PhD Thesis Arne Lüker final version V4 - Cranfield University

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Fig. 1.15: Michael Faraday 22 Introduction In the 19 th century two major developments occurred which were to prove foundation stones for sol-gel processing: the physical properties of colloids came under intensive development of the sol-gel materials field. study by such giants as Becquerel and Faraday (his gold sols were so stable that they are still in existence at the Royal Institution in London); and the chemistry became very sophisticated. In 1846 Ebelmen [28] prepared the first silicon alkoxides by the reaction between silicon tetrachloride and alcohol, observing that the product gelled on prolonged exposure to atmosphere with normal humidity. In 1876 Troost and Hautefeuille [29] made hydrolysed derivates of silicon alkoxides. In 1884 Grimaux [30] hydrolysed tetramethoxysilane to prepare silicic acid sols, and made colloidal iron oxides from iron alkoxide. However, for the next 50 years these developments had little scientific impact for the The one notable exception in this period was the work of W.A. Patrick, who pioneered the field of silica gel desiccants, catalysts and absorbent materials, starting with the drying and firing of a homogeneous silica gel at up to 700°C to produce a very porous form of silica [31]. In 1923 he went on to show that impregnation of the partially dried materials with metal salts led to the formation of supporting catalysts [32], and by 1930 he had filed many patents for supporting catalysts, including the use of sol-gel methods [33]. This signaled the beginning of an intensive period of technological development using sol-gel methods, which produced a very large number of patents, many useful materials, and a large body of data and experience. Only later, with the advent of modern characterisation methods, could a set of firmly-based theoretical principles begin to be established to interpret all these data. Indeed it could be said that despite the new methods now available, the subject is still strongly influenced by this empirical approach. It is therefore important that the large amount of early work in the field is not overlooked, only to be re-discovered as the topics attract interest for a new reason, just as was the case with the Roman cement.
Sol-Gel derived Ferroelectric Thin Films for Voltage Tunable Applications 1.3 Materials for Tunable Applications – State of the Art The appropriate level of tunability for tunable applications (n ≥ 1.5 or n(%) ≥ 0.3, see Section 2.1.2) is practically achievable in materials with a relatively high dielectric permittivity at the frequencies of choice. For non-composite bulk materials, a value of the dielectric constant exceeding some 1000 is required whereas in thin films (where the application of higher fields is possible) the dielectric constant should be lager than some 300. This limitation together with the requirement of relatively low values of the loss (below a few percent) narrows the scope of the materials of interest to the displacive type ferroelectrics (incipient or regular in the paraelectric phase). Up to now, many materials have been addressed as possible candidates for tunable applications, the most attention has being paid to SrTiO3 and its solid solution with BaTiO3: (Ba,Sr)TiO3. In the present section the state of the art of this scientific field is reviewed Motivated by increasing speed and bandwidth requirements, the III-V semiconductor industry has recently made considerable progress in developing transistors with transit time and maximum oscillation frequencies well above 100 GHz. The traditional digital silicon industry also, driven by increasing market demands and cost constraints, has put substantial effort in developing analog microwave circuits. Silicon based bipolar and CMOS transistors with transit times and maximum oscillation frequencies above 100 GHz are now available, making it possible to develop monolithic microwave ICs (MMICs) for frequencies up to 50 GHz. Along with the transistors, varactors are the other widely used components used in microwave technology for analog (nondigital) tuning purposes. In spite of the progress achieved in the transistor technology, no semiconductor varactors with high enough Q factor and tunability are available for frequencies above 10–20 GHz. Typically, the Q factors of semiconductor varactors decrease with the increased frequency as Q ~ f −1 , and at 50 GHz it is of the order 10 or less. There is a lack of varactors with adequate (compatible with the transistors) performance in this frequency range. The current progress indicates that, to a certain degree, the “varactor gap” may be filled in by ferroelectric varactors. After several decades of research considerable progress has 23
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Intensity [arb. Units] (g) (f) (e)
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Relative diel. diel. constant Const
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Intensity [arb. Units] Intensity [a
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Tunability [%] 100 90 80 70 60 50 4
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(g) (f) (e) (d) (c) (b) (a) Sol-Gel
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Internships: Sol-Gel derived Ferroe
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PhD Thesis Arne Lüker final version V4 - Cranfield University

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