Inorganic Microstructures

This research group currently comprises:

Dr Kevin M. Knowles (Head of Group)

Majed Ali (Ph.D. student)

Steve Lainé (Ph.D. student)

Hassan Qarra (Ph.D. student)

Professor John A. Fernie (Visiting Scientist)

Dr Anjan Sil (Visiting Scientist from IIT Roorkee, India)

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:

● Deformation twinning in titanium alloys

● Devitrite, Na₂Ca₃Si₆O₁₆

● Elastic moduli of crystalline materials

● Hamaker constants of isotropic thin films between highly anisotropic materials

● Joining of engineering ceramics for high temperature applications

● MAX phases in zirconium-based carbides

● Microstructure of spherulites in crystalline glazes

● Molecular dynamics simulation of twinning in devitrite and plagioclase feldspars

● 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 2017

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 and thin films 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 example 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. A by-product of these spherulites is their ability to diffuse light, reported recently in the paper K.M. Knowles, H. Butt, A. Batal, A. Sabouri and C.J. Anthony, ‘Light scattering and optical diffusion from willemite spherulites’, Optical Materials 52, 163-172 (2016).

Recent Research Group Publications

1. K.M. Knowles, ‘The plane strain Young’s modulus in cubic materials’, Journal of Elasticity, in the press (2017).

2. M. Ali, K.M. Knowles, P.M. Mallinson and J.A. Fernie, ‘Evolution of the interfacial phases in Al₂O₃–Kovar^® joints brazed using a Ag–Cu–Ti-based alloy’, Philosophical Magazine, in the press (2017).

3. S.J. Lainé, K.M. Knowles, P.J. Doorbar, R.D. Cutts and D. Rugg, ‘Microstructural characterisation of metallic shot peened and laser shock peened Ti–Al–4V’, Acta Materialia 123, 350–361 (2017).

4. H. Butt, A.K. Yetisen, A.A. Khan, K.M. Knowles, M.M. Qasim, S.K. Yun and T.D. Wilkinson, ‘Electrically tunable scattering from devitrite-liquid crystal hybrid devices’, Advanced Optical Materials 5, 1600414-1–1600414-7 (2017).

5. S.V. Harb, S. H Pulcinelli, C.V. Santilli, K.M. Knowles and P. Hammer, ‘A comparative study on graphene oxide and carbon nanotube reinforcement of PMMA-siloxane-silica anticorrosive coatings’, ACS Applied Materials and Interfaces 8, 16339–16350 (2016).

6. S.V. Harb, F.C. dos Santos, S. H Pulcinelli, C.V. Santilli, K.M. Knowles and P. Hammer, ‘Protective coatings based on PMMA-silica nanocomposites reinforced with carbon nanotubes’, in Carbon Nanotubes – Current Progress of their Polymer Composites, ed. Mohamed Reda Berber and Inas Hazzaa Hafez, Intech publications, Chapter 7: pp. 195-225 (2016).

7. K.M. Knowles, ‘The biaxial moduli of cubic materials subjected to an equi-biaxial strain’, Journal of Elasticity 124, 1-25 (2016).

8. K.M. Knowles, H. Butt, A. Batal, A. Sabouri and C.J. Anthony, ‘Light scattering and optical diffusion from willemite spherulites’, Optical Materials 52, 163-172 (2016).

9. M. Ali, K.M. Knowles, P.M. Mallinson and J.A. Fernie, ‘Interfacial reactions between sapphire and Ag–Cu–Ti-based active braze alloys’, Acta Materialia 103, 859-869 (2016).

10. M. Ali, K.M. Knowles, P.M. Mallinson and J.A. Fernie, ‘Microstructural evolution and characterisation of interfacial phases in Al₂O₃/Ag-Cu-Ti/Al₂O₃ braze joints’, Acta Materialia 96, 143-158 (2015).

11. S.J. Lainé and K.M. Knowles, ‘{1 1 -2 4} deformation twinning in commercial purity titanium at room temperature’, Philosophical Magazine 95, 2153-2166 (2015).

12. K.M. Knowles and P.R. Howie, ‘The directional dependence of elastic stiffness and compliance shear coefficients and shear moduli in cubic materials’, Journal of Elasticity 120, 87-108 (2015).

13. B. Kahr and K.M. Knowles, ‘Polarizing films’, Chapter 26 of Tatsuo Kaiho (ed.) Iodine Chemistry and Applications, pp: 479-488, Wiley (2014).

14. K.M. Knowles and R.P. Thompson, ‘Growth of devitrite, Na₂Ca₃Si₆O₁₆, in soda-lime-silica glass’, Journal of the American Ceramic Society 97, 1425-1433 (2014).

15. A. Bhowmik , K.M. Knowles and H.J. Stone, ‘Discontinuous precipitation of Co₃V in a complex Co-based alloy’, Philosophical Magazine 94, 752-763 (2014).

16. H. Butt, K.M. Knowles, Y. Montelongo, G.A.J. Amaratunga and T.D. Wilkinson, ‘Devitrite-based optical diffusers’, ACS Nano 8, 2929-2935 (2014).

17. B. Li and K.M. Knowles, ‘Molecular dynamics simulation of albite twinning and pericline twinning in low albite’, Modelling and Simulation in Materials Science and Engineering 21, 055012 (18pp) (2013).

18. B. Li and K.M. Knowles, ‘Molecular dynamics simulation of twinning in devitrite, Na₂Ca₃Si₆O₁₆’, Philosophical Magazine 93, 1582-1603 (2013).

19. K.M. Knowles and C.N.F. Ramsey, ‘Type II twinning in devitrite, Na₂Ca₃Si₆O₁₆’, Philosophical Magazine Letters 92, 38-48 (2012).

20. 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’, Journal of the American Ceramic Society, 93, 2713-2719 (2010).

Crystallography and Crystal Defects (Second edition)

Published February 2012 ISBN 978-0-470-75014-8 (Paperback) Detailed worked solutions for all the problem in this book are available on the Wiley web page accompanying it at http://booksupport.wiley.com

Needles of devitrite crystals nucleated heterogeneously on the surface of soda-lime-silica glass viewed by

transmitted light polarised light microscopy with a sensitive tint at 45° to the polarizer and analyser

(highlighted in the ACerS blog Ceramic Tech Today in April 2014)

Research Group, December 2012:

left to right: Samantha O’Callaghan, Steve Lainé, Kevin Knowles, Bin Li, Majed Ali

Former members of the research group

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