Tim W. Button

Bio: Tim W. Button is an academic researcher from University of Birmingham. The author has contributed to research in topics: Ceramic & Dielectric. The author has an hindex of 24, co-authored 168 publications receiving 2009 citations. Previous affiliations of Tim W. Button include Central European Institute of Technology.

Dielectric and microwave properties of barium strontium titanate (BST) thick films on alumina substrates

Abstract: The dielectric and microwave properties of barium strontium titanate (BST) thick films on alumina substrates have been investigated. The BST films were screen printed and sintered at temperatures below 1300 °C. At temperatures below the Curie point the BST films exhibit tunability in the range 15–35% under a DC bias field of 2 kV/mm. The dielectric loss is critically dependent on film thickness with lower losses ( 100 μm). A relaxation process appears to take place for the BST films in the MHz to GHz frequency regime. The variation of permittivity with bias field exhibits hysteretic behaviour in both the ferroelectric and paraelectric regions. This is believed to arise due to the non-uniform composition and existence of micro/nano-polar phases in the films.

The processing and properties of high T/sub c/ thick films

Abstract: The influence of processing conditions on the microstructure and properties of YBa/sub 2/Cu/sub 3/O/sub x/ thick films on yttria-stabilized zirconia substrates is reported Films processed below the peritectic temperature are fine grained, exhibit little preferred orientation, and generally have low critical currents Above the peritectic temperature, the morphology changes dramatically and large, highly textured, spherulitic crystals are observed, with an associated increase in critical current and an improved T/sub c/ Properties in these materials are generally much better than those of bulk YBa/sub 2/Cu/sub 3/O/sub x/ The relationship between microstructure and properties is shown to be complex The influence of silver additions to the films is examined, and examples are given of thick-film devices which are being evaluated Thick-film devices under evaluation include flux transformers, TE/sub 011/ cavities, coaxial and helical resonators, microstrip resonators, current limiters, and shields

Low surface resistance in YBa2Cu3Ox melt-processed thick films

Abstract: A LOWsurface resistance,Rs, is the key to successful development of radio-frequency and microwave applications of high-temperature superconductors. Here we report the Rs of YBa2Cu3Ox thick films on yttria-stabilized zirconia substrates at frequencies up to 50 GHz. Films processed below the peritectic temperature are fine grained, have Rs similar to bulk YBa2Cu3Or, generally have low critical current density (Jc) and exhibit little preferred orientation of crystallographic axes. Films processed above the peritectic temperature exhibit preferred orientation in large spherulitic grains, have higher Jc and far lower Rs. For these films the crossover frequency at which Rs equals that of copper is 50 GHz, a factor of two higher than the best bulk material or thick film yet reported and only a factor of ∼4 lower than high-quality thin films. At the frequencies used for mobile communications (900 MHz and 1.8 GHz), the superconductor losses would be two orders of magnitude lower than those of normal metals. The particular advantages of the thick-film route are the speed and low cost of the process, and the ability to apply the films on curved surfaces and on large areas, the largest so far being >200 cm2.

Temperature-Stable Relative Permittivity from −70°C to 500°C in (Ba0.8Ca0.2)TiO3–Bi(Mg0.5Ti0.5)O3–NaNbO3 Ceramics

Abstract: Temperature-stable relaxor dielectrics have been developed in the solid solution system: 0.45Ba0.8Ca0.2TiO3-(0.55-x) Bi(Mg0.5Ti0.5)O3-xNaNbO3. Ceramics of composition x = 0 have a relative permittivity ?r = 950 ± 15% over a wide temperature range from +70°C to 600°C. Modification with NaNbO3 at x = 0.2 decreases the lower limiting temperature to -70°C, but also decreases relative permittivity such that ?r ∼ 600 ± 15% over the temperature range -70°C to 500°C. For composition x = 0.3, the low-temperature dispersion in loss tangent, tan δ, (at 1 kHz) shifts to lower temperature, giving tan δ values ≤0.02 across the temperature range -60°C to 300°C in combination with ?r ∼ 550 ± 15%. Values of dc resistivity for all samples are of the order of 1010 Ω m at 250°C and 107 Ω m at 400°C.

Consolidation/synthesis of materials by electric current activated/assisted sintering

Abstract: This review article aims to provide an updated and comprehensive description of the development of the Electric Current Activated/assisted Sintering technique (ECAS) for the obtainment of dense materials including nanostructured ones. The use of ECAS for pure sintering purposes, when starting from already synthesized powders promoters, and to obtain the desired material by simultaneously performing synthesis and consolidation in one-step is reviewed. Specifically, more than a thousand papers published on this subject during the past decades are taken into account. The experimental procedures, formation mechanisms, characteristics, and functionality of a wide spectrum of dense materials fabricated by ECAS are presented. The influence of the most important operating parameters (i.e. current intensity, temperature, processing time, etc.) on product characteristics and process dynamics is reviewed for a large family of materials including ceramics, intermetallics, metal–ceramic and ceramic–ceramic composites. In this review, systems where synthesis and densification stages occur simultaneously, i.e. a fully dense product is formed immediately after reaction completion, as well as those ones for which a satisfactory densification degree is reached only by maintaining the application of the electric current once the full reaction conversion is obtained, are identified. In addition, emphasis is given to the obtainment of nanostructured dense materials due to their rapid progress and wide applications. Specifically, the effect of mechanical activation by ball milling of starting powders on ECAS process dynamics and product characteristics (i.e. density and microstructure) is analysed. The emerging theme from the large majority of the reviewed investigations is the comparison of ECAS over conventional methods including pressureless sintering, hot pressing, and others. Theoretical analysis pertaining to such technique is also proposed following the last results obtained on this topic.

Ultrasonic arrays for non-destructive evaluation: A review

Abstract: An ultrasonic array is a single transducer that contains a number of individually connected elements. Recent years have seen a dramatic increase in the use of ultrasonic arrays for non-destructive evaluation. Arrays offer great potential to increase inspection quality and reduce inspection time. Their main advantages are their increased flexibility over traditional single element transducer methods, meaning that one array can be used to perform a number of different inspections, and their ability to produce immediate images of the test structure. These advantages have led to the rapid uptake of arrays by the engineering industry. These industrial applications are underpinned by a wide range of published research which describes new piezoelectric materials, array geometries, modelling methods and inspection modalities. The aim of this paper is to bring together the most relevant published work on arrays for non-destructive evaluation applications, comment on the state-of the art and discuss future directions. There is also a significant body of published literature referring to use of arrays in the medical and sonar fields and the most relevant papers from these related areas are also reviewed. However, although there is much common ground, the use of arrays in non-destructive evaluation offers some distinctly different challenges to these other disciplines.

(K, Na)NbO3-Based Lead-Free Piezoceramics: Fundamental Aspects, Processing Technologies, and Remaining Challenges

Abstract: Environment-friendly lead-free piezoelectric ceramics have been studied extensively in the past decade with great progress particularly in systems based on a niobate perovskite compound formulated as (K, Na)NbO3 (abbreviated as KNN). A comprehensive review on the latest development of KNN-based piezoelectric ceramics is presented in this article, including the phase structure, property enhancement approaches, and sintering processes as well as the status of some promising applications. The phase structure of KNN was reexamined and associated with the effect of chemical modification on its tetragonal-to-orthorhombic transition. Then, a special focus is placed on the temperature dependence of piezoelectric properties of KNN-based ceramics, followed by reviewing the recent approaches devoted to the temperature-stability enhancement. The processing fundamentals related to the sintering of KNN-based ceramics are also presented with an emphasis on compositional and microstructural control. Finally, this review introduces several industrial attempts of traditional piezoceramic products using KNN-based ceramics and the studies on some promising application in authors' laboratory.

Oxygen redox chemistry without excess alkali-metal ions in Na2/3[Mg0.28Mn0.72]O2

Abstract: The search for improved energy-storage materials has revealed Li- and Na-rich intercalation compounds as promising high-capacity cathodes. They exhibit capacities in excess of what would be expected from alkali-ion removal/reinsertion and charge compensation by transition-metal (TM) ions. The additional capacity is provided through charge compensation by oxygen redox chemistry and some oxygen loss. It has been reported previously that oxygen redox occurs in O 2p orbitals that interact with alkali ions in the TM and alkali-ion layers (that is, oxygen redox occurs in compounds containing Li+-O(2p)-Li+ interactions). Na2/3[Mg0.28Mn0.72]O2 exhibits an excess capacity and here we show that this is caused by oxygen redox, even though Mg2+ resides in the TM layers rather than alkali-metal (AM) ions, which demonstrates that excess AM ions are not required to activate oxygen redox. We also show that, unlike the alkali-rich compounds, Na2/3[Mg0.28Mn0.72]O2 does not lose oxygen. The extraction of alkali ions from the alkali and TM layers in the alkali-rich compounds results in severely underbonded oxygen, which promotes oxygen loss, whereas Mg2+ remains in Na2/3[Mg0.28Mn0.72]O2, which stabilizes oxygen.