Survey of Metallic & Non-Metallic Materials Used in Electronics – Detailed Study Notes

Importance of Materials in Electrical & Electronic Engineering

• Development Link
  • Advancements in any engineering discipline are tightly coupled to improvements in materials science & engineering.
  • Progress requires understanding both the science (atomic / electronic structure) and the technology (processing / fabrication) of materials.
• Drivers of Recent Progress
  • Rapid evolution of electron-based computers (miniaturization, speed, power density).
  • Replacement of vacuum valves by VLSI integrated circuits.
  • Emergence of conducting polymers (organic conductors) enabling flexible electronics.
  • Ferroelectrics providing a modern class of high-$
    ϵ_r$ dielectrics for capacitors & memory.
  • Ferrites establishing superior magnetic cores for high-frequency devices.
• 21st-Century (Smart) Materials Vision
  • Hair-sized conductors capable of carrying high-voltage transmission current.
  • Hybrid magnets exceeding $100\,\text{T}$ for research & medical systems.
  • Magnetic refrigerators operating above $100\,\text{K}$ using magnetocaloric effects.
  • Infra-red sensor crystals such as $\text{Hg}<em>{1-x}\text{Cd}</em>x\text{Te}$.
  • Smart-material family includes ferrites, ferroelectrics, garnets, whiskers, ceramic superconductors, ruby lasers, super-alloys, crystalline $\text{C}_{60}$ (bucky-balls), etc.
  • Futuristic breakthroughs hinge on tuning specific material properties (conductivity, magnetism, etc.).

Broad Classes of Engineering Materials

• Metals
  • Elemental substances; plastically deformable; excellent electrical & thermal conductors.
  • Sub-classes: Ferrous (iron-based) & Non-ferrous (no Fe-C matrix).
  • Ferrous: Classification controlled by carbon % and its phase / constituents (graphite, cementite, etc.). Difference between mild steel, cast iron, etc. entirely carbon-dependent.
• Non-Ferrous Metals
  • Examples & special traits:
  • $\text{Al},\;\text{Cu},\;\text{Ag},\;\text{Au}$ → high conductivity.
  • $\text{Ag}$: most malleable; $\text{Au}$: most ductile.
  • $\text{Cr}$: corrosion-resistant (protective oxide).
  • $\text{Zn}$: galvanizing / plating; $\text{Sn}$: bearing bushes; $\text{Ni}$: strength + creep resistance.
• Ceramics
  • Mostly metal/non-metal oxides; may include mixed phases yet chemically homogeneous.
  • Conventional: rocks, glasses, fireclay, cement, lime.
  • Modern: ferrites, garnets, ferroelectrics, ceramic superconductors.
• Organic Polymers
  • Hydrocarbon-derived chains linked by covalent $\text{C}!! - !!\text{H}!! - !!\text{O}$ bonds; light, inert, high plasticity.
  • “Poly” (many) + “mer” (units): monomers polymerize → high molecular weight (Bakelite, polyethylene, nylon, teflon).
• Alloys
  • Metallic mixtures whose overall properties differ from constituents; tailored for specific applications (ferrous or non-ferrous based).
• Composites
  • Two+ constituents with dissimilar properties (metal/ceramic/polymer combinations).
  • Reinforcement forms: particulate, fibrous, flake.
  • Current: fibrous composites; Future: whisker-reinforced.

Classification of Electrical & Electronic Materials

• Conductors, Semiconductors, Dielectrics (Insulators), Superconductors, Magnetic materials, Ferroelectrics, Piezoelectrics, Perovskites, Spinels.

Conductors

• Possess “plenty” of free electrons; valence & conduction bands overlap.
• Resistivity ≈ $10^{\,-9}\,\Omega\,\text{m}$ (Au, Ag, Cu, Al).
• Influencing factors: temperature (positive coefficient), impurities, alloying, plastic deformation.
• Economic choices: Cu & Al for wires, machine windings; OFHC Cu for cryogenic use.

Semiconductors

• Energy gap ($E_g$) intermediate between conductors & insulators.
• \sigma{\text{ins}} < \sigma{\text{semi}} < \sigma_{\text{cond}}.
• Varieties:
  • Intrinsic: pure elemental Si, Ge (require $\approx10^8\,\text{V}$ to bridge gap → low practical value).
  • Extrinsic: doped $n/p$-type; compound (GaAs, CdTe), alloy (HgCdTe), oxides/halides/sulphides.
• Properties: hard, brittle, negative temperature coefficient of resistance.
• Key devices: rectifiers, amplifiers, diodes, transistors, photocells, power electronics, computing hardware.

Dielectrics (Insulators)

• Valence electrons tightly bound; E_g > 3\,\text{eV}; full valence band, empty conduction band.
• Forms & examples:
  • Solid: mica, bakelite, elastomeric fibre, paper.
  • Liquid: transformer oil, silicone oil, varnish.
  • Gas: air, $\text{N}_2$, ozone.
• Applications: capacitors, insulators, strain gauges, sonar.
• Important parameters:
  • Dielectric constant $\kappa$, dielectric strength, loss tangent $\tan\delta = \frac{\varepsilon''}{\varepsilon'}$.
  • Good dielectric → high $\kappa$, high strength, low $\tan\delta$ (transformer oil & fused silica $\tan\delta \approx 0.0001$).
  • Polarization mechanisms governed by time, frequency, temperature; relaxation time quantifies temporal response.

Superconductors

• Exhibit abrupt $R \rightarrow 0$, $B \rightarrow 0$ below a critical temperature $T_c$; hence $I^2 R$ losses vanish.
• Material forms: elemental, compound, alloy; metallic or ceramic; low-$T<em>c$ or high-$T</em>c$.
• Need remains to attain room-temperature superconductivity.
• Examples:
  • Metallic/compound: $\text{Nb}<em>3\text{Sn},\; \text{La}</em>{80}\text{Au}_{20},\; \text{Nb}!-!\text{Zr}!-!\text{Ti}$ alloys.
  • Ceramic (cuprate) high-$T<em>c$: $\text{YBa}</em>2\text{Cu}<em>3\text{O}</em>{7-x},\; \text{BiSrCaCuO},\; \text{ThBaCaCuO}$.
• Applications: levitated (maglev) trains, nuclear-fusion magnets, MRI, cryogenic electronics, MHD power.

Magnetic Materials

• Magnetism originates from electron spins + orbital motion + their interaction.
• Categories by magnetic moment alignment:
  • Diamagnetic: no net moment (Au, Bi, Cu).
  • Paramagnetic: weak moment (Al, Pt, $\text{NiSO}_4$).
  • Ferromagnetic: strong moment (Fe, Ni, Co, Gd).
  • Antiferromagnetic: antiparallel spins via exchange (metal oxides/sulphides).
  • Ferrimagnetic (Ferrites): unequal antiparallel sub-lattices → net moment.
• Ferromagnets by B-H area:
  • Soft (low hysteresis) → transformer cores.
  • Hard (high remanence) → permanent magnets for motors, alternators.

Ferrites (Special Class)

• Ceramic compounds of two metallic oxides, e.g. $\text{FeO}\cdot\text{Fe}<em>2\text{O}</em>3,\;\text{BaO}\cdot6\text{Fe}<em>2\text{O}</em>3$.
• High resistivity $\approx10^{7}\,\Omega\,\text{m}$, $\mu_r \approx10^{4}$; relaxation $10^{\,-9}!\text{--}!10^{\,-10}\,\text{s}$.
• Exist as soft or hard ferrites.

Ferroelectrics

• Dielectrics with spontaneous polarization; \varepsilon_r > 2000.
• Analogous to ferromagnets but electric instead of magnetic dipole ordering.
• Examples: Rochelle salt, $\text{SrTiO}<em>3$, $\text{PbZrO}</em>3$.
• Uses: miniature capacitors, electro-optic modulators, sonar.

Piezoelectrics

• Exhibit interconversion of mechanical strain ⇆ electric field (piezoelectric effect).
• Examples: Quartz, CdS, ZnO.
• Serve as transducers converting electrical ↔ mechanical signals (microphones, actuators).

Energy-Gap-Based Classification of Solids

• Conductors: overlapping bands, $E_g \approx 0$.
• Semiconductors: $E_g \sim 0.7\,\text{eV}$ (Ge) to $1.1\,\text{eV}$ (Si).
• Insulators: E_g > 3\,\text{eV} (diamonds $\sim6\,\text{eV}$).

Representative Electrical Resistivities (\Omega m)

• Conductors: $10^{\,-9}$ (Ag, Au, Cu) → $10^{\,-4}$ (Sb) → $10^{\,-2}$ (Nichrome).
• Semiconductors: $10^{\,-3}$ (doped Ge) → $10^{\,-1}$ (pure Ge) → $10^{\,-2}!\text{--}!10^{\,-3}$ (pure Si).
• Insulators: $10^{5}$ (glass) → $10^{17}$ (pure SiO$_2$).

Comparison Matrix

• Conductivity: High (conductors) vs. medium (semiconductors) vs. negligible (insulators).
• Temperature coefficient of resistance: Positive (conductors) vs. negative (semiconductors) vs. variable (insulators).
• Breakdown under high voltage: none → yes.
• Bond strength of valence electrons: loose → moderate → tight.

Application Domains & Material Choices

• Electrical Engineering: contacts, cables, magnets, transformers, capacitors, piezoelectrics, etc.
• Electronics: amplifiers, ICs, antennas, printed circuits, rectifiers, photoconductive cells.
• Computer Engineering: hardware, memory, displays, disks.
• Robotics: sensors, actuators, manipulators.
• Biotechnology / Medical: ECG, fibre-optic endoscopy, MRI.
• Instrumentation: transducers, oscilloscopes, thermistors.
• IT: networking cables, routers, optical fibre.
• Mechanical: furnaces, thermocouples, welding, X-ray systems.
• Consumer: bulbs, heaters, remote controls, microwave ovens.

Desired Property ↔ Application Mapping

1. Electrical: $\rho,\;\sigma,\;\kappa,\;E<em>b,\;\tau</em>{relax},\;\tan\delta$.
2. Magnetic: hysteresis, $B_r$, $\mu$, $\chi$, coercivity.
3. Electronic: drift, diffusion, $E<em>g$, $E</em>F$, mobility $\mu<em>n$, carrier density $n</em>i$.
4. Optical: reflection, refraction, transmission, luminescence.
5. Physical: density, $T_m$, shape, porosity.
6. Thermal: expansion $\alpha<em>T$, $k</em>T$, fatigue, shock.
7. Mechanical: creep, toughness, ductility.
8. Chemical: corrosion resistance, passivity.
9. Cryogenic: ductile→brittle transition, superconductivity.
10. Acoustical: damping, absorption.
11. Structural: strength, stiffness.
12. Surface: wear, erosion.
13. Metallurgical: phase rule, diffusion.
14. Technological: weldability, machinability.
15. Aesthetic: texture, lustre.
16. Nuclear: half-life $t_{1/2}$, decay constant $\lambda$, absorptivity.

Illustrative Required-Property Cases

• Porosity: beneficial for semiconductor doping, harmful for outdoor insulators.
• Unequal thermal expansion: exploited in bimetallic strips (thermostats, thermocouples).
• Fatigue resistance: lamp filaments facing voltage fluctuation.
• Corrosion resistance: universally desirable.
• Strength: vital for motor/alternator casings.
• Elasticity & ductility: wires, coils.
• Abrasion & wear resistance: electrical contacts (sparking, rubbing).
• Diffusion capability: Si/Ge doping.
• Weldability: laminated transformer cores.
• Damping: shielding against mechanical shocks & external EM fields.

Operational Requirement Spectrum

• Materials must match extremes of: voltage, current, temperature, frequency, polarization, remanence, resistivity, emission.
• Hence categories: high/low voltage, high/low conduction, high/low $T$, high/low frequency, high/low resistivity, high/low emission.

High-Voltage Service Materials (\ge110\,kV)

• High voltage lowers line losses $\Rightarrow$ economical power systems.
• Transmission benchmarks: $400\,\text{kV}$ (present), $800\,\text{kV}$ (India), $1000\,\text{kV}$ (Russia).
• Traditional gear bulky & costly ➜ demand new HV materials:
  • Overhead conductors: ACSR (Al reinforced by steel).
  • Underground cables: N$_2$-gas-filled pressure cables + oil-impregnated paper.
  • Insulators: purified benzene, hexane, silicone oil, ferroelectrics, mica, vacuum.
  • HF coils: powdered metals.
  • Circuit breaker contacts: Pd-Rh, W-Pt; insulating oil.
  • Low-loss capacitors: ferroelectrics, vacuum.

High- / Low-Temperature Service Materials

• Measurement or operation above metals’ melting points demands refractory materials.
• Melting points: W $3410\,^{\circ}\text{C}$, Ir $2454\,^{\circ}\text{C}$; Pt $1769\,^{\circ}\text{C}$, Cu $1083\,^{\circ}\text{C}$; Ga $30\,^{\circ}\text{C}$, F $-220\,^{\circ}\text{C}$.
• Thermocouples / heating coils use nichrome ($80\%$ Ni + $20\%$ Cr), Constantan, Kanthal, Pt-Rh.
• Cryogenic steels (Ni-alloys) for < -100^{\circ}\text{C} (service temperatures at $-100, -120, -190^{\circ}\text{C}$ with $3.5, 5.0, 9.0\%$ Ni respectively).

Selected Product–Material Pairings

• Film sound recording → magnetism → metallic glass.
• Accelerometer → piezoelectric effect → quartz.
• Solar cell → photovoltaic → compound semiconductor.
• Automatic door → photoconductivity → CdSe/CdS/CdTe.
• Computer memory → soft magnetism → magnetic bubbles.
• Maglev train → superconductivity → ceramic cuprates.
• Oscillographic recorder → electromagnetic → FeCoNi alloy, $\text{Gd}<em>3\text{Fe}</em>5\text{O}_{12}$.
• Capacitive transducer → saline-impregnated paper dielectric.
• Glow discharge tube → neon gas ionization.
• LVDT core → high-$\mu$ → permalloy/supermalloy/Si-Fe.
• Piezo microphone → piezoelectrics (quartz, Rochelle salt).
• Antenna rod → conductive metals (Al, Cu).

Emerging & Exotic Material Themes

• Spintronic Materials: exploit electron spin + charge for non-volatile memory.
• Ferromagnetic Semiconductors: combine magnetism with semiconductor electronics.
• Left-Handed (Negative-Index) Materials: exhibit simultaneous negative $\mu$ & $\varepsilon$ → reverse Snell refraction; metamaterial lenses.