Electrical and Electronic Materials (EX-504 (C)) - Unit 3 Short Notes

UNIT 3: ELECTRICAL AND ELECTRONIC MATERIALS

I. INTRODUCTION AND CLASSIFICATION OF ENGINEERING MATERIALS

A. Broad Classification

Engineering materials are primarily classified into five categories based on their atomic structure and properties:

B. Distinction Between Metals and Alloys

• Metal: A pure elemental substance (e.g., pure copper, pure iron) with metallic bonding.

• Alloy: A homogeneous mixture of two or more elements, where at least one is a metal. Created to enhance properties (strength, corrosion resistance, conductivity).

  • Ferrous Alloys: Iron-based (e.g., steel, cast iron). Magnetic, high strength.

  • Non-Ferrous Alloys: No iron base (e.g., brass, bronze, aluminum alloys). Non-magnetic, often lighter, better corrosion resistance.

[!TIP] Exam Focus: Questions often ask for examples and reasons for alloying (e.g., why Cu is alloyed with Sn to make bronze? → Improved strength & corrosion resistance).

C. Ceramic Materials

• Definition: Inorganic, non-metallic solids formed by heating and cooling. Bonding is a mix of ionic and covalent.

• Examples & Applications:

  • Traditional: Clay products (bricks, tiles), cement, glass.

  • Advanced/Engineering: Alumina (substrates, insulators), Silicon Carbide (abrasives, high-temp), Barium Titanate (capacitor dielectric), Zirconia (thermal barrier coatings).

II. CRYSTAL STRUCTURE AND MATERIAL SYNTHESIS

A. Crystal Structures

• Unit Cell: The smallest repeating unit that defines the crystal structure.

• Bravais Lattices: 14 distinct 3D lattice types.

• Crystal Systems: 7 systems (Cubic, Tetragonal, Orthorhombic, Hexagonal, Trigonal, Monoclinic, Triclinic) based on unit cell parameters (a, b, c, α, β, γ).

• Common Structures:

  • Simple Cubic (SC): Atoms at corners.

  • Body-Centered Cubic (BCC): Atoms at corners + body center. (e.g., α-Fe, Cr, W).

  • Face-Centered Cubic (FCC): Atoms at corners + face centers. (e.g., γ-Fe, Cu, Al, Au).

  • Hexagonal Close-Packed (HCP): (e.g., Mg, Zn, Ti).

B. Crystal Defects

• Point Defects: Vacancies, interstitials, substitutional impurities (dopants in semiconductors).

• Line Defects (Dislocations): Edge and screw dislocations. Crucial for plastic deformation.

• Surface/Planar Defects: Grain boundaries, stacking faults, twin boundaries.

C. Material Purification & Crystal Growth

• Zone Refining: A purification technique for metals/semiconductors. A molten zone (heater) moves along a solid rod, carrying impurities to one end. Repeated passes yield high purity.

  DiagramSEARCH: zone refining process diagram

• Bridgman Technique: Crystal growth from melt. A crucible with molten material is slowly lowered through a temperature gradient, allowing a single crystal to solidify from a seed crystal.

  DiagramSEARCH: Bridgman crystal growth technique diagram

III. CONDUCTING MATERIALS

A. Electrical Properties

• Electrical Conductivity (σ): Measure of a material's ability to conduct electric current.

$$\sigma = n e \mu_e$$

where $n$ = charge carrier density (m⁻³), $e$ = electron charge ($$\displaystyle 1.6 \times 10^{-19} $$ C), $$\displaystyle \mu_e $$ = electron mobility (m²/V·s).

• Resistivity (ρ): $$\displaystyle \rho = 1/\sigma $$ (Ω·m).

• Drift Velocity ($$\displaystyle v_d $$): Average velocity of electrons under an electric field $\vec{E}$.

$$v_d = \mu_e E$$

• Scattering Mechanisms & Mobility:

  Electrons scatter off lattice vibrations (phonons) and ionized impurities. Matthiessen's Rule states:

$$\boxed{\frac{1}{\mu_e} = \frac{1}{\mu_i} + \frac{1}{\mu_l}}$$

where $$\displaystyle \mu_i $$ = mobility limited by ionized impurities, $$\displaystyle \mu_l $$ = mobility limited by lattice vibrations.

> [!TIP] At **low temperatures**, $$\displaystyle \mu_i $$ dominates (impurities fixed). At **high temperatures**, $$\displaystyle \mu_l $$ dominates (phonon density increases).

B. Common Conducting Materials

• Copper (Cu): Highest conductivity among non-precious metals (IACS ≈ 100%). Ductile, good thermal conductivity. Used in windings, cables, busbars.

• Aluminum (Al): ~61% IACS of Cu. Lighter, cheaper, forms protective oxide layer. Used in overhead lines, busbars, windings (larger cross-section needed).

• Alloys:

  • Cu Alloys: Brass (Cu-Zn), Bronze (Cu-Sn) – higher strength, lower conductivity.

  • Al Alloys: Al-Mg-Si (e.g., 6000 series) – good strength, weldability.

C. Material Selection for Specific Applications

D. Numerical Problem (Example)

Given: $$\displaystyle l=0.2 $$ m, $$\displaystyle A=1 \text{ mm}^2 = 10^{-6} \text{ m}^2 $$, $$\displaystyle V=4 $$ V, $$\displaystyle \mu_e=4.5 \times 10^{-6} \text{ m}^2/\text{V·s} $$, $$\displaystyle n=8.5 \times 10^{28} \text{ m}^{-3} $$. Find: Current $I$. Solution:

1. $$\displaystyle \sigma = n e \mu_e = (8.5 \times 10^{28}) \times (1.6 \times 10^{-19}) \times (4.5 \times 10^{-6}) = 6.12 \times 10^{4} \text{ S/m} $$.

2. $$\displaystyle R = \rho l / A = (1/\sigma) \times l / A = (1/6.12 \times 10^{4}) \times 0.2 / 10^{-6} = 3.27 \text{ Ω} $$.

3. $$\displaystyle I = V/R = 4 / 3.27 \approx 1.22 \text{ A} $$.

IV. SEMICONDUCTING MATERIALS

A. Energy Band Theory

• Conductors: Valence band (VB) and conduction band (CB) overlap or VB is partially filled. No band gap ($$\displaystyle E_g \approx 0 $$).

• Semiconductors: VB is full, CB is empty at 0K. Small band gap ($$\displaystyle E_g \sim 1 $$ eV). Thermal energy excites electrons to CB.

  DiagramCANVAS: Energy band diagrams side-by-side for conductor, semiconductor (intrinsic), and insulator. Show VB, CB, Fermi level, and band gap.

B. Intrinsic Semiconductors

• Pure semiconductor (Si, Ge). Electron-hole pairs generated thermally: $$\displaystyle n = p = n_i $$.

• Carrier Concentration ($$\displaystyle n_i $$):

$$n_i^2 = N_c N_v e^{-E_g/kT}$$

where $$\displaystyle N_c, N_v $$ = effective density of states in CB/VB, $k$ = Boltzmann constant, $T$ = temperature.

At room temperature, $$\displaystyle n_i $$ for Si $$\displaystyle \approx 1.5 \times 10^{10} \text{ cm}^{-3} $$, for Ge $$\displaystyle \approx 2.4 \times 10^{13} \text{ cm}^{-3} $$.

C. Extrinsic Semiconductors

• n-type: Pentavalent dopant (P, As in Si). Donor level $$\displaystyle E_d \approx 0.05 $$ eV below CB. Majority carriers = electrons ($n \gg p$).

• p-type: Trivalent dopant (B, Al in Si). Acceptor level $$\displaystyle E_a \approx 0.05 $$ eV above VB. Majority carriers = holes ($p \gg n$).

• Comparison:

  | Feature | Intrinsic | Extrinsic (n/p-type) | | :--- | :--- | :--- | | Carrier Source | Thermal generation | Impurity ionization | | Majority Carriers | $$\displaystyle n = p = n_i $$ | $n \gg p$ (n-type) / $p \gg n$ (p-type) | | Conductivity | Low | High (at same T) | | Fermi Level | Near mid-gap | Closer to CB (n) / VB (p) |

D. Hall Effect

• Principle: When current $I$ flows through a semiconductor in x-direction and magnetic field $B$ is applied in z-direction, a transverse Hall voltage $$\displaystyle V_H $$ develops in y-direction due to Lorentz force.

  DiagramSEARCH: Hall effect setup semiconductor sample

• Hall Coefficient ($$\displaystyle R_H $$) for n-type:

$$R_H = \frac{1}{n e} \quad (\text{Negative for n-type})$$

Derivation:

1.  Equilibrium: $$\displaystyle e E_H = e v_d B \Rightarrow E_H = v_d B $$.

2.  $$\displaystyle V_H = E_H \cdot w = v_d B w $$.

3.  $$\displaystyle I = n e (v_d \cdot t \cdot w) \Rightarrow v_d = I/(n e t w) $$.

4.  Substitute: $$\displaystyle V_H = (I B)/(n e t) \Rightarrow R_H = V_H t / (I B) = 1/(n e) $$.

• Applications & Importance:

  • Determine type (sign of $$\displaystyle R_H $$), concentration ($n$ or $p$), and mobility ($$\displaystyle \mu = \sigma |R_H| $$).

  • Magnetic field sensors, current sensors.

E. Compound Semiconductors

• Gallium Arsenide (GaAs):

  • Direct band gap ($$\displaystyle E_g \approx 1.42 $$ eV at 300K) → Efficient light emission/absorption.

  • Higher electron mobility ($$\displaystyle \mu_n \approx 8500 \text{ cm}^2/\text{V·s} $$) than Si.

  • Applications: High-frequency devices (microwave ICs), solar cells, LEDs, laser diodes.

• Gallium Phosphide (GaP):

  • Indirect band gap ($$\displaystyle E_g \approx 2.26 $$ eV). Emits green/yellow light when doped with N.

  • Applications: LEDs (low efficiency), optoelectronic substrates.

F. Semiconductor Optoelectronic Devices

• Semiconductor Laser (Laser Diode):

  • Principle: Stimulated emission in forward-biased p-n junction with direct band gap. Requires optical cavity (cleaved ends).

  • Characteristics: Coherent, monochromatic, fast switching, low power.

  • Applications: Fiber optic communications, CD/DVD players, laser printers, barcode scanners.

• Avalanche Photodiode (APD):

  • Principle: Reverse-biased p-n⁺ junction. Photogenerated carriers gain enough energy to create secondary electron-hole pairs (avalanche multiplication). High internal gain.

  • Applications: High-sensitivity optical receivers (long-haul fiber optics), LIDAR.

• Photoconductive Cell (LDR) vs. Photovoltaic Cell (Solar Cell):

  | Feature | Photoconductive Cell (LDR) | Photovoltaic Cell (Solar Cell) | | :--- | :--- | :--- | | Operation Mode | Variable resistor (conductivity changes with light) | Power source (generates voltage/current) | | Bias | Usually zero or small bias | No external bias (photovoltaic mode) | | Output | Change in resistance/current | DC voltage & current (power) | | Material | CdS, CdSe | Si (crystalline, amorphous), GaAs, CdTe | | Response Time | Slow (ms) | Fast (µs-ns) | | Primary Use | Light detection, switching, alarms | Power generation |

V. DIELECTRIC MATERIALS

A. Polarization in Dielectrics

• Definition: Alignment of induced or permanent dipoles under an electric field $\vec{E}$, creating a dipole moment per unit volume $\vec{P}$.

• Types:

  1. Electronic Polarization ($$\displaystyle \vec{P}_e $$): Displacement of electron cloud relative to nucleus. Present in all materials. Very fast ($$\displaystyle 10^{-15} $$ s).

  2. Ionic Polarization ($$\displaystyle \vec{P}_i $$): Displacement of positive/negative ions in ionic crystals. ($$\displaystyle 10^{-12} $$-$$\displaystyle 10^{-13} $$ s).

  3. Orientational (Dipole) Polarization ($$\displaystyle \vec{P}_o $$): Alignment of permanent molecular dipoles. ($$\displaystyle 10^{-10} $$-$$\displaystyle 10^{-2} $$ s). Absent in non-polar dielectrics.

  4. Space Charge (Interfacial) Polarization ($$\displaystyle \vec{P}_s $$): Accumulation of charges at interfaces (grain boundaries, electrodes). Very slow, significant at low frequencies.

B. Electric Susceptibility and Relative Permittivity

• Electric Susceptibility ($$\displaystyle \chi_e $$): Measure of ease of polarization. $$\displaystyle \vec{P} = \varepsilon_0 \chi_e \vec{E} $$.

• Relative Permittivity ($$\displaystyle \varepsilon_r $$): $$\displaystyle \varepsilon_r = 1 + \chi_e $$. Also $$\displaystyle \vec{D} = \varepsilon_0 \varepsilon_r \vec{E} $$.

• Numerical Example (Polar Molecules):

  Given: $$\displaystyle N = 2 \times 10^{28} \text{ m}^{-3} $$, $$\displaystyle p = 1.8 \times 10^{-27} \text{ C·m} $$, $$\displaystyle E = 10^5 \text{ V/m} $$.

  1. $$\displaystyle \vec{P} = N \vec{p} = (2 \times 10^{28}) \times (1.8 \times 10^{-27}) = 36 \text{ C/m}^2 $$.

  2. $$\displaystyle \chi_e = P / (\varepsilon_0 E) = 36 / (8.85 \times 10^{-12} \times 10^5) \approx 4.07 \times 10^6 $$.

  3. $$\displaystyle \varepsilon_r = 1 + \chi_e \approx 4.07 \times 10^6 $$.

  [!TIP] This is an ideal case assuming complete alignment. Real $$\displaystyle \chi_e $$ is much lower due to thermal agitation.

C. Dielectric Loss and Dissipation Factor

• Dielectric Loss ($$\displaystyle P_L $$): Power dissipated as heat in a dielectric under AC field due to lag of $\vec{P}$ behind $\vec{E}$ (loss tangent).

$$P_L = \omega C_0 \varepsilon_0 \varepsilon_r'' V E^2 = \omega C_0 \varepsilon_0 \varepsilon_r \tan\delta \cdot V E^2$$

where $$\displaystyle \varepsilon_r'' $$ = imaginary part of permittivity, $$\displaystyle \tan\delta = \varepsilon_r''/\varepsilon_r' $$ = **dissipation factor**.

• Factors Affecting Loss:

  • Frequency (resonance peaks).

  • Temperature (increases molecular motion).

  • Material purity and moisture content.

  • Applied electric field strength.

D. Dielectric Strength

• Definition: Maximum electric field that a dielectric can withstand without breakdown (V/m or kV/mm).

• Factors Affecting:

  • Material thickness, temperature, humidity.

  • Electrode shape, applied voltage waveform (duration).

  • Impurities, voids, manufacturing defects.

E. Classes of Insulating Materials (Thermal Classes)

Classified by maximum operating temperature (IEC 60085):

F. Applications

• Capacitor Dielectrics: High $$\displaystyle \varepsilon_r $$, low loss (e.g., Ceramic: BaTiO₃; Film: Polypropylene).

• Power Systems: Transformer oil (mineral oil), pressboard, porcelain, SF₆ gas (GIS), XLPE/EPR cables.

VI. MAGNETIC MATERIALS

A. Classification Based on Magnetic Susceptibility ($$\displaystyle \chi_m $$)

B. Hard vs. Soft Magnetic Materials

• Soft Magnetic Materials:

  • Characteristics: Low coercivity ($$\displaystyle H_c $$), high permeability ($\mu$), narrow hysteresis loop → low hysteresis loss.

  • Examples: Silicon steel (transformer cores), Permalloy (Ni-Fe), Ferrites.

  • Applications: Transformer cores, motor/Generator stators, electromagnet yokes.

• Hard Magnetic Materials (Permanent Magnets):

  • Characteristics: High coercivity ($$\displaystyle H_c $$), high remanence ($$\displaystyle B_r $$), high energy product $$\displaystyle (BH)_{max} $$, wide hysteresis loop.

  • Examples: Alnico (Al-Ni-Co), Hard Ferrites (Ba/Sr ferrite), Rare-earth (NdFeB, SmCo).

  • Applications: Permanent magnets in motors, speakers, magnetic separators, storage.

[!TIP] Key Difference: Soft → easy to magnetize/demagnetize (low $$\displaystyle H_c $$). Hard → hard to demagnetize (high $$\displaystyle H_c $$).

C. Permeability and Hysteresis

• Permeability ($\mu$): $$\displaystyle \mu = B/H $$. Initial permeability (low H), maximum permeability (peak of B-H curve), differential permeability (dB/dH).

• Factors Affecting Permeability: Composition, heat treatment, grain orientation (grain-oriented silicon steel), mechanical stress, temperature (Curie point $$\displaystyle T_c $$).

• Hysteresis Loss: Energy dissipated per cycle = area of hysteresis loop. Increases with frequency.

  • Factors: Material (loop area), frequency, peak flux density ($$\displaystyle B_m $$). Steinmetz equation: $$\displaystyle P_h = \eta f B_m^x $$ (x ≈ 1.6-2.5).

D. Magnetization Curve (B-H Curve)

• Plots magnetic flux density $B$ vs magnetic field intensity $H$.

• Regions:

  1. Initial magnetization: Domain wall motion.

  2. Approach to saturation: Domain rotation.

  3. Saturation: All moments aligned, $B$ increases only with $$\displaystyle \mu_0 H $$.

  4. Demagnetization (reversal): Hysteresis loop.

• For soft materials, curve is steep and loop narrow. For hard materials, loop is wide.

E. Applications

• MHD Generator Principle: Hot ionized gas (plasma) flows perpendicular to $\vec{B}$. Lorentz force separates charges → direct electricity generation without moving parts.

  DiagramSEARCH: MHD generator principle diagram

• Transformers/Motors: Soft magnetic cores (Si steel, ferrites) to channel flux with low loss.

• Magnetic Storage: Hard disk drives (Co-Cr-Pt alloy), magnetic tapes (γ-Fe₂O₃).

VII. SUPERCONDUCTING MATERIALS

A. Critical Parameters

• Critical Temperature ($$\displaystyle T_c $$): Temperature below which material becomes superconducting (zero resistivity). e.g., NbTi ($$\displaystyle T_c \approx 9.2 $$ K), YBCO ($$\displaystyle T_c \approx 93 $$ K).

• Critical Magnetic Field ($$\displaystyle H_c $$): Maximum field that can be applied before superconductivity destroys. Decreases with T: $$\displaystyle H_c(T) = H_c(0)[1 - (T/T_c)^2] $$.

  Numerical Example (May 2024): Given $$\displaystyle H_c(12\text{K})=15 $$ T, $$\displaystyle H_c(10\text{K})=18 $$ T.

  1. Use formula: $$\displaystyle H_c(T) = H_c(0)[1 - (T/T_c)^2] $$.

  2. Set up equations:

  $$\displaystyle 15 = H_c(0)[1 - (12/T_c)^2] $$
  

  $$\displaystyle 18 = H_c(0)[1 - (10/T_c)^2] $$
  

  3. Divide: $$\displaystyle 15/18 = [1 - (144/T_c^2)] / [1 - (100/T_c^2)] \Rightarrow 5/6 = (T_c^2 - 144)/(T_c^2 - 100) $$.

  4. Solve: $$\displaystyle 5(T_c^2 - 100) = 6(T_c^2 - 144) \Rightarrow 5T_c^2 - 500 = 6T_c^2 - 864 \Rightarrow T_c^2 = 364 \Rightarrow T_c \approx 19.1 \text{ K} $$.

  5. $$\displaystyle H_c(0) = 15 / [1 - (12/19.1)^2] \approx 15 / [1 - 0.394] \approx 24.8 \text{ T} $$.

• Critical Current Density ($$\displaystyle J_c $$): Maximum current density before superconductivity breaks down (due to self-field).

B. Meissner Effect

• Definition: Complete expulsion of magnetic flux from the interior of a superconductor when cooled below $$\displaystyle T_c $$ in a field (perfect diamagnetism, $$\displaystyle \chi_m = -1 $$).

• Significance: Distinguishes superconductivity from perfect conductivity. A perfect conductor would trap flux, a superconductor expels it.

C. Types of Superconductors

D. Applications

• MRI: High-field (1.5-3 T) superconducting magnets (NbTi/Nb₃Sn).

• Particle Accelerators: LHC (CERN) uses NbTi cables for 8.3 T dipoles.

• Power Transmission: Experimental cables (high $$\displaystyle J_c $$, low loss).

• Maglev Trains: Superconducting magnets for levitation.

• SQUIDs: Ultra-sensitive magnetometers (biomagnetism, geophysics).

VIII. SPECIAL AND ADVANCED MATERIALS

A. Gaseous Insulating Materials: SF₆

• Properties: Electronegative gas (captures free electrons → negative ions), high dielectric strength (2-3× air at same pressure), chemically inert, non-flammable, arc-quenching.

• GIS (Gas Insulated Switchgear) Specifications:

  • Purity: >99.9% (moisture < 15 ppm).

  • Pressure: Typically 0.4-0.6 MPa (gauge).

  • Advantages: Compact size, high reliability, safe in polluted environments, silent operation.

  DiagramSEARCH: GIS gas insulated switchgear substation

B. Liquid Insulating Materials: Transformer Oils

• Primary Function: Insulation and cooling in power transformers.

• Testing Methods:

  1. Breakdown Voltage (BDV): Measures dielectric strength. Low BDV indicates contaminants/water.

  2. Dissolved Gas Analysis (DGA): Detects gases (H₂, CH₄, C₂H₂, CO, CO₂) dissolved in oil. Indicates internal faults (thermal, electrical) via key gas ratios (Rogers ratio, Duval triangle).

  3. Water Content: Measured in ppm. High water drastically reduces BDV.

  4. Acidity (Neutralization Value): Oxidation products form acids → corrode paper insulation.

C. Lightweight and Porous Materials

• Ultralight Materials: Metallic foams, aerogels. Extremely low density (< 0.1 g/cm³).

• Metallic Foams: Metal (Al, Ni) with closed or open cells. Structure: solid matrix + gas pores.

  • Properties: Low density, high surface area, good energy absorption, sound/heat insulation.

  • Applications: Lightweight structural panels (aerospace), heat exchangers, battery electrodes, sound absorbers.

D. Nanomaterials

• Definition: Materials with at least one dimension < 100 nm.

• Significance: High surface-to-volume ratio → quantum effects, enhanced mechanical/electrical/optical properties.

• Examples: Carbon nanotubes (high strength, conductivity), Quantum dots (size-tunable optoelectronics), Nanocrystalline metals (high strength).

E. Polymeric and Display Materials

• Polymers:

  • Electrical Properties: Generally insulating ($$\displaystyle \rho > 10^{10} $$ Ω·cm). Can be made conductive via doping (e.g., polyacetylene).

  • Uses: Insulation (PVC, PE), capacitors (polypropylene), PCBs (epoxy-glass), packaging.

• Liquid Crystal Displays (LCDs):

  • Principle: Liquid crystals (LCs) are anisotropic fluids. Their orientation (and thus light transmission) is controlled by electric field.

  • Materials: Twisted Nematic (TN) LCs, In-Plane Switching (IPS) LCs.

  • Structure: Polarizers, glass substrates with ITO electrodes, alignment layers, LC layer, color filters.

  DiagramSEARCH: LCD pixel structure cross-section

F. Voltage-Dependent Resistors (Varistors)

• Material: Metal Oxide Varistor (MOV) – mainly Zinc Oxide (ZnO) grains with Bi₂O₃ additive.

• V-I Characteristic: Highly non-ohmic. Low resistance at high voltage (clipping), high resistance at low voltage.

$$I = k V^\alpha \quad (\alpha \gg 1, \text{ typically } 20-50)$$

• Principle: Grain boundaries form potential barriers. High voltage causes avalanche breakdown of barriers.

• Applications: Surge protection in power supplies, substations, telecom lines.

IX. MATERIALS FOR SPECIFIC ELECTRONIC/POWER DEVICES

A. Resistor Materials

B. Materials for Power Electronics

• Busbar Materials: Copper (highest conductivity) or Aluminum (weight/cost advantage). Often plated (Sn, Ag) for oxidation resistance and contact reliability.

• Substrates for Power Devices:

  • Need high thermal conductivity, good electrical insulation, CTE match to Si/SiC.

  • Direct Bonded Copper (DBC): Alumina (Al₂O₃) or AlN ceramic with copper bonded.

  • Insulated Metal Substrate (IMS): Aluminum base with thin dielectric (epoxy) and copper foil.

  • Advanced: Si₃N₄, diamond composites for extreme power density.

X. INTEGRATED APPLICATIONS AND CASE STUDIES

A. Material Selection for Power Systems

• GIS: SF₆ gas for insulation/arc quenching. Enables compact substations for urban areas.

• Transformers:

  • Core: Grain-oriented silicon steel (low hysteresis loss).

  • Windings: Copper (or Al for large transformers).

  • Insulation: Mineral oil (cooling/insulation) + cellulose paper/pressboard (solid insulation).

• Cables:

  • Conductor: Cu (urban, underground) / Al (overhead).

  • Insulation: XLPE (cross-linked polyethylene) for high voltage, EPR for medium voltage.

  • Sheath: PVC or PE for moisture protection.

B. Material Challenges in Modern Devices

• Optoelectronics: Need direct band gap materials (GaAs, InP) for efficient lasers/LEDs. Challenges: lattice matching, defect control for high-efficiency solar cells.

• High-Frequency/Power: Wide bandgap semiconductors (SiC, GaN) for high-temperature, high-frequency, high-power devices (EV inverters, RF amplifiers). Challenges: substrate cost, defect density.

• High-Voltage DC (HVDC): Cable insulation (XLPE) must withstand high DC stress → space charge accumulation management.

• Energy Storage: Battery electrodes require high capacity, stability, safety (Li-ion: NMC, LFP; Solid-state: sulfide/oxide electrolytes).

[!TIP] Exam Strategy: For "material selection" questions, always link property → application requirement. E.g., "For transformer core, low hysteresis loss (soft magnetic) is required → use grain-oriented Si steel."