If an endothermic (heat-absorbing) process occurs, the sample will momentarily become cooler than the reference material; the small temperature difference is detected by the pair of thermocouples and a downward deflection, termed an endotherm, is produced in the plot of AT (temperature difference between sample of rapid heating of a confined sample, and must be recognized as and reference) versus T (temperature of the heating block).
such.
Evolution of heat by the sample will similarly produce an upward Some representative thermograms of high-energy materials are deflection, termed an exotherm. The printed output produced by shown in Figures 2.4-2.6.
the instrument, a thermogram, is a thermal "fingerprint" of the material being analyzed. Thermal analysis is quite useful for determining the purity of materials; this is accomplished by exam-LIGHT EMISSION
ining the location and "sharpness" of the melting point. DTA is also useful for qualitative identification of solid materials, by com-The pyrotechnic phenomena of heat, smoke, noise, and motion are paring the thermal pattern with those of known materials. Reac-reasonably easy to comprehend. Heat results from the rapid re-tion temperatures, including the ignition temperatures of high-lease of energy associated with the formation of stable chemical energy materials, can be quickly (and safely) measured by ther-bonds during a chemical reaction. Smoke is produced by the dis-mal analysis. These temperatures will correspond to conditions persion in air of many small particles during a chemical reaction.
42
Chemistry of Pyrotechnics
Basic Chemical Principles
43
FIG. 2.5 Ballistite, a "smokeless powder" consisting of 60% nitrocellulose and 40% nitroglycerine, produces a thermogram with no FIG. 2.6 Black powder was the first "modern" high-energy mix-transitions detectable prior to exothermic decomposition above ture, and it is still used in a variety of pyrotechnic applications.
150 0C.
It is an intimate blend of potassium nitrate (75%), charcoal (15%), I
and sulfur (10%). The thermogram for the mixture shows endotherms near 105° and 119°C corresponding to a solid-solid phase transition and melting for sulfur, a strong endotherm near 130°
representing a solid-solid transition in potassium nitrate, and a Noise is produced by the rapid generation of gas at high tempera-violent exotherm near 330°C where ignition of the mixture occurs.
ture, creating waves that travel through air at the "speed of sound," 340 meters/second. Motion can be produced if you direct the hot gaseous products of a pyrotechnic reaction out through an We can represent these allowed electronic energies by a diagram exit, or nozzle. The thrust that is produced can move an object of considerable mass, if sufficient propellant is used.
such as Figure 2.7.
The theory of color and light production, however, involves Logic suggests that an electron will occupy the lowest energy the energy levels available for electrons in atoms and molecules, level available, and electrons will successively fill these levels as according to the beliefs of modern chemical theory. In an atom they are added to an atom or molecule. "Quantum mechanics"
or molecule, there are a number of "orbitals" or energy levels restricts all orbitals to a maximum of two electrons (these two have opposite "spins" and do not strongly repel one another), that an electron may occupy. Each of these levels corresponds to a discrete energy value, and only these energies are possible.
and hence a filling process occurs. The filling pattern for the The energy is said to be quantized, or restricted to certain val-sodium atom (sodium is atomic number 11 - therefore there will ues that depend on the nature of the particular atom or molecule.
be 11 electrons in the neutral atom) is shown in Figure 2.7).
Basic Chemical Principles
45
This energy can be lost as heat upon return to the ground state, or it can be released as a unit, or "photon," of light.
Light, or electromagnetic radiation, has both wave and particle or unit character associated with its behavior. Wavelengths range from very short (10 -12 meters) for the "gamma rays" that accompany nuclear decay to quite long (10 meters) for radio waves.
All light travels at the same speed in a vacuum, with a value of 3 X 108 meters/second - the "speed of light." This value can be used for the speed of light in air as well.
The wavelength of light can now be related to the frequency, or number of waves passing a given point per second, using the speed of light value:
frequency (v) = speed (c)/wavelength (A)
(2.6)
(waves/second) = (meters/second) /(meters /wave) The entire range of wavelengths comprising "light" is known as the electromagnetic spectrum (Figure 2.8).
FIG. 2.7 The energy levels of the sodium atom. The sodium atom contains 11 electrons. These electrons will successively fill the lowest available energy levels in the atom, with a maximum popu-ULTRAVIOLET
lation of two electrons in any given "orbital." The experimentally-and VISIBLE
determined energy level sequence is shown in this figure, with the 11th (and highest-energy) electron placed in the 3s level. The lowest vacant level is a 3p orbital. To raise an electron from the 3s to the 3p level requires 3.38 X 10 -19 joules of energy. This energy corresponds to light of 589 nanometer wavelength - the yellow portion of the visible spectrum. Sodium atoms heated to high temperature will emit this yellow light as electrons are thermally excited to the 3p level, and then return to the 3s level and give off the excess energy as yellow light.
ULTRAVIOLET
200-380 nm (1nm = 10-9m)
VISIBLE
380.780nm
When energy is put into a sodium atom, in the form of heat or FIG. 2.8 The electromagnetic spectrum. The various regions of light, one means of accepting this energy is for an electron to be the electromagnetic spectrum correspond to a wide range of wave-
"promoted" to a higher energy level. The electron in this "ex-lengths, frequencies, and energies. The radiofrequency range is cited state" is unstable and will quickly return to the ground at the long-wavelength , low energy end, with gamma rays at the state with the release of an amount of energy exactly equal to short-wavelength, high-frequency, high-energy end. The "vis-the energy difference between the ground and excited states.
ible" region - that portion of the spectrum perceived as color by For the sodium atom, the difference between the highest occu-the human visual system -- falls in the narrow region from 380-pied and lowest unoccupied levels is 3.38 X 10 -19 joules/atom.
780 manometers (1 nm = 10 -9 m).
46
Chemistry of Pyrotechnics
Basic Chemical Principles