B1+xT) ceramics (0.0 ≤ x ≤ 0.03) were synthesized by a conventional solid-state reaction process. X-ray diffraction shows that the compositions are at the morphotropic phase boundary where rhombohedral and tetragonal phase coexist. Grain size slightly changes with the increase of Ba2+ content and reaches the minimum at x = 0.02. The depolarization temperature (Td) decreases with the extra Ba2+content but the lowest Td was obtained at x = 0.01–0.02. The pyroelectric coefficient (p) was measured as a function of Ba2+ content, and increased from 2.90 × 10−4 to 3.54 × 10−4 C m−2 °C−1, and from 55.3 × 10−4 to 740.7 × 10−4 C m−2 °C−1 for x = 0.00 and 0.02 at RT, and depolarization temperature (Td) respectively. The pyroelectric coefficient (p) shows a large increase with rising the temperature and reaches the maximum value at the depolarization temperature (Td). The figures of merits of Fi, Fv and FD have all been improved with the addition of extra barium. These improved pyroelectric properties indicate that NBT–0.06B1+xT is a promising material for pyroelectric applications or a wide range of temperature.
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    Electrical properties of Sb and Cr-doped PbZrO3-PbTiO3-PbMg1/3Nb2/3O3 ceramics
    (Elsevier, 2003-04) Whatmore, Roger W.; Molter, O.; Shaw, Christopher P.
    The pyroelectric, dielectric and DC resistive properties of Sb and Cr-doped ceramics with a base composition of Pb(Mg1/3Nb2/3)0.025(Zr0.825Ti0.175)0.975O3 have been studied. Sb doping has been shown to produce a linear reduction in Curie temperature (TC=−22z+294 °C) with concentration (z) and to give an increase in pyroelectric coefficient from 250 to 310 μCm−2 K−1 for z increasing from 0 to 3 at.%. It also produces first a reduction and then an increase in both dielectric constant and loss, so that the 33 Hz pyroelectric figures of merit (FOM's) are as follows: FV peaks at 3.8×10−2 m2 C−1 and FD peaks at 1.2×10−5 Pa−1/2. The resistivity is increased substantially from 1.1×1011 to ca 6×1011 Ωm with 1 at.% Sb, thereafter changing little. The behaviour has been explained in terms of Sb acting as a donor ion, reducing oxygen vacancy concentrations up to 1 at.%, with conductivity dominated by hole hopping between traps (Ea=0.59±0.05 eV) that are not changed by the Sb doping. It is concluded that additions of higher levels of Sb do not produce electron-mediated hopping conduction. The Cr additions have no effect upon TC, but reduce dielectric constant and loss, pyroelectric coefficient and resistivity at doping levels up to 3 at.%. The FOM FV peaks at 3.6×10−2 m2 C−1 and FD at 1.9×10−5 Pa−1/2. The behaviour of the electrical resistivity as a function of dopant level is shown to produce a linear ln(σo) vs z−1/3 dependence (σo=DC conductivity), as would be expected for hole hopping conduction between Cr3+ sites, with an Ea=0.38±0.03 eV.
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    Enhancement of pyroelectric properties of lead-free 0.94 Na 0.5 Bi 0.5 TiO 3-0.06 BaTiO 3 ceramics by La doping
    (Elsevier, 2016-12-13) Balakt, Ahmed M.; Shaw, Christopher P.; Zhang, Qi
    Lead-free 0.94NBT-0.06BT-xLa ceramics at x = 0.0–1.0 (%) were synthesized by a conventional solid-state route. XRD shows that the compositions are at a morphotropic phase boundary where rhombohedral and tetragonal phases coexist. With increasing La3+ content pyroelectric coefficient (p) and figures of merits greatly increase; however, the depolarization temperature (Td) decreases. p is 7.24 × 10−4C m−2 °C−1 at RT at x = 0.5% and 105.4 × 10−4C.m−2 °C−1 at Td at x = 0.2%. Fi and Fv show improvements at RT from 1.12 (x = 0%) to 2.65 (x10 −10 m v−1) (x = 0.5%) and from 0.021 to 0.048 (m2.C−1) respectively. Fi and Fv show a huge increase to 37.6 × 10−10 m v−1 and 0.56 m2 C−1 respectively at Td at x = 0.2%. FC shows values of 2.10, 2.89, and 2.98 (x10−9C cm−2 °C−1) at RT at 33, 100 and 1000 (Hz) respectively. Giant pyroelectric properties make NBT-0.06BT-xLa at x = 0.2% and 0.5% promising materials for many pyroelectric applications
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    Giant pyroelectric properties in La and Ta co-doped lead-free 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 ceramics
    (Elsevier, 2017-03-14) Balakt, Ahmed M.; Shaw, Christopher P.; Zhang, Qi
    La and Ta co-doped Lead-free 0.94NBT-0.06BT ceramics were synthesized by a conventional solid-state route. The compositions remain at a morphotropic phase boundary. The depolarization temperature (Td) decreased with increasing doping contents. The room temperature (RT) pyroelectric coefficient (p) was highly enhanced compared with undoped material (3.15 × 10−4 C m−2 °C−1) and reached 12.9 × 10−4 C m−2 °C−1, whereas, at Td, 58.6 × 10−4 C m−2 °C−1 could be obtained rather than 23.9 × 10−4 C m−2 °C−1 for undoped materials. The pyroelectric figure of merits, Fi and Fv, also showed a huge improvement from 1.12 × 10−10 m v−1 and 0.021 m2 C−1 of undoped material to 4.61 × 10−10 m v−1 and 0.078 m2 C−1 of doped materials at RT, and to 20.94 × 10−10 m v−1 and 0.28 × 10−10 m2.C−1 at Td. RT FC values are ∼2.40, 2.46, and 2.57 (×10−9 C cm−2 °C−1) at frequency 33, 100 and 1000 (Hz) respectively, at La = Ta = 0.2%. The pyroelectric coefficient achieved at RT in this study is almost one order of magnitude higher than PZT materials, furthermore, the figure of merits of the new compositions are comparable with or even better in those of PZT materials and other lead-free ceramics. The improvement in the pyroelectric properties makes La and Ta co-doped NBT-0.06BT ceramics possible materials to replace lead-containing PZT ceramics for infrared detector materials at a wide temperature range.
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    Polymeric materials for piezoelectricity and second harmonic generation
    (Cranfield University, 1991-09) Shaw, Christopher P.; Block, H.
    Studies have been undertaken to utilise the possible potential of the rodlike, helical structures associated with polyglutamates and polyisocyanates for piezoelectric (PE) and nonlinear optical (second harmonic generation (SHG)) applications. Various techniques have been employed to form samples/films of these polymers containing oriented helices whose bulk structures are non-centrosymmetric, (an important criterium for the aforementioned applications). Owing to the poor yields obtained for certain intermediates in the synthetic stages of these polymers, only poly('S-benzyl-L-glutamate) (PBzLG) and poly(n- hexylisocyanate) (PHIC) were available for subsequent studies. Piezoelectric work was limited to hydrostatic measurements (d3h) made on electrically poled, composite (guest host), polymer samples. Phase separation was common in all the samples, but no piezoelectric response was observed. However, low concentration (not greater than 10% w/w) of active polymer (PBzLG or PHIC) and high conductivity during the poling stage, may have accounted for the lack of response. Electrically poled, homopolymer samples of PBzLG and PHIC were achieved using a solvent evaporation technique. Nonlinear optical studies of these poled samples revealed threshold fields above which detectable levels of SHG could be observed, (i. e. PBzLG > 40 V/mm, PHIC > 150 V/mm) - The variation of SHG signal versus. the angle of polarisation of the incident laser light (1064 nm) relative to the poling direction of the film,, implied biaxial symmetry was present in the case of PBzLG, whereas the more conventional uniaxial symmetry was observed for PHIC. The variation in SHG signal observed for a fixed thickness was attributed to varying degrees of alignment, resulting from uneven poling. Although PHIC and PBzLG showed low SHG activity (less than value for urea), little ageing of this activity occurred over the 6 month testing period. Langmuir Blodgett studies carried out on chemically modified, low molecular weight PBzLG molecules showed poor transference of the monolayer to a quartz substrate once ten layers had been deposited. In addition, areas per molecule (or per residue) calculated from the pressure-area isotherms, proved inconclusive when deducing possible orientations for the PBzLG helices. No SHG signal was observed for the L. B. films.
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    Porous, functionally gradient pyroelectric materials
    (Blackwell Publishing, 2007-01) Shaw, Christopher P.; Whatmore, Roger W.; Alcock, Jeffrey R.
    Properties of a new type of pyroelectric ceramic structure containing a layer of known porosity laminated between two dense layers, to form a functionally gradient material (FGM), are reported. The combination of theoretical models for pyroelectric, dielectric, and thermal properties gave a model for the pyroelectric voltage figure of merit (FV) in good agreement with experiment, which had shown a 20% improvement for an introduced central layer porosity of 27%. Preliminary pyroelectric responsivity measurements on FGM infrared detectors indicated an even better improvement. It is postulated that this is due to the porous layer acting as a thermal barrier in the structure.
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    Pyroelectric arrays using ceramics and thin films integrated radiation collectors: design fabrication and testing
    (Taylor and Francis, 2005-07) Whatmore, Roger W.; Landi, Spartaco; Shaw, Christopher P.; Kirby, Paul B.
    Pyroelectric infra-red detectors have been of-interest for many years because of their wide wavelength response, good sensitivity and lack of need for cooling. Arrays of such detectors, comprising a pyroelectric material interfaced to an application specific integrated circuit for signal amplification and read out, provide an attractive solution to the problem of collecting spatial information on the IR distribution in a scene. Sol gel deposition provides an excellent technique for the growth of ferroelectric thin films and Mn-doped PZT30/70 films can be grown at 560°C with FD = 3.85 × 10-5 Pa-1/2. A new concept is presented here: the use of arrays of thin film pyroelectric detector elements with integrated radiation collectors designed to enhance the intensity of the radiation falling on the element. Two collector designs are presented, one based on the use of wet-chemically-etched pyramidal cavities, the second based on the Compound Parabolic Concentrator (CPC). Approximations to truncated CPC structures were SF6-dry-etched into a silicon wafer, upon which had been defined pyroelectric IR sensors with low thermal conductance (spiral leg structure) fabricated in a high sensitivity PZT thin film. First experimental assessment of the performance of these structures is presented.
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    Pyroelectric ceramics and thin films for applications in uncooled infra-red sensor arrays
    (Royal Swedish Academy of Sciences, 2007-11-22T00:00:00Z) Whatmore, Roger W.; Zhang, Qi; Shaw, Christopher P.; Dorey, Robert A.; Alcock, Jeffrey R.
    Pyroelectric infra-red detector arrays provide an attractive solution to the problem of collecting spatial information in the far IR. They are only sensitive to changes in the IR flux and are well suited to sensing movements of people. The applications of low cost arrays with limited ( a few hundred) elements for people sensing and radiometry will be illustrated. The performances of uncooled pyroelectric arrays are ultimately driven by the materials used. For this reason, continuous improvements in materials technology and figures-of-merit (FoM) are important. The performance of dense, bulk pyroelectric ceramics has not increased for several years, but nevertheless it is possible to obtain significant improvements in performance through the use of tape-cast, functionally gradient materials (FGMs) in which controlled porosity is used to control the permittivity and heat capacity of the material. A model for the performance of such a material will be presented, and compared with experimentally determined properties of FGMs based on a modified-PZT pyroelectric ceramic. The use of ferroelectric thin films is offering considerable potential for low cost and high performance. It will be shown that the introduction of controlled amounts of porosity can have a significant positive effect on the relevant pyroelectric FoM.
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    Sol-gel derived lead zirconate titanate thick films and their improved pyroelectric properties
    (Taylor and Francis, 2004) Zhang, Qi; Shaw, Christopher P.; Huang, Zhaorong; Whatmore, Roger W.
    The single-layer thickness of Pb(Zr0.3Ti0.7)O3(PZT 30/70) thin films made by the sol-gel process is often restricted by the appearance of cracks when a single layer is thicker than 100 nm. Making a film with 1μm thick needs more than 10 times coating cycles, which is sometimes unacceptable. In this study, a sol modified with an additive was used for the preparation of thick PZT films. The thickness of the films depends upon the rotation rate. Up to 700 nm crack-free single layers could be obtained. The deposited films crystallised as the perovskite structure on platinum-buffered silicon by heating at 550°C. The porosity of the thick films was a function of the single layer thickness. A dense PZT film with a single layer 140 nm thick was found to have a very high pyroelectric coefficient (p=2.36×10− 4CK− 1m− 2) and figure of merit (1.88×10− 5Pa− 0.5). The introduction of pores in the films reduced p. However, the figure of merit of the films exhibited a reasonably high value due to a dramatic reduction of dielectric constant.
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    Ultrahigh resolution of lead zirconate titanate 30/70 domains as imaged by piezoforce microscopy.
    (2006-11-14T15:31:35Z) Dunn, Steve; Shaw, Christopher P.; Huang, Zhaorong; Whatmore, Roger W.
    Piezoforce microscopy (PFM) has been used to determine the domain structure of lead zirconate titanate (PZT) (30/70) on an indium tin oxide (ITO)/glass substrate with a TiO2 boundary layer. The PZT nucleates into the perovskite form in a random crystallographic manner, which leads to a random domain structure in the final film. Using PFM it has been possible to visualize the domain structure of the PZT and determine that the domain structure has features as fine as 8 nm herringbone patterns. The possible impact of these structures for future devices utilizing nanoscale features of PZT and especially FeRAM developments is highlighted.