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Senior Lecturer

MA University of Cambridge
DPhil University of Oxford

Inorganic Microstructures

We focus on the relationship between microstructure and the mechanical and electronic properties of engineering ceramics. In addition to mathematical modelling, transmission electron microscope techniques are routinely used, as well as scanning electron microscopy, X-ray diffraction, mechanical testing and electrical characterization.

Devitrite

Historically, this triclinic phase appeared as one of many unwanted crystalline forms founds in commercial glassware. With improvements in commercial manufacturing methods for glasses in the mid-twentieth century, interest in devitrite largely disappeared in the scientific literature. However, recent work at Cambridge has shown that the optical scattering properties of fans of needle-like devitrite crystals deliberately produced in soda-lime-silica glass are such that glass sections containing these crystals can act as efficient optical diffusers.

Joining of engineering ceramics

Engineering ceramics such as alumina, zirconia, silicon nitride and silicon carbide can now be manufactured reliably with reproducible properties. As such, they are of increasing interest to industry, particularly for use in demanding environments, where their thermomechanical performance is of critical importance, with applications ranging from fuel cells to cutting tools. One aspect common to virtually all applications of engineering ceramics is that eventually they must be joined with another material, most usually a metal. Work has been focused on an understanding of the active metal brazing of engineering ceramics to ceramics and metals, and how successful joining is achieved through sequences of complex nanometre scale interfacial reactions at both the braze-ceramic and braze-metal interfaces.

Twinning

Twinning in crystalline materials can arise as a consequence of growth from the vapour, liquid or solid, or as a consequence of mechanical deformation. Work in this area has been concentrated on an understanding of twinning in non-cubic materials, such as the plethora of deformation twinning modes possible in h.c.p. titanium alloys, of relevance to the aerospace industry, and twinning arising during crystal growth, such as the Type II rotation twinning mode recently discovered in devitrite.

A low magnification photograph of devitrite needles nucleated on the surface of a float glass block after a heat treatment of 17 h at 850°C observed in transmitted polarized light with a sensitive tint at 45° to the polarizer and analyzer (K.M. Knowles and R.P. Thompson, J. Am. Ceram. Soc., 97, 1425-1433 (2014)).
  • S. Shrikanth, K.M. Knowles, S. Neelakantan and R. Prasad, ‘Planes of isotropic Poisson’s ratio in anisotropic crystalline solids’, International Journal of Solids and Structures, 191-192, 628-645 (2020)
  • R.C. Pond, J.P. Hirth and K.M. Knowles, ‘Topological model of type II deformation twinning in NiTi martensite’, Philosophical Magazine, 99, 1619-1632 (2019)
  • K.M. Knowles, A. Sil, B. Stöger and M. Weil, ‘Crystal structure of the thortveitite-related M phase, (MnxZn1-x)2V2O7 (0.75 < x < 0.913): a combined synchrotron powder and single-crystal X-ray study’, Acta Crystallographica C, 74, 1079-1087 (2018)
  • S.J. Lainé, K.M. Knowles, P.J. Doorbar, R.D. Cutts and D. Rugg, and D.A. Smith, ‘Microstructural characterisation of metallic shot peened and laser shock peened Ti-6Al-4V’, Acta Metallurgica, 123, 350-361 (2017)
  • 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, 97, 718-742 (2017)

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