How materials evolve: a defective story
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Materials science is the art of optimising materials for applications, and manipulating crystal defects is the knob for doing so.
That was the key message from Professor David Srolovitz, Head and Chair Professor of Department of Materials Science and Engineering, at his engaging talk on 20 March as part of the President’s Lecture Series: Excellence in Academia.
The talk was delivered in front of an audience of scholars from City University of Hong Kong (CityU) and other institutions as well as online in real-time in light of public health concerns about COVID-19. During the Q&A, several participants from around the world submitted questions for the speaker to address in real time.
“We learn to understand one type of microstructure evolution from the atomic to the continuum scales, which is what makes materials so scientifically interesting,” said Professor Srolovitz, who is concurrently Chair Professor in the Department of Mechanical Engineering and in the Department of Physics; and a Senior Fellow at Hong Kong Institute for Advanced Study.
In his talk, Professor Srolovitz said that many material properties depended on their structure at a scale much larger than atoms and electrons (i.e., microstructure). Controlling microstructure evolution is (one of) the main challenges of materials science.
Among his several key points is that “crystal defect” means an interruption in the periodic order of a crystal rather than a defect in the sense of something broken.
“Defects are an intrinsic property of all materials,” he said. “A ‘microstructure’ means the spatial arrangement of defects and materials scientists manipulate how crystal defects form, move and are arranged,” he said.
Materials scientists, he continued, aim to understand the basic mechanisms of how things works (defects move/microstructure forms); as materials engineers, their role is to translate this understanding into materials processing for applications.
“Materials scientists also manipulate the composition of the material. This is the other knob,” he said.
In terms of applications, Professor Srolovitz said materials were important across a range of industries because they are both the enabler and limiter of all engineering systems.
For example, in terms of applications in engineering for aerospace, materials can retain high strength at very high temperature and allow better turbine blades for jet engines, which leads to greater engine efficiency, less fuel burned, less fuel carried in flight, lighter weight structures and thus even more efficiency.
For microelectronics, materials science helps to make them more efficient (faster) and the basic components can be put closer and closer together. This means more heat is generated in a smaller and smaller chip. What limits this drive for efficiency is our ability to make and operate smaller and smaller (faster and faster) microelectronics. This depends on how to make devices with near atomic scale features and how to get the heat out so that the chips don’t melt.
“These are both materials issues and depend on our ability to manipulate and control defects,” he said.