47
transitions possible for the particular atom. The pattern is char-We can readily tell that light is a form of energy by staying acteristic for each element and can be used for qualitative iden-out in the sun for too long a time. Elegant experiments by Einstein and others clearly showed that the energy associated with tification purposes.
light was directly proportional to the frequency of the radiation:
• = by = h c/A
(2.7)
Molecular Emission
A similar phenomenon is observed when molecules are vaporized where
and thermally excited. Electrons can be promoted from an oc-
• = the energy per light particle ("photon") cupied ground electronic state to a vacant excited state; when
• = a constant, Planck's Constant, 6.63 X 10 -34 joule-seconds an electron returns to the ground state, a photon of light may v = frequency of light (in waves - "cycles" - per second) be emitted.
• = speed of light (3 X 10 8 meters /second) Molecular spectra are usually more complex than atomic spec-A = wavelength of light (in meters)
tra. The energy levels are more complex, and vibrational and rotational sublevels superimpose their patterns on the electronic spectrum. Bands are generally observed rather than the sharp This equation permits one to equate a wavelength of light with the lines seen in atomic spectra. Emission intensity again increases energy associated with that particular radiation. For the sodium as the flame temperature is raised. However, one must be con-atom, the wavelength of light corresponding to the energy differ-cerned about reaching too high a temperature and decomposing ence of 3.38 X 10 -19 joules between the highest occupied and low-the molecular emitter; the light emission pattern will change if est unoccupied electronic energy levels should be: this occurs. This is a particular problem in achieving an in-
• =hd=hc/A
tense blue flame. The best blue light emitter - CuCl - is unstable at high temperature (above 1200°C).
rearranging,
A=hc/E
"Black Body" Emission
The presence of solid particles in a pyrotechnic flame can lead to
-7
a substantial loss of color purity due to a complex process known
= 5.89 X 10
meters
as "black body radiation." Solid particles, heated to high tem-
= 589 nm (where 1 nm = 10 -9 meters)
perature, radiate a continuous spectrum of light, much of it in the visible region - with the intensity exponentially increasing Light of wavelength 589 nanometers falls in the yellow portion of with temperature. If you are attempting to produce white light the visible region of the electromagnetic spectrum. The charac-
(which is a combination of all wavelengths in the visible region), teristic yellow glow of sodium vapor lamps used to illuminate many this incandescent phenomenon is desirable.
highways results from this particular emission.
Magnesium metal is found in most "white light" formulas. In To produce this type of atomic emission in a pyrotechnic sys-an oxidizing flame, the metal is converted to the high-melting tem, one must produce sufficient heat to generate atomic vapor magnesium oxide, MgO, an excellent white-light emitter.
Also,
in the flame, and then excite the atoms from the ground to vari-the high heat output of magnesium-containing compositions aids ous possible excited electronic states. Emission intensity will in-in achieving high flame temperatures.
Aluminum metal is also
crease as the flame temperature increases, as more and more atoms commonly used for light production; other metals, including ti-are vaporized and excited. Return of the atoms to their ground tanium and zirconium, are also good white-light sources.
state produces the light emission. A pattern of wavelengths, The development of color and light-producing compositions will known as an atomic spectrum, is produced by each element. This be considered in more detail in Chapter 7.
pattern - a series of lines - corresponds to the various electronic
48
Chemistry of Pyrotechnics
REFERENCES
h
1.
R. C. Weast (Ed.), CRC Handbook of Chemistry and Physics, 63rd Ed., CRC Press, Inc. , Boca Raton, Florida, 1982.
2.
A. A. Shidlovskiy, Principles of Pyrotechnics, 3rd Edition,
Moscow, 1964. (Translated as Report FTD-HC-23-1704-74
by Foreign Technology Division, Wright-Patterson Air Force Base, Ohio, 1974.)
3.
L. Pytlewski, "The Unstable Chemistry of Nitrogen," presented at Pyrotechnics and Explosives Seminar P-81, Franklin Research Center, Philadelphia, Penna. , August, 1981.
4.
U.S. Army Material Command, Engineering Design Handbook, Military Pyrotechnic Series, Part Three, "Properties of Materials Used in Pyrotechnic Compositions," Washington, D .C . , 1963 (AMC Pamphlet 706-187).
5.
T. L. Davis, The Chemistry of Powder and Explosives, John Wiley & Sons, Inc., New York, 1941.
6.
U.S. Army Material Command, Engineering Design Handbook, Military Pyrotechnic Series, Part One, "Theory and Application, Washington, D.C., 1967 (AMC Pamphlet 706-185).
7.
J. H. McLain, Pyrotechnics from the Viewpoint of Solid State Chemistry, The Franklin Institute Press, Philadelphia, Penna., 1980.
8.
R. L. Tuve, Principles of Fire Protection Chemistry, National Fire Protection Assn., Boston, Mass., 1976.
9.
W. J. Moore, Basic Physical Chemistry, Prentice Hall, Englewood Cliffs, NJ, 1983.
10.
W. W. Wendlandt, Thermal Methods of Analysis, Inter-science, New York, 1964.
A "pinwheel" set piece, reflected over water. Cardboard tubes are loaded with spark-producing pyrotechnic composition.
The "pin-
wheel," attached to a pole, revolves about its axis as hot gases are vented out the end of a "driver" tube to provide thrust. Sparks are produced by the burning of large particles of charcoal or aluminum.
(Zambelli Internationale)
3COMPONENTS OF
HIGH-ENERGY MIXTURES
I NTRODUCTION
Compounds containing both a readily-oxidizable and a readily-reducible component within one molecule are uncommon. Such species tend to have explosive properties.
A molecule or ionic
compound containing an internal oxidizer/reducer pair is inherently the most intimately-mixed high energy material that can be prepared. The mixing is achieved at the molecular (or ionic) level, and no migration or diffusion is required to bring the electron donor and electron acceptor together.