Imperatives in a materials education Vikram Jayaram Materials - - PowerPoint PPT Presentation

Curricula in Materials and Chemical Sciences TEQIP IIT-KANPUR Feb 2014

Vikram Jayaram Materials Engineering IISc

Imperatives in a materials education

ENGINEERING SCIENCE

PHYSICS CHEMISTRY BIOLOGY EARTH SCIENCE 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

• ver-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?

Mud Wood Cement / Stone Pottery & Ceramic

THE ANCIENT

WORLD

Glass Textiles Blacksmithery and iron

The Modern world of Materials It’s all the same today, except for….

Glass fibre Rayon Aluminium Ceramic spark plug Composite ???????

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-Fe2O3 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
• vercoming 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
• f 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.,
• xide 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???

Characterisation

• Structural characterisation, thermal analysis 

combine theory with extensive hands-on exposure

• Avoid obsession with diffraction  spectroscopy and

scanning probe methods needed in a first course

• Electrical characterisation  handling equipment as

well as examining material properties

What do we need from others?

• Mathematics (on-going debate! Taught by

friendly engineers? Physicists? Mathematicians?)

• Physical Chemistry, solution thermodynamics,
• rbitals, chemical bonds and some solid state

chemistry

• Basic quantum mechanics
• Electrical systems, basic electronics
• Solid mechanics, fluid mechanics
• Manufacturing

Length scales, Dimensional Analysis and backs of envelopes

The interplay between competing energies leading to a critical length scale

• domain widths in ferromagnetism, critical nuclei and

crack lengths, dislocation core width, twins in a displacive transformation, coercivity maxima in nanoparticles

• when do length scale effects really start to play a role,

i.e., when is “nano” really the harbinger of something new as opposed to something just small?

Error analysis, behaviour of functions, dimensional analysis with practical examples (soft course, but must be practical; perhaps over a summer)

Modifications to teaching

• Avoid ppts (always crammed with info and go

too fast)

• Frequent tests (allows for early detection of

problem cases); make them open book except at the end

• Make assignments (and exams) more thinking-

based

• Encourage situations wherein students ask

questions when they do not understand:

the lever rule why is there an Mf and an Ms?

Laboratory practice and projects

• Thorny implementation issue! Staff intensive!
• Need creative experiments (better to design a

fluidised bed with ping-pong balls to illustrate principles than make students draw blast furnaces)

• Examining how components are made
• Do not abolish projects for heaven’s sake!

Needs faculty discussion, PhD student involvement

At the end of the day

What are we trying to fix?

• Increase high quality intake into materials
• Materials graduates do not interact well with other

engineering disciplines; even when they become professors

• Rigid (inflexible) faculty recruitment strategies
• Clarity on the employment we are preparing them for

Finally: We rely on others (physicists, chemists, engineers) to provide the background we need; this requires negotiation!

The idea of an activated state

Thermal Activation: the notion of thermal excitations promoting the formation of an activated state is all-pervasive. Reaction rate theory fundamentals need to be strengthened