Imperatives in a materials education Vikram Jayaram Materials Engineering IISc Curricula in Materials and Chemical Sciences TEQIP IIT-KANPUR Feb 2014
PHYSICS CHEMISTRY BIOLOGY EARTH SCIENCE SCIENCE ENGINEERING MECHANICAL ELECTRICAL CHEMICAL CIVIL MEDICINE AEROSPACE
Subject range wide (metals, ceramics, polymers, semiconductors) Must convey lots of raw information (properties of materials) Attention spans are lower (need to make it interesting) Other engineering skills needed (for most employers) Therefore we must: teach less in each subject unify concepts bring in practical situations from day 1 Polymers probably require a separate approach . Attempts to over-unify have not been too successful elsewhere. But inorganic materials need commonality of approach
Who are our potential employers?? Primary metal makers, mineral exploration Not heavy employers, limited attraction for students Polymers and plastics Not the strengths of our curriculum Electronic materials / Thin films Not there in India except isolated cases in modeling With the exception of one or two multinationals, materials graduates employability demands collateral engineering skills broad education, logical thinking, mathematical skills, design. This carries over to masters as well.
What is the core unique to Materials? • A historical perspective on materials usage and development • Generalised solution thermodynamics and phase transformations • Defects and their influence on properties These must be taught rigorously!
Where did Materials Science & Engineering come from? Pottery & Ceramic Blacksmithery and iron Cement / Stone THE ANCIENT WORLD Mud Textiles Wood Glass
The Modern world of Materials Aluminium Rayon It’s all the ??????? same today, except for…. Composite Glass fibre Ceramic spark plug
Some illustrations from thermodynamics Interplay of Entropy and Enthalpy in phase equilibria Entropy drives mixing makes purity expensive Electricity transmission not possible before OFHC copper Five 9s Al is more expensive than gold Ga for GaAs, GaN, etc. must be 8 9s pure (99.999999) Fe removal from clay to avoid brown spots in sanitary ware Separating rare-earths! The Chinese problem! Entropy drives mixing dictates ion dissolution from solid into liquid and from liquid to liquid electrical double layer theory mineral processing stability of emulsions / suspensions in ceramics / paint making nanoparticles and bottom-up nanoassembly Osmosis / dialysis
Some illustrations from thermodynamics Interplay of Entropy and Enthalpy in phase equilibria Not just phase equilibria in solids Fractional distillation / crystallisation, steam distillation Zone refining Oil-water mixing and detergents Separation of fat from milk during denaturing Enclosed miscibility gaps Constant temperature baths Getting water from icebergs
Free Energy & Chemical Potential Energy and its conversion chemistry to heat to work OR electrochemistry directly to work Photons to work Low grade heat and high grade heat, no free lunch The origin of dissipation The free energy change when an atom is added •Darken experiment; Si and C like each other (think of SiC!) •Selective dissolution of Ag from a Au-Ag alloy •Maximum voltage for anodising before water starts splitting •Stability of electrodes or electrolytes when the EMFs for dissociation are approached
Conjugate variables Inter-relate traditionally distinct areas Cracks can propagate at constant load / displacement . The stability of mechanical systems internal energy change at constant volume (no work done) or at constant pressure (PV work done) OC voltage and SC current in fuel / solar cells Electrical analogues of the constrained stress or unconstrained strain developed in a particle undergoing a phase transformation Actuation from electro / magneto striction or SMA maximum work extractable is always less than the ideal finite current (displacement) resistances (dissipation) A high EMF is like a tall dam. High current is like a broad shallow barrage. Solar cells are like barrages and need electrical stepping up. Windmills need mechanical stepping up (gears). It is more convenient to spin a small turbine at high speed than a huge turbine slowly. But that’s why small is not beautiful.
Bonding and Structure Defer crystallography until bonding / packing are explained • Use the interatomic U-r curve as much as possible • Metals from packing considerations (including interstitials) • Ionic crystals and Madelung constants can be taught without Miller indices • Solutions Hume Rothery, link to property changes (band gap changes with electronegativity difference in II-VI and III- V, not just Vegard) • Solutions and constitutional defects (compensating ions) • Clearly distinguish compositions and phases; magnetite to γ -Fe 2 O 3 is an example of miscibility with the solute being electrons and oxygen vacancies
Transport & Transformations • Must we split transport into heat / mass transfer & solid state diffusion? • All transformations (solid-solid, vapour-solid, solid-liquid) • Examples from ceramics as well as metals: zirconia, ferroelectrics), shape memory alloys • Inter-relate driving forces and nucleation from different areas e.g., precipitation in aqueous solutions uses chemical potentials of reactants to control G (not temperature), e.g., aragonite platelets in shells, apatite in bones • e.g., nucleation of reverse domains analagous to overcoming a barrier with magnetostatic driving force • Commonplace examples like ice-cream and cloud formation
Mechanical properties Less obsession with crystallographic slip! Begin with ideas of work, force, stiffness, viscosity and displacement do not start with deformation of metals at low homologous temp. Start with generalised Voigt / Maxwell models, Standard linear solids Do anelasticty, creep without distinguishing between material classes Teach as much as possible with scalar quantities before getting into tensors. Mechanical properties need to be taught extensively in a phenomenological way before bringing in crystallography, texture and crystal plasticity
Dynamic effects (response to cyclic impulses) • anelastic response in general (Zener), the concept of resonance and a time scale of response matching the (inverse) frequency of excitation • relaxation in polymers and glasses, Snoek, PLC • impedance curves (electrochemistry) and different mechanisms of polarisation • atomic force microscopy in the dynamic mode (this is not formidable; any BSc Physics student or mechanical engineering student has the math to handle it) • Superparamagnetism and blocking temperature, the life of information in the hard disc are all related to switching times for domains. Once basic magnetism is taught, even NMR can be explained
Electronic structure and properties In 2014, if your students do not know the definition of a metal and why metals conduct electricity, they have no right to be called metallurgists or materials scientists! They will also not understand solar cells, GMR read heads, thermoelectrics, oxide sensors, infra red detectors, lasers, exchange spring magnets and the whole of the semiconductor industry Bite the bullet! Physicists will not do the whole job for you! • Build on basic quantum mechanics (a first course that goes up to the harmonic operator and hydrogen atom is essential, so too the principles of the chemical bond) • Introduce energy bands with minimal math., make all the important concepts plausible and then get on with it. Leave the rigour to the condensed matter physicists
Electronic materials and links with classical theories • Holes are conceptually easier for those who have been exposed to vacancies • Defect equilibria for charged species are easily linked to reaction rate theory • Fermi level equalisation, junctions <-> chemical potential Choice of Ohmic and Schottky contacts • Choosing electrodes and electrolytes, e.g., oxide for photocatalysis should not dissociate before water splits • Corrosion is simply the reverse of all this
Materials Processing • Primary material extraction from ores (strengthen links with surface chemistry, colloids, wetting, chemo- mechanical effects) • Unification of processes involving mass transfer (primary metal production (thermal as well as electrical) CVD, crystal growth and solidification) • Solid state processes (sintering, metal working) Concepts followed by selective detail. If you must teach entire courses on one single element, make it an elective.
Materials Design and Selection • Design : alloying additions, thermomechanical treatment, composites, multilayer architectures • Link to component design and manufacturability • Selection: criteria for property optimisation (Ashby maps) • Vast field: Not ideal for teaching, Select a few examples from different material and application classes, make students deliver a term paper
Modeling and numerical analysis Do we need it? Most certainly Do not begin with methods. Start with operations research type exercises in how to frame a problem, first conceptually and then mathematically What sort of modeling methods do we need to teach? Who should teach it? How do we connect with the rest of the syllabus? Most importantly: who will teach mathematics in a useful way to materials people???
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