PhD Thesis Arne Lüker final version V4 - Cranfield University

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2.5 Sol-Gel Chemistry 54 Theoretical Considerations and Literature Review There are many methods for ferroelectric thin film preparation, such as metal organic chemistry vapour deposition (MOCVD), pulsed laser deposition (PLD), magnetron sputtering and sol-gel. Thin films with high properties could be fabricated by all this methods, and comparatively perfect processing parameters were formed. The RF magnetron sputtering and sol-gel processing are relatively simple preparation methods while PLD produces films with superior quality. However, the drawback of PLD is the restriction to small sample sizes. The main advantage of the sol-gel method is the ease of doping (substitution of atoms on the A- and/or B site of the perovskite crystal) without purchasing new sputtering targets, and flexibility concerning the substrate size. In this chapter the sol-gel processing of ferroelectrics is reviewed. The underlying chemical and physical aspects of the sol-gel deposition of these materials will be discussed, with a focus on understanding solution preparation, film deposition, and phase transformation (to the crystalline ceramic) which are occuring during film synthesis. 2.5.1 Solution Preparation The general principle involved in the sol-gel deposition of perovskite films is to prepare a “homogeneous” solution of the necessary cation species that may later be applied to a substrate. The fabrication of thin films by this approach involves four basic steps: (1) synthesis of the precursor solution; (2) deposition by spin-coating where drying processes usually begin depending on the solvent; (3) low-temperature heat treatment for drying, pyrolysis of organic species (typically 300−400°C), and the formation of an amorphous film; (4) higher temperature annealing for densification and crystallization of the coating into the desired oxide phase (600−1100°C). Solution preparation of perovskite materials generally involves the use of metalloorganic compounds that are dissolved in a common solvent. The starting reagents are typically metal alkoxide compounds, M(OR)x, where M is a metal and R is an alkyl group, metal carboxylates, M(OOCR)x, and metal β-diketonates, MOx(CH3COCHCOCH3)x. The selection of the starting reagents is dictated by solubility and reactivity considerations and the type of solution precursor species desired. The
Sol-Gel derived Ferroelectric Thin Films for Voltage Tunable Applications synthetic strategy used will define solution precursor properties such as equivalent solids content, extent of oligomerisation and cation interaction, degree of homogeneity and reactivity, type and number of modifying ligands, and precursor size, structure, and shape. The solution route used will also determine the temperature at which pyrolysis of organic species occurs, the weight loss associated with oxide formation, the densification and crystallisation behavior of the film, and stress development within the film. While some of these effects are now well understood, others are less clear, partly because of the difficulty in characterizing the solution precursor and the resulting amorphous film. In addition to precursor characteristics, film processing behavior, such as substrate wetting, can also play a role in determining the solution chemistry that must be developed. Film properties that can necessitate changes in solution chemistry include poor thickness uniformity (striations), crack formation, crystallization behavior and phase purity, and compositional nonuniformities [51, 52]. In this study a modified sol-gel route was used which is a mixure of two classical approaches: 1) Sol−gel processes that use 2-methoxyethanol as a reactant and solvent [53 - 55]. 2) Chelate processes that use modifying ligands such as acetic acid [56 – 58]. Although other alcohols have been utilized, the solvent 2-methoxyethanol (CH3- OCH2CH2OH), is most extensively used in the chemical synthesis of perovskite materials. Processes based on 2-methoxyethanol [53, 54] are most appropriately considered sol-gel processes and the key reactions leading to the formation of the precursor species are hydrolysis and condensation of the alkoxide reagents, in which metal-oxygen-metal (M-O-M) bonds are formed: a) Hydrolysis: M(OR)x + H2O → M(OR)x-1(OH) + ROH [Eq. 2.26] b) Condensation (alcohol elimination): 2M(OR)x-1(OH) → M2O(OR)2x-3(OH) + ROH [Eq. 2.27] 55
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Internships: Sol-Gel derived Ferroe
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