Section 1: Review quiz
Instructions: Review each question prompt and evaluate its metallurgical principles before using the control button to reveal the answer key.
1. What are the two primary ores used for the commercial production of tungsten, and how is the metal extracted?
Tungsten is primarily produced from wolframite ((Fe,Mn)WO4) and scheelite (CaWO4). The extraction process involves a hydrometallurgical chemical route where the concentrated ore is converted to tungsten trioxide and subsequently reduced using hydrogen gas to yield pure tungsten powder.
2. Why is tungsten specifically chosen as the primary material for incandescent light bulb filaments?
Tungsten is selected due to its exceptionally high melting point of 3410 °C (3683 K), which allows filaments to operate at the extreme temperatures necessary to emit bright visible light. While other refractory metals could be considered, the severe service conditions—operating at approximately 80% of the material's melting threshold—limit the choice to those with the highest absolute thermal resistance.
3. Describe the historical significance of the Cligga Mine in Cornwall regarding tungsten production.
Originally focused on manufacturing explosives, the Cligga Mine and Nobels Munitions Factory shifted operations during the Second World War to tungsten mining for armaments. The high density of tungsten (19.3 g cm−3) made it ideal for defensive armour plating and the production of kinetic-energy projectiles capable of penetrating standard military armour.
4. How does the "coiling" of a filament affect the efficiency and temperature of a light bulb?
Coiling a filament creates a geometric enclosure where the local emissivity approaches unity, closely mimicking an ideal black body, while simultaneously reducing gas convection flow around the wire compared to a single strand. This configuration alters convective heat transfer, allowing the filament to reach significantly higher operating temperatures for the same length and thickness, thereby emitting more light in the useful visible range.
5. What is the role of potassium (K) in the manufacture of tungsten filaments?
Potassium is added to the metal oxide in solution to create minute, non-metallic gas bubbles that pin longitudinal grain boundaries during high-temperature service. These rows of bubbles restrain destructive transverse grain growth and prevent grain boundary sliding, significantly improving the structural creep stability and lifespan of the tungsten filament at high temperatures.
6. Explain the process of "sintering" in the production of tungsten filaments.
Because of its extreme melting point, tungsten must be processed via a solid-state powder metallurgy route where powder is mechanically compacted into an ingot and pre-heated to approximately 1150 °C in a protective hydrogen atmosphere. High-current electrical resistance heating is then applied to reach 90% of the melting current, driving out internal pores through vacancy diffusion mechanisms until the bar becomes a dense, solid mass.
7. What is "faceting," and how does it contribute to the failure of a tungsten filament?
Faceting is a failure mode caused by the continuous vaporisation and condensation of tungsten atoms, or their directional migration along the surface, which leads to a non-uniform cross-section. This creates localized "hot spots" where the wire becomes thinner and physically hotter, eventually generating a weak point that causes the filament to break under tension or melt.
8. How do alternative lighting sources like LEDs and fluorescent lamps compare to incandescent bulbs in terms of efficiency?
Tungsten filament bulbs are relatively inefficient, with the vast majority of electrical input dissipated as infrared heat radiation rather than visible light. In contrast, fluorescent lamps achieve 50–90 lumens per watt, and LEDs can exceed 100 lumens per watt, whereas standard incandescent bulbs typically offer only 10 to 20 lumens per watt.
9. Define the "Colour Rendering Index" (CRI) and explain how incandescent bulbs perform on this scale.
The CRI measures how accurately an artificial light source reveals object colours relative to natural sunlight on a standardised scale of 0 to 100, with 100 representing a perfect match. Incandescent bulbs have a very high CRI of approximately 95, outperforming most fluorescent options (55 to 75) and standard commercial LEDs (around 85).
10. What physical changes occur to a filament after it has fractured during service?
Micrographs indicate that when a filament fails, the tungsten undergoes localized melting at the exact point of fracture due to concentrated resistance heating. This molten metal rapidly solidifies upon circuit failure to form characteristic spherical globules; there is also often evidence of internal gas porosity escaping to the surface of these cooling features.
Section 2: Essay questions
Instructions: Review the detailed composition options below. Dynamic review hints focusing on thermal stress and microstructure evolution can be expanded for guidance.
1. The metallurgy of longevity
Discuss how the transition from fine-grained to elongated, large-grained structures (and the use of additives like potassium and thoria) impacts the creep strength and structural integrity of tungsten filaments.
Key points for formulation: Focus on how recrystallisation mechanisms modify grain shape boundaries. Discuss how potassium bubbles act as severe physical pinning points, stopping grain boundaries from migrating across the wire width and preventing high-temperature grain boundary sliding or sag.
2. Radiation and heat transfer
Analyze the relationship between Planck's law of radiation and the design of light bulb filaments. How do temperature and wavelength distribution dictate the shift from infrared to visible light?
Key points for formulation: Evaluate the mathematical progression of black body emission spectra under increasing temperature. Show that elevating operating temperatures shifts the peak emission wavelength toward the visible band according to Wien's displacement law, optimizing luminous output over infrared loss.
3. Tungsten in industry and defence
Compare the physical properties that make tungsten valuable for domestic use (lighting) versus its applications in military armaments (armor-piercing shells).
Key points for formulation: Contrast thermal parameters against pure kinetic density profiles. Show that while domestic lighting relies entirely on melting point barriers and electrical resistance, military applications leverage its exceptional mass density ($19.3\,\text{g cm}^{-3}$) to maximize kinetic energy delivery in non-explosive armor penetration.