Inorganic Microstructures

This research group currently comprises:

Dr Anjan Sil (Visiting Scientist – UKIERI link with IIT Roorkee)

The research interests of the group focus on the relationship between microstructure and the mechanical and electronic properties of inorganic engineering materials. Examples of research topics of interest are:

● Hamaker constants of isotropic thin films between highly anisotropic materials

● Joining of engineering ceramics for high temperature applications

● Microstructure of spherulites in crystalline glazes

● Novel thin film zinc oxide varistors (UKIERI project with IIT Roorkee)

● Optically transparent hard coatings

● Silicon nitride-silicon carbide particulate ceramics

● Silicon-pyrex bonding and aluminium-pyrex anodic bonding

● Sol-gel processing of lead-free NKN piezoelectric materials

Transmission electron microscope techniques are routinely used by the research group, as well as scanning electron microscopy, X-ray diffraction, mechanical testing and electrical characterisation.

Ph.D. projects for entry in October 2010

Rolls-Royce Ph.D. studentships

Novel zinc oxide varistors

Varistors derive their non-linear current-voltage characteristics from the addition of low levels of transition and heavy metal oxides. It is usual for commercial compositions to have five or six oxide additions. However, recent work here at Cambridge has shown that ZnO with additions of manganese oxide and vanadium pentoxide alone exhibits varistor behaviour rivalling that of commercial material. This Ph.D. project will establish microstructure-electrical property relationships in this ternary system by examining a range of ZnO-rich pellets by X-ray diffraction and transmission electron microscopy. An area of particular interest will be mapping of the electrical barriers at zinc oxide grain boundaries using electron holography and Fresnel fringe analyses.

Anodic bonding

Anodic bonding is a common process in microelectromechanical systems (MEMS). For example, it is used to manufacture pressure sensors. Two materials, such as silicon and pyrex, are bonded together by applying a high d.c. voltage across the bond at a temperature of ~ 400 ˚C. In this example, pyrex, the poorer electrical conductor, is cathodic with respect to silicon. A requirement of anodic bonding is that the ceramic material used in the process is a sufficiently good electrical conductor at the temperature used to make the bond. In this PhD project the principle of anodic bonding will be applied to the bonding of metals (e.g., Nb, Cu, Ni and Al) to other ceramics and glasses, such as soda-lime-silica glass and zirconia. The aims of the project will be to specify the range of temperature and d.c. voltage conditions within which good bonds can be formed between the metals and the ceramics and to characterise these bonds using a range of transmission electron microscopy techniques.

High temperature brazing of engineering ceramics

The aim of this PhD project is to examine by X-ray diffraction, scanning electron microscopy and transmission electron microscopy, the microstructure of novel particle-reinforced brazes for joining engineering ceramics such as alumina, silicon carbide and silicon nitride to metallic materials such as nickel, so that the joints can be used at service temperatures of > 500 ˚C. Experience at Cambridge with brazes for bonding SiC has shown that the introduction of small SiC particulates to high temperature braze compositions is beneficial for joining SiC to itself, producing mechanically sound joints. It is proposed to extend this methodology to other systems of engineering significance.

Microstructure of crystalline glazes

Crystalline glazes are glazes in which large spherulites, visible by eye, are formed in a glaze during the glazing process, such as in the examples shown here.

Typically these specialist glazes are used by potters for aesthetic effects when making vases. The crystal phase which grows in these glazes is usually willemite, Zn₂SiO₄, arising from the incorporation of zinc oxide into silica-rich glazes. Traces of other oxides such as cobalt oxide colour the willemite crystals. Other phases can also be produced in crystalline glazes, usually unintentionally, but arising as a consequence of the various glaze recipes used by different practitioners of the art. It is only recently that modern microstructural techniques of analysis have been applied to such glazes, e.g., K.M. Knowles and F.S.H.B. Freeman, ‘Microscopy and microanalysis of crystalline glazes’, Journal of Microscopy, 215, 257-270 (2004). This Ph.D. project will examine in depth further, more exotic, examples of crystalline glazes, with the aim of establishing the nature of the crystalline phases that are able to co-exist with willemite in crystalline glazes and characterising in detail the nature of the willemite spherulites. The experimental work will involve X-ray diffraction, transmission electron microscopy, scanning electron microscopy and polarised light microscopy of these fascinating glazes.

Recent Research Group Publications

1. B. Mŏgulkoç, H.V. Jansen, K.M. Knowles, H.J.M. ter Brake, and M.C. Elwenspoek ‘Surface devitrification and the growth of cristobalite in Borofloat^® (borosilicate 8330) glass’, J. Am. Ceram. Soc., in the press.

2. K.M. Knowles, ‘Commentary: Herapathite – the first man-made polarizer’, Philosophical Magazine Letters, 89, 745-755 (2009).

3. J.A. Fernie, R.A.L. Drew and K.M. Knowles, ‘Joining of engineering ceramics’, International Materials Reviews, 54, 283-331 (2009).

4. S.A. O’Callaghan and K.M. Knowles, ‘Synthesis and processing of nanosized sodium potassium niobate powders’, Materials Science and Technology, 25, 1302-1306 (2009).

5. A.E. Markaki, K.M. Knowles, R.A. Oliver and A. Gholinia, ‘Surface terracing on ferritic stainless-steel fibres and potential relevance to in-vitro cell growth’, Philosophical Magazine, 89, 2285-2303 (2009).

6. B. Mŏgulkoç, H.V. Jansen, J.W. Berenschot, H.J.M. ter Brake, K.M. Knowles and M.C. Elwenspoek ‘Characterization of MEMS-on-tube assembly: reflow bonding of borosilicate glass (Duran^®) . tubes to silicon substrates’, J. Micromech. Microeng., 19, 085027 (2009).

7. Y.-H. Han, A. Taylor and K.M. Knowles, ‘Scratch resistance and adherence of novel organic–inorganic hybrid coatings on metallic and non-metallic substrates’, Surface & Coatings Technology, 203, 2871-2877 (2009).

8. K.M. Knowles, M.E. Vickers, A. Sil, Y.-H. Han and P. Jaffrenou, ‘X-ray powder diffraction and electron diffraction studies of the thortveitite-related L phase, (Zn,Mn)₂V₂O₇’, Acta Crystallographica B: Structural Science, 65, 160–166 (2009).

9. Y.-H. Han, A. Taylor and K.M. Knowles, ‘Characterisation of organic-inorganic hybrid coatings deposited on aluminium substrates’, Surface & Coatings Technology, 202, 1859-1868 (2008).

10. Y.-H. Han, A. Taylor, M.D. Mantle and K.M. Knowles, ‘UV curing of organic-inorganic hybrid coating materials’, Journal of Sol-Gel Science and Technology, 43, 111-123 (2007).

11. Y.-H. Han, A. Taylor, M.D. Mantle and K.M. Knowles, ‘Sol-gel-derived organic-inorganic hybrid materials’, Journal of Non-Crystalline Solids, 353, 313-320 (2007).

12. K.M. Knowles and A.T.J. van Helvoort, ‘Anodic Bonding’, International Materials Reviews, 51, 273-311 (2006).

13. K.M. Knowles, ‘‘Modelling of the current-time characteristics in anodic bonding’, Advances in Science and Technology, 45, 1558-1567 (2006).

14. S.J.P. Longworth, K.M. Knowles and R.E. Dunin-Borkowski, ‘The measurement and interpretation of electrostatic potential profiles across grain boundaries in strontium titanate’, Journal of Physics: Conference Series, 26, 235-238 (2006).

15. S. Turan and K.M. Knowles, ‘Interfaces in non-oxide ceramic composites’, in Ceramic Matrix Composites, edited by I.M. Low (Woodhead Publishing Limited, Cambridge, England), pp. 461-490 (2006).

16. K.M. Knowles, ‘Dispersion forces at planar interfaces in anisotropic ceramics’, Journal of Ceramic Processing Research, 6, 10-16 (2005).

17. A.T.J. van Helvoort, K.M. Knowles and R. Holmestad, ‘Compositional characterisation of electrostatic bonds’, Inst. Phys. Conf. Ser., 179, 409-412 (2004).

18. K.M. Knowles, ‘Structure-property relationships in ceramic matrix composites’, in Metal and Ceramic Matrix Composites, edited by B. Cantor, F.P.E. Dunne and I.P. Stone, pp. 281-298. Institute of Physics Publishing, Bristol and Philadelphia (2004).

19. K.M. Knowles and F.S.H.B. Freeman, ‘Microscopy and microanalysis of crystalline glazes’, Journal of Microscopy, 215, 257-270 (2004).

20. A.T.J. van Helvoort, K.M. Knowles and J.A. Fernie, ‘Joining mechanisms in electrostatic bonding’, Key Engineering Materials, 264-268, 649-654 (2004).

21. H. Pfeiffer and K.M. Knowles, ‘Reaction mechanisms and kinetics of the synthesis and decomposition of lithium metazirconate through solid-state reaction’, Journal of the European Ceramic Society, 24, 2433-2443 (2004).

22. H. Pfeiffer and K.M. Knowles, ‘Effects of vanadium and manganese concentrations on the composition, structure and electrical properties of ZnO-rich MnO₂-V₂O₅-ZnO varistors’, Journal of the European Ceramic Society, 24, 1199-1203 (2004).

23. A.T.J. van Helvoort, K.M. Knowles, R. Holmestad and J.A. Fernie, ‘Anodic oxidation during electrostatic bonding’, Philosophical Magazine, 84, 505-519 (2004).

Former members of the research group

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