Bulb Filament Material: What You Need to Know

A bulb filament is a thin wire that glows when an electric current flows through it. That glowing wire makes an incandescent bulb work: the current heats the wire until it emits visible light. Filament materials need special properties to survive those temperatures and still deliver bright, stable light. This page covers the history, characteristics and uses of the main filament materials, plus the pros and cons of incandescent bulbs.

What is an Incandescent Light Bulb?

An incandescent light bulb is an electric lamp whose wire filament glows at high temperature. The filament sits inside a glass bulb evacuated or filled with inert gas so it cannot oxidize. Metal contacts at the base connect the bulb to its power supply, and two support wires hold the filament in place.

Inventors across the 18th and 19th centuries chased incandescent lighting, but Thomas Edison delivered the first commercially practical bulb in 1879. His October 1879 lamp used a carbonized cotton thread filament that burned about 13 and a half hours. A year later his team found that a carbonized Japanese bamboo filament could burn for more than 1200 hours.

What are the Properties of a Good Bulb Filament Material?

The bulb filament material must have the following properties to function well as an incandescent light source:

• High melting point: The filament must be able to withstand temperatures of up to 2500°C without melting or breaking.
• Low vapor pressure: The filament must not evaporate or sublimate at high temperatures, which would cause the bulb to blacken and reduce its brightness and efficiency.
• Free from oxidation: The filament must not react with oxygen or other gases in the bulb at high temperatures, which would cause it to corrode or burn out.
• High resistivity: The filament must have a high electrical resistance, which means it opposes the flow of electric current. This causes it to heat up and emit light when a current passes through it.
• Low thermal coefficient of expansion: The filament must expand and contract only slightly as it heats and cools, or it will deform or break.
• Low-temperature coefficient of resistance: The filament must hold its resistance nearly steady through temperature swings, keeping the current and brightness stable.
• High Young’s modulus and tensile strength: The filament must be able to withstand the mechanical stress caused by its own weight and vibration without sagging or snapping.
• Sufficient ductility: The filament must be able to be drawn into a very thin wire without breaking or cracking.
• Ability to be converted into a shape of filament: The filament must be able to be formed into a coil or a double coil, which increases its surface area and brightness without increasing its length or resistance.
• High fatigue resistance: The filament must be able to endure repeated heating and cooling cycles without weakening or failing.

What are the Types of Bulb Filament Materials?

Different types of materials have been used for making bulb filaments over the years. Some of these materials are listed below:

Carbon

Edison and other inventors first used carbon for bulb filaments. Carbon brings a high melting point near 3500°C, low vapor pressure and high resistivity (1000-7000 µΩ-cm). Its weaknesses are poor oxidation resistance, relatively high thermal expansion (2 to 6 ×10−6 per K), low tensile strength and heavy bulb blackening. Carbon filaments ran at up to about 1800°C with an efficiency of roughly 4.5 lumens per watt (lm/W) at best.

Carbon is also used for making pressure-sensitive resistors, which are used in automatic voltage regulators, and carbon brushes, which are used in DC machines.

Tantalum

Werner von Bolton introduced tantalum as a bulb filament material in 1902. It offers a high melting point (2900°C), low vapor pressure, high resistivity (12.4 µΩ-cm) and a modest thermal expansion coefficient (6.5 ×10−6 per K). Against that stand poor oxidation resistance, a noticeable temperature coefficient of resistance (0.0036 per °C), limited tensile strength and low efficiency (3.6 watts per candle power). Tantalum filaments ran at up to about 2000°C.

Tantalum is not widely used as a bulb filament material anymore due to its low efficiency and scarcity.

Tungsten

Tungsten is the most commonly used material for making bulb filaments today. William D. Coolidge first put ductile tungsten into lamps in 1910. The metal combines a very high melting point (3410°C), low vapor pressure, high resistivity (5.65 µΩ-cm), high tensile strength and little tendency to blacken the bulb because it evaporates so slowly inside the protective vacuum or inert atmosphere. Its drawbacks are a strong temperature coefficient of resistance (about 0.005 per °C) and thermal expansion of 4.3 ×10−6 per K. Tungsten filaments reach roughly 2500°C and deliver about 12 lm/W.

Tungsten is also used as an electrode in X-ray tubes and as an electrical contact material in certain applications.

How are Bulb Filaments Made?

Bulb filaments are made by various processes depending on the material used. Some of these processes are described below:

Carbon

Carbon filaments are made by carbonizing organic materials like bamboo, cotton thread or paper pulp in an inert atmosphere at high temperatures (1000-1500°C). The carbonized material is then stretched into thin wires and wound into coils.

Tantalum

Tantalum filaments are made by powder metallurgy techniques. Tantalum powder is mixed with a binder and pressed into rods or wires. The rods or wires are then sintered at high temperatures (2000-2500°C) in a vacuum or inert gas atmosphere. The sintered rods or wires are then drawn into thin wires and wound into coils.

Tungsten

Tungsten filaments are made by several steps:

• Tungsten ore is extracted from wolframite or scheelite minerals and converted into tungstic acid or ammonium para tungstate.
• Tungstic acid or ammonium para tungstate is reduced with hydrogen gas to form tungsten powder.
• Tungsten powder is mixed with a binder and pressed into rods or wires.
• The rods or wires are sintered at high temperatures (2000-3000°C) in a vacuum or inert gas atmosphere.
• The sintered rods or wires are swaged (hammered) into thinner rods or wires.
• The swaged rods or wires are drawn through diamond dies into very thin wires (10-50 µm).
• The thin wires are annealed (heated) at moderate temperatures (1000-1500°C) in hydrogen gas to improve their ductility and strength.
• The annealed wires are wound into coils or double coils.

What are the Advantages and Disadvantages of Incandescent Light Bulbs?

Incandescent light bulbs have some advantages and disadvantages compared to other types of light sources, such as fluorescent and LED lamps. Some of these are listed below:

Advantages

• They produce warm white light with a good color rendering index (CRI).
• They are cheap and easy to manufacture and use.
• They can be dimmed easily without affecting their color temperature.
• They can operate at low voltages and frequencies without flickering.
• Their simple construction shrugs off shocks and vibrations better than some other types of lamps, such as long fluorescent tubes.

Disadvantages

• They have low efficiency (10-20 lm/W) compared to other types of lamps (50-200 lm/W).
• They have short lifespan (1000-2000 hours) compared to other types of lamps (10,000-50,000 hours).
• Most of their energy (around 90 percent) leaves as heat, which can increase cooling costs and fire hazards.
• They consume more electricity than other types of lamps, which can increase greenhouse gas emissions and environmental impact.
• They contain fragile components such as glass bulbs and thin filaments which can break easily.

Conclusion

Bulb filaments produce light by heating up when an electric current passes through them. Carbon, tantalum and tungsten each took a turn as the leading material, and tungsten still wins today thanks to its high melting point, slow evaporation inside the bulb and manageable working properties. Incandescent bulbs nevertheless waste most of their energy as heat, burn out within a couple of thousand hours and break easily, so fluorescent and LED alternatives now dominate most applications.