Globar
Definition and meaning of Globar in chemistry.
A Globar is a rod of sintered silicon carbide that is electrically heated to incandescence and used as a continuous infrared radiation source in spectrophotometers.
In more detail
When current passes through the SiC rod, resistive heating raises its temperature to roughly 1000-1650 degrees Celsius, causing it to emit infrared radiation approximating a graybody spectrum over a broad range, from the near-infrared through the far-infrared. Because silicon carbide has a negative temperature coefficient of resistance, the rod's ends must be water- or air-cooled at the electrical contacts to prevent runaway heating and arcing while the center glows hottest. Globars are rugged, require no preheating period (silicon carbide is conductive even at room temperature), and were widely used in dispersive and early FTIR spectrometers as an alternative to the Nernst glower.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Composition | Sintered silicon carbide (SiC) |
| Operating temperature | ~1000-1650 degrees Celsius |
| Typical use | Broadband infrared source in IR/FTIR spectrometers |
In a mid-infrared FTIR spectrometer, a Globar source heated to about 1300 degrees Celsius supplies the broadband infrared beam that is split, passed through the sample and reference paths, and recombined to produce an absorption spectrum used to identify functional groups.
Frequently asked questions
Why does a Globar need cooling if it works by heating?
Silicon carbide has a negative temperature coefficient of resistance, so as it heats up its resistance drops and current tends to increase; the electrode ends are water- or air-cooled to prevent thermal runaway and contact damage while the central rod remains glowing hot.
How does a Globar differ from a Nernst glower?
Both have a negative temperature coefficient of resistance once operating, so both need current-limiting to avoid thermal runaway. The key difference is at startup: a silicon carbide Globar is already conductive at room temperature (it behaves as a semiconductor) and needs no preheating, while a Nernst glower is made of sintered rare-earth oxides that are electrical insulators when cold, so it must be externally preheated to red heat before enough current can flow for it to self-sustain and glow.