Combustion Temperature
A pyrotechnic reaction generates a substantial quantity of heat, and the actual flame temperature reached by these mixtures is an area of study that has been attacked from both the experimental and theoretical directions.
Flame temperatures can be measured directly, using special high-temperature optical methods. They can also be calculated estimated) using heat of reaction data and thermochemical values for heat of fusion and vaporization, heat capacity, and transition temperatures. Calculated values tend to be higher than the actual experimental results, due to heat loss to the surroundings as well as the endothermic decomposition of some of the reaction products. Details regarding these calculations, with several examples, have been published.
Considerable heat will be used to melt and to vaporize the reaction products. Vaporization of a reaction product is commonly the limiting factor in determining the maximum flame temperature. For example, consider a beaker of water at 25°C. As the water is heated, at one atmosphere pressure, the temperature of the liquid rises rather quickly to a value of 100 0C. To heat the water over this temperature range, a heat input of approximately 1 calorie per gram per degree rise in temperature is required. To raise 500 grams of water from 25° to 100°C will require
Once the water reaches 100°C, however, the temperature increase stops. The water boils, as liquid is converted to the vapor state, and 540 calories of heat is needed to convert 1 gram of water from liquid to vapor. To vaporize 500 grams of water, at 100°C,
(500 grams)(540 cal/gram) = 270,000 calories
of heat is required. Until this quantity of heat is put into the system, and all of the water is vaporized, no further temperature increase will occur. Similar phenomena involving the vaporization of reaction products such as magnesium oxide (MgO) and aluminum oxide (A1 20 3 ) tend to limit the temperature attained in pyrotechnic flames. The boiling points of some common combustion products are given in Table 1.
TABLE 1. Melting and Boiling Points of Common Non-Gaseous Pyrotechnic Products
Mixtures using organic (carbon-containing) fuels usually attain lower flame temperatures than those compositions consisting of an oxidizer and a metallic fuel. This reduction in flame temperature can be attributed to the lower heat output of the organic fuels versus metals, as well as to some heat consumption going towards the decomposition and vaporization of the organic fuel and its by-products. The presence of even small quantities of organic components can markedly lower the flame temperature, as the available oxygen is consumed by the carbonaceous material rather than metallic fuel. Table 2 illustrates this behavior, with data reported by Shimizu.
TABLE 2. Effect of Organic Fuels on Flame Temperature of Magnesium /Oxidizer
Mixtures
This reduction of flame temperature can be minimized somewhat by using binders with as high an oxygen content as possible. In such binders, the carbon atoms are already partially oxidized, and they will therefore consume less oxygen in going to carbon dioxide during the combustion process. The balanced chemical equations for the combustion of hexane (C 6H1 ,,) and glucose (C 6 H 12O6) illustrate this (both are six-carbon molecules)
Pyrotechnic flames typically fall in the 2000-3000°C range. Table 3 lists approximate values for some common classes of high-energy reactions.
TABLE 3. Maximum Flame Temperatures of Various Classes of Pyrotechnic Mixtures
Binary mixtures of oxidizer with metallic fuel yield the highest flame temperatures, and the choice of oxidizer does not appear to make a substantial difference in the temperature attained. For compositions without metal fuels, this does not seem to be the case. Shimizu has collected data for a variety of compositions and has observed that potassium nitrate mixtures attain substantially lower flame temperatures than similar mixtures made with chlorate or perchlorate oxidizers. This result can be attributed to the substantially -endothermic decomposition of KNO 3 relative to the other oxidizers. Table 4 presents some of the Shimizu data.
TABLE 4. Flame Temperatures for Oxidizer/Shellac Mixtures
A final factor that can limit the temperature of pyrotechnic flames is unanticipated high-temperature chemistry. Certain reactions that do not occur to any measurable extent at room temperature become quite probable at higher temperatures. An example of this is the reaction between carbon (C) and magnesium oxide (MgO). Carbon can be produced from organic molecules in the flame.
This is a strongly endothermic process, but it becomes possible at high temperature due to a favorable entropy change - formation of the random vapor state from solid reactants. Such reactions provide another reason for the lower flame temperatures achieved when organic binders are added to oxidizer/metal mixtures.