Classification of Electrical Conducting Materials

Electrical conducting materials carry current, provide controlled resistance or make a moving electrical interface. Resistivity is one useful property, but selection also depends on temperature, strength, corrosion, mass, joining, wear, geometry and cost. The sections below classify conductors by resistivity and by application.

Based on Resistivity or Conductivity

• Low resistivity materials for efficient current paths
• Higher-resistivity materials for controlled resistance or heat generation

The chart shows this broad division. Conductivity is the reciprocal of resistivity, so the two labels describe the same electrical property from opposite directions. The boundary is application-specific rather than a formal material class.

Low-Resistivity or High-Conductivity Materials

Low-resistivity materials reduce conductor loss and voltage drop. They are used as conductors in cables, busbars, machine windings and electronic interconnects. Copper and aluminium dominate bulk power applications. Silver and gold are used selectively where contact behaviour, corrosion resistance or thin conductive layers justify their cost.
Common high-conductivity metals include:

• Silver, for specialised contacts, coatings and high-performance conductors
• Copper, for wiring, busbars, windings and cables
• Gold, mainly as a corrosion-resistant contact or connector finish
• Aluminium, for lightweight cables, busbars and overhead conductors

Higher-Resistivity Conducting Materials

The topic High resistivity or Low conductivity conducting covers materials that create a required resistance in a practical length and cross-section. Resistance alloys are used in heaters, shunts and precision resistors. Tungsten is used for an incandescent lamp filament because of its high-temperature properties, not because it has the highest room-temperature resistivity. Carbon and graphite serve specialised resistor, electrode, contact and brush roles.
Representative materials include:

• Iron-chromium-aluminium resistance-heating alloys
• Nickel-chromium resistance-heating alloys
• Copper-nickel resistance alloys
• Manganin and related precision-resistance alloys
• Tungsten, carbon and graphite for specialised high-temperature or contact uses

Based on Area of Application

• Conductors for electrical-machine coils
• Resistance-heating materials
• Incandescent-lamp filament materials
• Overhead transmission line conductors
• Thermostatic bimetals
• Electrical contact materials
• Electrical carbon and graphite materials
• Brush materials for electrical machines
• Fuse-element materials

The application chart groups materials by function. Several materials appear in more than one group, and one product may combine a conductive metal, coating, core, binder or substrate to achieve the required behaviour.

Conductors for Electrical-Machine Coils

Copper is the usual winding conductor because it combines high conductivity with established insulation, joining and manufacturing methods. Aluminium reduces mass and material cost but needs a larger cross-section for the same resistance and requires suitable joints. Silver is not a normal bulk winding material. Designers also consider slot fill, cooling, mechanical forces, operating temperature, fatigue and insulation compatibility.

Materials for Heating Elements

Resistance heaters commonly use nickel-chromium, iron-chromium-aluminium or copper-nickel alloys, selected for the required temperature and atmosphere. Resistivity sets element dimensions, but usable temperature, surface load, oxidation behaviour, hot strength, support, thermal cycling and manufacturability determine service life.

• A maximum continuous element temperature above the design temperature
• A stable protective oxide or other corrosion resistance in the operating atmosphere
• Adequate hot strength, creep resistance and dimensional stability
• Sufficient ductility and formability for wire, strip or fabricated elements

Materials for Lamp Filaments

Tungsten became the standard metallic material for an incandescent lamp filament because it can operate at very high temperature with manageable evaporation and useful hot strength. Carbon and tantalum were used historically. A filament material and its support system require these properties:

• High melting temperature and usable recrystallisation behaviour
• Low vapour pressure at the intended filament temperature
• Compatibility with the lamp’s vacuum or inert fill gas and getter system
• Resistivity that permits the required power in a compact filament
• Thermal expansion compatible with supports and feed-throughs
• A predictable temperature coefficient of resistance for start-up and steady operation
• Adequate elastic modulus and tensile strength at temperature
• Manufacturability as fine wire before final heat treatment
• Ability to retain a coiled filament geometry
• Resistance to thermal-cycle fatigue and vibration
• Acceptable cost, availability and process yield

Overhead Transmission-Line Conductors

An overhead conductor for a transmission line must meet electrical, mechanical, thermal and environmental requirements:

• Low AC resistance and adequate ampacity
• Rated tensile strength for span, ice and wind loads
• Acceptable mass, diameter, sag and creep
• Corrosion resistance suited to the route environment
• Stable strength and sag at normal and emergency temperatures
• Thermal expansion compatible with clearance requirements
• Whole-life cost, availability, fittings and installation compatibility

Common overhead conductor families include:

• All-aluminium conductor (AAC)
• All-aluminium-alloy conductor (AAAC)
• Aluminium conductor steel reinforced (ACSR)
• Aluminium conductor steel supported (ACSS)
• Aluminium-alloy conductor steel reinforced (AACSR)
• Aluminium conductor with a composite or aluminium-clad core
• Copper or copper-alloy conductors in specialised or legacy installations

Bimetals

A thermostatic bimetal bonds two layers with different coefficients of linear thermal expansion. A temperature change makes one layer expand more than the other, so the strip bends. Material pairs are engineered for deflection, force, resistivity, temperature range, corrosion and bond durability.

• A high-expansion layer, often an iron-nickel-manganese or copper-based alloy
• A low-expansion iron-nickel alloy layer, often from the Invar family

Electrical Contact Materials

Contact material cannot be selected from bulk conductivity alone. A low-level connector, a relay and a high-current contactor encounter different films, forces, arcs and temperatures. Silver, silver alloys, gold finishes, copper alloys and refractory-metal composites each address different failure modes.

• Stable interface resistance, film formation and required contact force
• Hardness, wear, material transfer, bounce and welding resistance
• Load voltage, current, inrush, DC or AC duty, arc energy and switching frequency

Electrical Carbon Materials

Electrical carbon products include natural graphite, synthetic graphite, carbon-graphite, electrographite and metal-graphite composites. Their conductivity, lubricity, temperature capability and chemical stability suit different interfaces and high-temperature parts.
Applications in electrical Engineering include:

• Historical carbon filaments for the incandescent lamp
• Sliding electrical contacts and current collectors
• Carbon-film, composition and high-energy resistors
• Brushes and grounding contacts for DC machines, slip-ring machines and alternators
• Electrodes and conductive additives in batteries and electrochemical cells
• Graphite electrodes for electric furnaces
• Arc, gouging and selected welding electrodes
• High-temperature furnace, vacuum and semiconductor-processing components
• Pantograph strips, seals, bearings and antistatic components

Material for Brushes Used in Electrical Machines

A brush grade must match the machine, current density, peripheral speed, commutator or ring material, cooling and atmosphere. Required properties include:

• A suitable contact voltage drop and stable current distribution
• Thermal stability and heat dissipation
• Film-forming and self-lubricating behaviour
• Mechanical strength with controlled wear
• Resistance to chipping, vibration, dust and commutation damage

Common brush material families are:

• Carbon-graphite
• Natural-graphite grades
• Electrographite
• Metal-graphite, including copper-graphite and silver-graphite
• Resin-bonded or pitch-bonded carbon grades

Materials Used for Fuse Elements

The fuse element is calibrated with the body, filler and arc-control system to produce a specified time-current characteristic and breaking capacity. Material alone does not define fuse performance.
Controlled normal-state loss – the element must limit unwanted voltage drop and heating at rated current while still responding predictably to overcurrent.
Predictable melting and arcing – element material, thickness, notches, joints and any low-melting deposit control pre-arcing time, peak current, arc voltage and I²t. A low melting point by itself is not sufficient.
Element constructions use several metals, alloys and composites, including:

• Silver
• Copper
• Tin-based or other low-melting alloys
• Aluminium or zinc in suitable fuse classes
• Silver- or copper-based composite elements
• Low-melting deposits used to create the M-effect