Candoluminescence
   HOME

TheInfoList



OR:

Candoluminescence is the
light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...
given off by certain materials at elevated temperatures (usually when exposed to a
flame A flame (from Latin ''flamma'') is the visible, gaseous part of a fire. It is caused by a highly exothermic chemical reaction taking place in a thin zone. When flames are hot enough to have ionized gaseous components of sufficient density they ...
) that has an intensity at some wavelengths which can, through chemical action in flames, be higher than the
blackbody A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The name "black body" is given because it absorbs all colors of light. A black body ...
emission expected from
incandescence Incandescence is the emission of electromagnetic radiation (including visible light) from a hot body as a result of its high temperature. The term derives from the Latin verb ''incandescere,'' to glow white. A common use of incandescence is ...
at the same temperature.H.F. Ivey, "Candoluminescence and radical-excited luminescence," ''Journal of Luminescence'' 8:4, pp. 271–307 (1974) The phenomenon is notable in certain
transition-metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...
and
rare-earth The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides (yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silve ...
oxide materials (
ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...
s) such as
zinc oxide Zinc oxide is an inorganic compound with the formula . It is a white powder that is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, food supplements, rubbers, plastics, ceramics, glass, cemen ...
,
cerium(IV) oxide Cerium(IV) oxide, also known as ceric oxide, ceric dioxide, ceria, cerium oxide or cerium dioxide, is an oxide of the rare-earth metal cerium. It is a pale yellow-white powder with the chemical formula CeO2. It is an important commercial produc ...
and
thorium dioxide Thorium dioxide (ThO2), also called thorium(IV) oxide, is a crystalline solid, often white or yellow in colour. Also known as thoria, it is produced mainly as a by-product of lanthanide and uranium production. Thorianite is the name of the minera ...
.


History

The existence of the candoluminescence phenomenon and the underlying mechanism have been the subject of extensive research and debate since the first reports of it in the 1800s. The topic was of particular interest before the introduction of electric lighting, when most artificial light was produced by fuel combustion. The main alternative explanation for candoluminescence is that it is simply "selective" thermal emission in which the material has a very high emissivity in the visible spectrum and a very weak emissivity in the part of the spectrum where the blackbody thermal emission would be highest; in such a system, the emitting material will tend to retain a higher temperature because of the lack of invisible radiative cooling. In this scenario, observations of candoluminescence would simply have been underestimating the temperature of the emitting species. Several authors in the 1950s came to the view that candoluminescence was simply an instance of selective thermal emission, and one of the most prominent researchers in the field, V. A. Sokolov, once advocated eliminating the term from the literature in his noted 1952 review article, only to revise his view several years later. The modern scientific consensus is that candoluminescence does occur, that it is not always simply due to selective thermal emission, but the mechanisms vary depending on the materials involved and the method of heating, particularly the type of flame and the position of the material relative to the flame.


Mechanism

When the fuel in a flame combusts, the energy released by the combustion process is deposited in combustion products, usually molecular fragments called
free radicals In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemically reactive. Many radicals spont ...
. The combustion products are excited to a very high temperature called the
adiabatic flame temperature In the study of combustion, the adiabatic flame temperature is the temperature reached by a flame under ideal conditions. It is an upper bound of the temperature that is reached in actual processes. There are two types adiabatic flame temperature: ...
(that is, the temperature before any heat has been transferred away from the combustion products). This temperature is usually much higher than the temperature of the air in the flame or which an object inserted into the flame can reach. When the combustion products lose this energy by radiative emission, the radiation can thus be more intense than that of a lower-temperature blackbody inserted into the flame. The exact emission process involved varies with the material, the type of fuels and oxidizers, and the type of flame, though in many cases it is well established that the free radicals undergo
radiative recombination In the solid-state physics of semiconductors, carrier generation and carrier recombination are processes by which mobile charge carriers (electrons and electron holes) are created and eliminated. Carrier generation and recombination processes are ...
.D. M. Mason
"Candoluminescence"
in ''Proc. Am. Chem. Soc., Div. Fuel Chem.'', V. 11:2, pp. 540–554, (1967).
This energetic light emitted directly from the combustion products may be observed directly (as with a blue gas flame), depending on the wavelength, or it may then cause
fluorescence Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...
in the candoluminescent material. Some free-radical recombinations emit
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
light, which is only observable through fluorescence. One important candoluminescence mechanism is that the candoluminescent material catalyzes the recombination, enhancing the intensity of the emission. Extremely narrow-wavelength emission by the combustion products is often an important feature in this process, because it reduces the rate at which the free radicals lose heat to radiation at invisible or non-fluorescence-exciting wavelengths. In other cases, the excited combustion products are thought to directly transfer their energy to luminescent species in the solid material. In any case, the key feature of candoluminescence is that the combustion products lose their energy to radiation without becoming thermalized with the environment, which allows the effective temperature of their radiation to be much higher than that of thermal emission from materials in
thermal equilibrium Two physical systems are in thermal equilibrium if there is no net flow of thermal energy between them when they are connected by a path permeable to heat. Thermal equilibrium obeys the zeroth law of thermodynamics. A system is said to be in ...
with the environment.


Welsbach lights

Early in the 20th century, there was vigorous debate over whether candoluminescence is required to explain the behavior of Welsbach
gas mantle A Coleman white gas lantern mantle glowing at full brightness An incandescent gas mantle, gas mantle or Welsbach mantle is a device for generating incandescent bright white light when heated by a flame. The name refers to its original heat sou ...
s or
limelight Limelight (also known as Drummond light or calcium light)James R. Smith (2004). ''San Francisco's Lost Landmarks'', Quill Driver Books. is a type of stage lighting once used in theatres and music halls. An intense illumination is created when ...
. One counterargument was that since thorium oxide (for example) has much lower emissivity in the near infrared region than the shorter wavelength parts of the visible spectrum, it should not be strongly cooled by infrared radiation, and thus a thorium-oxide mantle can get closer to the flame temperature than can a blackbody material. The higher temperature would then lead to higher emission levels in the visible portion of the spectrum, without invoking candoluminescence as an explanation. Another argument was that the oxides in the mantle might be actively absorbing the combustion products and thus being selectively raised to combustion-product temperatures.Comment by C. P. Steinmetz to H. E. Ives and W. W. Coblentz, "The Light of the Firefly" in ''Transactions of the Illuminating Engineering Society'' V. 4, p. 677–679, (1909). Some more recent authors seem to have concluded that neither Welsbach mantles nor limelight involve candoluminescence (e.g. Mason), but Ivey, in an extensive review of 254 sources, concluded that catalysis of free-radical recombination does enhance the emission of Welsbach mantles, such that they are candoluminescent.


See also

*
Incandescent light bulb An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light with a wire filament heated until it glows. The filament is enclosed in a glass bulb with a vacuum or inert gas to protect the filament from oxida ...
*
Catalytic converter A catalytic converter is an exhaust emission control device that converts toxic gases and pollutants in exhaust gas from an internal combustion engine into less-toxic pollutants by catalyzing a redox reaction. Catalytic converters are usually ...


References

{{Color topics Luminescence