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Home , Pyrotechnic composition

CHAPTER I

INTRODUCTION 1.1 GENERAL

For centuries pyrotechnics has been merely an art for display, but has recently developed into an important branch of science undergoing rapid development. The evolution of pyrotechnics as a branch of science originated in China or India prior to 1000 A. D. where a mixture resembling was used. In Europe, black powder was used as a in the 14th century. Production of good quality black powder in large quantities was done in Russia during the 15th century and by the 16th century had been extensively studied. First published formula dating back to 1560 A. D. gave a ratio of 75 /15/10 of chile salt petre (KNO3)/ / which is the same mixture as the one used today. Black powder was used as a safety fuze wrapped by lead covers in the early 1800s. This was used even in World War I and early World War II. In modern days also, gunpowder still finds applications in the form of igniters and initiators in rockets and missiles. As new problems arose, new requirements were formulated and as a result new pyrotechnic compositions were designed. A pyrotechnist should be able to evaluate the possibility of the occurrence of a given chemical reaction in the form of combustion. A pyrotechnic process differs from ordinary combustion by not requiring the presence of ambient air. Once a small external force triggers it, a pyrotechnic composition will take its course in complete isolation of external influences.

During the last fifty years, the field of pyrotechnics, which was limited mainly to applications in military ordnance, , and rock blasting, has developed into a highly advanced science and to a widely used technology. Today, pyrotechnics find extensive applications in space, missile systems and military ordnance. Many space missions would be impossible without pyrotechnic devices and systems. Pyrotechnic devices are also widely used in missile systems for ignition, booster separation, jettison, and for destruction in cases of malfunction. A pyrotechnic mixture is a suitable combination of fuel and oxidizer. Generally, the fuels used are reactive like , and aluminum. During storage pyrotechnic mixtures give rise to undesirable side reactions. Coating the fuel with a suitable can reduce the reactivity of these compositions. Also, pyrotechnic mixtures of solid fuels with roughly the same particle size can be granulated with the help of a binder. The granules obtained show good homogeneity. The metals in pyrotechnic mixtures are mostly used as 1 powders. Pyrotechnic mixtures of fuel and oxidizer are capable of producing an explosive self-sustaining reaction when heated to their ignition temperature. Such mixtures readily undergo an exothermic reaction that generates considerable energy in a relatively short time. These mixtures are incorporated in different pyrotechnic devices that are used as incendiaries, luminous sources for signalling, screens and delays. 1.2 FUELS A number of metals and non-metals are used as fuels in pyrotechnics. 1.2.1 Metallic Fuels Aluminum, magnesium, , and more recently , tantalum and are the commonly used metallic fuels for pyrotechnic applications. Magnesium and aluminum powders are important combustible ingredients usedwherever high output of energy is required in the form of light or heat. The pyrotechnic behavior of magnesium powder is affected by its properties governing the ease with which oxidation can take place, the shape and size of the particles, the proportion of fine and coarse particles present, the degree of surface oxidation etc. These powders also have some drawbacks like several oxidizers cannot be used with these metal powders on grounds of hygroscopicity, the sensitiveness of the mixtures or their violent combustion nature after mixing. Titanium metal offers some attractive properties. It is quite stable in the presence of moisture and most chemicals. It produces a brilliant silver-white spark and light effects with oxidizers. Cost and lack of basic research data on titanium seem to be the major factors holding titanium from being a much more widely used fuel. Zirconium is another reactive metal, but being considerably expensive is a major problem restricting its wider use in pyrotechnic compositions. Among the advantages of zirconium are the small amount of oxygen consumed by its combustion and its high corrosion resistance. It is easily ignited and therefore is quite hazardous - as a fine powder, and must be used with great care. The ignition temperatures of metal powders depend on its size and shape of the powder particles as well as on the quality of the film coating them. Higher the non-uniformity in coating, lower the ignition temperature.

1.2.2 Non - Metallic Fuels (free or in combined form, mainly as a hydrocarbon), hydrogen (in combined form in a compound), phosphorus, , sulfur etc. are also used as fuels for pyrotechnic formulations.

2 1.2.3 Silicon andboron are favored because of their high heats of combustion Heat evolved in the combustion of metals is important from the point of view of their applications to the pyrotechnic systems. Both heat evolved per unit mass and heat evolved per unit volume are important. They are called gravimetric and volumetric heating values, respectively. Heat evolved per unit mass and per unit volume for a few metals is given in the Table 1.1. Table 1.1: Heat Evolved per Unit Mass and Unit Volume of Fuels Commonly Used in Pyrotechnic Compositions

Gravimetric Volumetric Metal heat heat kcal /g kcal / cm

Boron 14.0 33 Aluminum 7.4 20 Magnesium 5.9 10 Silicon 7.7 18 Titanium 4.7 20 Zirconium 2.9 18 Tantalum 1.4 23 Tungsten 0.86 16.7 Carbon 7.83 17.6 Nickel 0.97 8.63

1.3 OXIDIZERS Commonly known oxidizers are , and . Ideally oxidizers should contain maximum oxygen, which is readily given up during combustion. It should be readily available and have no toxic effect on human beings. Oxidizers that are commonly used for pyrotechnic formulations are:

3 1.3.1 Salts a) Nitrates - , , nitrate, nitrate. b) Chlorates - Potassium , . c) Perchlorates - Potassium , sodium perchlorate, ammonium perchlorate, barium perchlorate. 1.3.2 Peroxides Barium peroxide, strontium peroxide, lead peroxide. 1.3.3 Ferric oxide, (Fe2C>3 and Fe3C>4) dioxide, molybdenum trioxide. red lead, litharge are the oxides preferred. 1.3.4 Secondary Oxides Bismuth trioxide, lead chromate, , barium chromate, potassium etc are preferred as secondary oxides. 1.4 BINDERS A pyrotechnic composition usually contains a small percentage of an organic polymer that functions as binder, holding all components together in a homogenous blend. Binders being organic compounds also serve as fuels in the mixture. Without a binder, solid components might segregate during manufacture and storage due to variations in density and particle size. The granulation process, in which the oxidizer, fuel and other components are blended with a binder (and usually a suitable solvent) to produce grains of homogenous composition is a critical step in the manufacturing process. , waxes, plastics, oils and rubbers have been used as binders that fill interstices of the particles and bind them together by adhesion. Besides, binders also frequently desensitize the mixture that would otherwise be very sensitive to impact, friction and static charge. They also protect the fuel from corrosion by moisture, modify the burning rate and enhance the luminous intensity of the in case of illuminating compositions. In the olden days natural resins like , laminae, gums etc., were used as binders. In modern days synthetic polymers like polyester, polyether, polybutadiene etc. are used as binders. Some commonly used binders arc , vinyl acetate alcohol , ethyl cellulose, phenolic resins etc

4 1.5 COMBUSTION OF PYROTECHNIC COMPOSITIONS The major factors that differentiate the process of combustion apart from other forms of chemical reactions are: 1. Presence of a moving reaction zone of high temperature separates the still unreacted substances from the reaction products. This differentiates the combustion processes from chemical reactions where the temperature is the same or nearly the same at all points of the reaction system. 2. Absence of a pressure differential in the reaction zone (in the flame); this clearly differentiates combustion reactions from explosive processes. The combustion of pyrotechnic compositions is an oxidation - reduction reaction in which the oxidation of some components of the mixture proceeds simultaneously with the reduction of other components, of the same mixture. The combustion proceeds in parallel layers, the rate of advancement of the reaction front into the unreacted mixture is called the burning rate. Spice and Staveley ' studied the propagation of exothermic reactions in solid systems by the identification of pre-ignition reactions and classification of pyrotechnics according to their stoichiometry. Further work in this area was done by Hill et al. ' They made attempts to link the kinetics of chemical reactions to temperature - time history measurements through the combustion wave and were the first to derive values for activation energies. 1.6 BURNING RATE The linear burning rate is the rate at which the combustion front advances into the unreacted mixture, is the measure of performance of the system. The burning rate is defined as the speed at which the burning layer recedes and is expressed in mm/s. For pyrotechnic delay systems the inverse of linear burning rate, called the inverse burning rate is commonly used and is expressed in s/mm. Burning rate is known to depend both on compaction and the composition, being particularly sensitive to the percentage of fuel. The proportion in which the fuel and the oxidizer are mixed determines the amount of heat evolved during an exothermic reaction. Ellern states that the percentage of metal fuel in a delay composition has a decisive influence on the burning time. Burning time decreases with increase in fuel content or inverse burning rate increases with increase in fuel content. This is attributed to the increased heat conductivity of the mixture rather than the increased 5 surface area of the fuel. In a delay composition faster burning time is dependent on high metal powder content than calorific output irrespective of the reaction mechanism. However, many studies have shown that the burning rate does not linearly increase with fuel content, as there is an optimum fuel-oxidant ratio at which the burning rate is the fastest. g The burning rate of the composition depends on the following factors • Nature of the reactants and products • Particle size of the ingredients • Relative amount of the ingredients • Effect of pressure • Effect of temperature • Compaction • Nature of the burning rate measurement tube Of the various factors affecting the burning rate of the composition mentioned above, the three most important factors are particle size, ambient temperature and ambient pressure. i) Particle size : The composition usually burns faster with the increasing fineness of the ingredients used. The particle size considerations of the oxidizers are somewhat less critical than those of the fuels. Thus burning rate, initiation sensitivity and other characteristics are fuel directed " in the majority of the formulations such as for light sources and delay compositions and only in exceptional cases " oxidizer directed." ii) Temperature : The burning rate of the pyrotechnic composition is also affected by the surrounding temperature. It has been found that as the initial temperature increases, the burning rate also increases. iii) Pressure : During combustion of a pyrotechnic composition large quantities of gases are produced and normally these gases escape into the atmosphere, but if combustion takes place in a confined space then the burning rate increases. As the pressure increases the burning rate increases and at reduced pressure the composition burns slowly. The effect of additives in molybdenum delay on its burning rate was studied by Ywenkeng in which the effect of various additives like molybdenum trioxide, barium chromate, sulphide, and selenium on the burning rate of molybednum delay with is discussed. Bernard et al. have given a theoretical

6 model for the parameters effecting the burning rate of solid - solid propagation reactions of zirconium, tungsten and lead chromate systems. 1.7 All pyrotechnic compositions produce heat during combustion, which is one of the important consequences of the combustion process. Since the pyrotechnic formulations contain metallic fuels, the heat of combustion of the metallic fuels becomes an important aspect for calculating the heat of combustion of pyrotechnic formulations. The amount of heat evolved in the combustion of pyrotechnic composition substantially determines their specialized applications. The heat of combustion is determined in two ways. i) By using Hess Law. ii) Experimentally by burning the composition in a Bomb Calorimeter. According to Hess law, the heat of formation of the combustion products from their elements is equal to the sum of the heat of formation of the components, which should be added to the heat evolved in the reaction. In other words, it can be written as

AH°C = S(AH°f) - I(AH°f) Heat of combustion Heat of formation Heat of formation Products Reactants

A theoretical estimation of heats of combustion can thus be made from the data on heat of formation available from standard tables. However, it is not always easy to calculate the combustion temperature because accurate data on latent heats may not be always available for all combustion products. Secondly specific heat varies with temperature and variation of specific heats of all the combustion products at different temperatures is required for correct evaluation of the heat capacities of the reaction products. Experimentally the combustion temperature can be measured by using high temperature thermocouples or various types of pyrometers but the accuracy is not more than ± 5%. Using a standard bomb calorimeter, the experimental determination of the heat of combustion is 12 done in an inert medium. Cackett has reported the heat of combustion of compositions with varying percentage of magnesium powder and incendiary materials.

7 1.8 THERMAL CONSTANTS Thermal conductivity, specific heat and thermal diffusivity are the three thermal constants of a substance. 1.8.1 Thermal Conductivity Thermal conductivity (A.) is an important thermo - physical parameter of explosives, , explosives and pyrotechnics, and its measurement is important for studying the combustion and deflagration phenomena. Thermal conductivity measurements are also needed to model the ignition and combustion mechanisms of propellants, explosives and pyrotechnics. Measurement of thermal conductivity has been regarded as the most difficult of the measurements of thermo-physical properties. Previously, thermal conductivities of materials were commonly determined experimentally by heating relatively large samples for hours in methods such as the guarded hot plate (GHP) test. The GHP test for determination 13 of thermal conductivity already has the corresponding military standard. Due to the big size of the samples required in this test method (8 in X 8 in X 1 in) the measurement of thermal conductivity of energetic materials becomes a dangerous operation which needs special safety precautions. In addition, the rather long equilibrium time required before the measurement and the steady time required during the measurement results in too long a test period. Researchers need to develop a new method that would use a small amount of sample by which measurement of thermal conductivity can be done precisely, safely and rapidly. The measurement of thermal conductivity with DSC meets this demand. The thermal conductivity of ingredients used in the present study is given in Table 3.8 (Chapter - HI)

1.8.2 Thermal Diffusivity

Thermal diffusivity studies are an important aspect in the characterization of pyrotechnic materials. The thermal diffusivity D is defined by the relation D = K/p c, where K, p and c are the thermal conductivity, density and the specific heat, respectively. It is an important aspect when describing heat transfer in "gasless" pyrotechnic systems, where the mechanism is one of thermal conduction. Boddington et al. have described the use of a modified DTA head to measure the thermal diffusivity of mixtures of tungsten and potassium dichromate. A disadvantage of the technique is that large samples need to be 8 taken which for some materials would involve a risk hazard. Thermal diffusivity along with density and specific heat can be used to calculate another important thermo - physical property, viz. thermal conductivity. In principle, measurements are made of the temperature gradient established in the sample when the ambient temperature is raised at a constant rate or suddenly from one constant value to another. The method consists of measurements of the temperatures at the center and walls of a substance versus time. The heat flow from the heating system thus establishes in the substance a radial temperature gradient. Knowing the central temperature profile versus time and the boundary and initial conditions the thermal diffusivity can be found out. The details of the measurements are given in Section 2.3.4 At present there is an almost complete lack of thermal diffusivity data on pyrotechnics and propellant ingredients. Calculations of thermal diffusivity of pyrotechnics and related materials are not straight forward and simple and require a lot of assumptions because the temperature in the sample is determined not only by the thermal diffusivity but also by the surface heat transfer characteristics. 1.9 THERMAL ANALYSIS

During a heating or cooling process many substances undergo chemical as well as physical changes that involve heat transfer. These thermal effects can be detected by the thermal analysis technique, differential thermal analysis (DTA). The technique consists of measuring the temperature difference between a sample and an inert reference usually alumina. Both are situated in a homogeneous temperature zone of a furnace and are heated according to pre - defined temperature program. When a reaction involving heat transfer takes place, a temperature difference occurs between the sample and the reference. This temperature difference is measured as a voltage between the sample and the reference thermocouples. As both the sample and reference receive the same heat from the furnace, then for a sample undergoing an endothermic reaction, more heat is absorbed by the sample in comparison to the reference. Thus, the sample temperature remains lower than the reference. Conversely, the sample undergoing an exothermic reaction releases heat and hence the sample temperature is higher than the reference. Thermogravimetry measures the change of mass of a sample when subjected to a controlled heating program. Pyrotechnic reactions have been widely studied using thermal 9 analysis techniques. These studies are useful for understanding the reaction mechanism, phase transformations, dehydration, decomposition, thermal stability, kinetics, solid state reactions, , ignition temperature etc. The differential scanning calorimetry (DSC) is a technique for measuring heat flow associated with a thermal transition during the programmed heating of a sample. In this technique, the difference in energy inputs into a substance and the reference material is measured as a function of temperature/time. It is available in two types i) Power compensation DSC ii) Heat flux DSC. In power compensation DSC, the sample and reference material is maintained at the same temperature during the entire heating program. In heat flux DSC, the difference in temperature between the sample and reference material is measured. It provides information on heat changes associated with transition temperature, heat of transition, purity of sample, heat capacity and kinetics of chemical reactions. 1.9.1 Applications The technique of thermal analysis has a significant influence on the study of the mechanism of pyrotechnic reactions. Pioneering studies on thermal analysis have been carried out at Picatinny Arsenal. ' Conklin has described various uses of DTA for pyrotechnic applications. A number of researchers have described DTA apparatus that have been modified with specialized measurements in mind. Gordon and Campbell19'20 used such modified DTA apparatus to measure the effect of confinement of pyrotechnic samples but a common pattern was not observed. Beardell et al. described a high heating rate apparatus capable of raising the temperature of samples at rates upto 10 °C/min, this technique offered the opportunity of studying pyrotechnic reactions at a temperature rise similar to that in an ignited sample. Boddington et al. have described the use of a modified DTA head to measure the thermal diffusivity of mixtures of tungsten and potassium dichromate. Thermal analysis has been used to investigate the factors which influence the ignition of pyrotechnics and this has proved to be a successful tool for investigating the role of materials that play an integral part in the chemistry of pyrotechnic reactions 1.10 IGNITION TEMPERATURE Ignition temperature or deflagration temperature is the lowest temperature to which an explosive or a pyrotechnic mixture must be heated so as to cause its spontaneous

10 inflammation associated with a clearly visible luminous, sound or smoke effect. At a temperature lower than the ignition temperature, the rate of exothermic reaction in a pyrotechnic combustion is slow and all heat is dissipated into the ambient medium. As the temperature rises the reaction rate and inflow of heat to the composition increases rapidly and at a certain temperature, the heat inflow exceeds the heat outflow. At this stage, the temperature of the pyrotechnic material begins to rise rapidly; simultaneously the rate of chemical reaction also rises sharply resulting in self- ignition. Ignition temperature for one and the same composition is not always constant but depends on the conditions of heat inflow / outflow, prevailing in the experiment. Therefore in practice the design of the ignition temperature measurement device has to be done very carefully. Ignition temperature studies have been well reported. The effect of binders on the ignition behavior of pyrotechnic compositions containing titanium/ and 22 titanium/ was studied by employing DTA. These studies showed that the presence of a binder produced exotherms. DTA curves were recorded for different compositions containing titanium/ sodium nitrate, Bees wax-NaN03 and titanium/ sodium nitrate -Bees wax. The effect of binders on ignition temperatures in argon medium is given below. Table 1.2 : Effect of Binder on Ignition Temperature 22

Binder Ignition remperature °C

Ti-NaN03 Ti-Sr(N03)2

Binary mix 772 Poly-vinyl acetate 768 Bees wax 748 - PVC 719 663 709 665 Carbon black 533 506 Boiled linseed oil 407 465

11 It was observed that the binder had caused ignition to occur generally at a lower temperature as compared to the binary mixture of titanium - NaNC>3 reaction. In the case of Ti - Sr(N03)2 mixtures the binary mixtures did not ignite but several of the ternary mixtures containing binder readily ignited under the same experimental conditions. It was observed that carbon and containing binders significantly lowered the ignition temperature where as oxygen containing binders did not show significant change of the 22 binary mixtures. Barton et al. also studied the effect of binder percentage on the ignition temperature of titanium/sodium nitrate, and titanium strontium nitrate mixtures using DTA. Titanium-sodium nitrate mixtures with varying percentage of boiled linseed oil from 2-12% were studied using DTA. As the binder content increased the temperature of ignition decreased. In the case of titanium/ strontium nitrate-alloprene, the temperature of ignition was lowered from 688°C to 463°C when the binder content was increased from 2 to 12%. 1.11 LASER IGNITION Recent developments in lasers and optical fibers have made it possible to make explosive initiation system that use light rather than electricity as their primary initiation stimulus. Microelectronic fabrication techniques have produced laser diodes of the size of a transistor that can fire ordnance. Most pyrotechnic materials are easily ignited using only a small fraction of the energy a miniature laser can produce. Laser initiation systems have a great safety advantage due to their intrinsic insensitivity to natural and man-made hazards in the environment. Fiber Lasers have successfully initiated pyrotechnic reactions and have the potential to be very inexpensive. Laser ignition of explosives and pyrotechnics is a relatively new technique. Pioneering work was done by Brich Menichelli and Yang," whose principal studies concerned laser ignited detonations. A review of earlier work in the field of laser ignition of 25 explosives and ignition diagnostics was published by Ostmark. Fetherolf et al. have studied CO2 laser ignition behavior of several pyrotechnic mixtures. They have investigated the laser ignition behavior of three zirconium-based compositions containing zirconium, KCIO4, red oxide (Fe2C>3), , and graphite. The materials were formulated and pressed into standard primer cups. Ignition delay times and luminous emission profiles were obtained with near IR photodiode. 12 High speed video photography was used to record both direct and schlierain movies of the ignition sequence. All tests were conducted in air at one atmosphere. The output signal was recorded by a Nicholet digital oscilloscope that was triggered by a synchronization signal from the laser at the onset of the laser heating. Tests were conducted at heat flux levels of 75, 160, 260, 400 W/cm with lasing times varied to investigate the effect of duration of laser heating on the post ignition luminous emission. 27 Chow and Mohler studied the thermal ignition of pyrotechnics with lasers. They studied the transient phenomena of thermal ignition using laser energy and have used the data obtained with existing theory to characterize pyrotechnic materials and to develop more precise kinetic models of the ignition process. In this paper they have presented a formula derived from the thermal explosion theory that allows one to determine the kinetic constants, with the surface heat flux and the inflection temperature as the only input parameters. They investigated three systems Fe3CVAl, Fe2CVAl and Ti/2B.The whole ignition process consists of two stages. In the first stage a laser acts as an external heat source that heats the surface of a pellet, an inert body. When the temperature reaches a certain level, a second stage chemical reaction occurs. Inflection point of the temperature Vs time trace separates the two stages. 28 Ostmark has studied laser ignition of explosives. In this paper a CO2 laser has been used to heat pyrotechnic mixtures and high explosives, with different particle size to measure the variations in ignition energy. Three different varieties of magnesium powders were tested with BaC>2 and NaNC>3. By using a laser to heat the explosive it is possible to precisely control the amount of energy necessary to achieve ignition. With this approach a 29 30 method for studying the ignition was developed. ' This method makes it possible to measure the ignition energy in physical units (e.g. J or J/m2), The earlier studies involved the external parameters, irradiated area and duration of energy deposition in the explosive and their influence on the sensitivity of the explosive. These studies included a simple theory which explains the achieved results. The laser ignition method is a convenient and fast method for customizing the ignitability of a pyrotechnic mixture, as well as being useful for obtaining the sensitivity of an explosive in physical units. 13 1.12 END PRODUCT ANALYSIS 1.12.1 X - Ray Diffraction (XRD) The most powerful application of x - ray diffraction is crystal structure determination. Families of planes of atoms in a crystal have the ability to reflect an x - ray beam when the Bragg equation 2dSin0 = nk is fulfilled, where d is the interplaner spacing, 0 is the angle between the planes and the x - ray beam ( Bragg angle ) k is the x - ray wavelength and n is an integer called the order of reflection. Families of planes are identified by a system of Miller Indices (hkl) these take integer values and correspond to the number of times a family of planes strike the a, b, c edges of the unit cell. The application of x - ray diffraction studies include structure determination, phase analysis, solid solution analysis, detection of preferred orientation and order / disorder, determination of the particle size etc. A variety of instruments have been devised for recording x - rays diffracted by polycrystalline solids. The reflections are either recorded electronically using an instrument called powder diffractometer, or they are recorded on film using powder diffraction cameras. Every crystalline substance has a unique x - ray powder diffraction pattern from which a characteristic set of d and I/Ij values can be derived . Materials are identified from these values in conjunction with the JCPDS ( Joint Committee on Powder Diffraction 32 Standards ) powder diffraction file. This contains a set of cards containing x - ray data for most known crystalline phases. The data include d and I/Ii values, Miller indices, unit cell dimensions etc. Manual search / procedures for the identification of a single substance involves matching the d and I/I] values of eight strongest lines of the pattern by systematically scanning a search manual. The appropriate data card is retrieved and if the experimental data match the standard data then the analysis is complete. 1.12.2 X - Ray Photo Electron Spectroscopy Studies on combustion mechanism have benefited greatly from the recent developments of various surface sensitive techniques and instrumentation. Among these devices are electron spectra for chemical analysis (ESCA) which is commonly termed X - ray photoelectron spectroscopy (XPS). In addition to its surface sensitivity ESCA is uniquely capable of providing chemical information such as Oxidation State and chemical bonding as well as elemental composition. In conjunction with ion milling or chemical 14 etching it has the ability to furnish a depth profile of both the surface and subsurface. It also allows the investigation of electron structure, providing a picture of molecular orbital for gas phase species, valence bond density of states and core level electron binding energies for solids. The characteristic electron energies allow elemental analysis as well as chemical state identification. In XPS if a sample is irradiated with monochromatic photons of energy v, the photons may be absorbed resulting in emission of electrons determined by the Einstein

-3-3 relation hv =EBE + E K E BE is the ionization energy or binding energy of the K species of the electron in the material and E K is the kinetic energy of the ejected electron. The kinetic energy of the ejected electron is measured using an electrostatic or magnetic analyzer.

1.13 THEORETICAL MODELLING The combustion process and the rate of propagation are at the heart of pyrotechnic systems; The rate of propagation may be influenced by any one of a number of design parameters. These include the chemical nature of the components, their purity and particle size, the uniformity of mixing, pressing load etc. The prediction of burning velocity is a much sought after goal, bringing together as it does practical requirements and the results of fundamental research both theoretical ^nd experimental. Current practice in which the solutions are computed for models of the combustion, leans heavily on the knowledge of reaction kinetics where information is very difficult to obtain and the results are often open to dispute. Earliest of models was presented for the calculation of the burning rates of gasless pyrotechnic compositions.34 This model was based on the measurement of the thermal diffusivity of the composition, the particle diameter and the combustion temperature. A further development of the theory on which the formula was based was presented by Widlund.35 He attempted to deal indirectly with the varying amounts of gases produced during the burning of pyrotechnic compositions. The resulting model was applied to an illuminating flare composition (Mg / NaN03 / Binder - 60/29/8) in order to compare the predicted and observed burning rates.

15 Boddington et al. has also developed a computer model to predict the behavior of gasless pyrotechnics. Using one-dimensional treatment incorporating heat transfer by thermal conduction. In this equation they had to consider the thermal conductivity, density, specific heat capacity, burning velocity, exothermicity, lateral heat loss coefficient and the fractional extent of reaction. They have derived the results by choosing a simple first order isothermal law and an Arrhenius temperature dependence. 37 38 Hardt and Phung ' have proposed diffusion mechanism in their analysis of ignition and propagation in and bimetallic pyrotechnics. Thermal analysis has been used to obtain macroscopic features of the chemistry but only at temperatures and heating rates far removed from those in the combustion regime. An alternative route is the 39 40 41 measurement and analysis of temperature- time data for the combustion wave. ' ' The use of theoretical models to predict various parameters of pyrotechnics is gaining importance as more complex reactions are being evaluated and understood. Miller has outlined the theory and modeling in combustion chemistry. Classical equilibrium thermodynamic computer programs are reasonably common methods that have been used to study the chemistry of pyrotechnic combustion reactions. Leo de Yong et al. have outlined the shortcomings of the most popular type of program that is the NASA Lewis CEC 76 program which is based purely on equilibrium thermodynamics, hence a new program Chemkin was developed, which allows the kinetics of gas phase reactions to be modeled. Its 44 potential use in modeling pyrotechnic combustion reactions was investigated. REAL is a new computer program useful for pyrotechnics since the number of species in this program are more than 2600 which is far more than the NASA program which has only 750 species. REAL is also useful to calculate transport properties like thermal conductivity and thermal diffusivity of the materials. It is very easy to operate and can be assigned for different conditions. It can be used for constant temperature (T = 300°K) and either at constant volume and constant pressure. The data obtained can directly be correlated with the data obtained from the bomb calorimeter. In the present study REAL program at constant pressure conditions has been used to predict values for flame temperature, volume of gaseous products, total number of moles of

16 products and the condensed species or pyrotechnic compositions cased on titanium 45 potassium perchlorate and HTPB binder. 1.14 PYROTECHNIC COMPOSITIONS FOR IR Literature survey on IR flares indicates that a number of pyrotechnic compositions have been formulated and studied for IR output, thermal behavior etc., but detailed technical information are either in patent form or classified. Two pyrotechnic compositions consisting of magnesium, teflon, rosin and magnesium, , rosin have been studied for radiant intensity, mass flow rate, effect of combustion temperature by Gonpei et al. in which it was concluded that teflon based compositions emit high order of IR intensity as compared to barium nitrate based compositions in the 3 - 5 urn and 8-13 u,m regions because of the halogen atoms present therein. Increase in combustion temperature increases the radiant intensity in the 3 - 5 urn region only. Radiant intensity increases with increased mass flow rate upto a certain limit, beyond that it decreases. Radiant intensity also increases with addition of metals like iron, aluminum, copper, titanium and silicon in both the wavebands of 3 - 5 urn and 8-13 urn. However addition of copper and aluminum show highest radiant intensity. A decoy flare composition containing magnesium powder - 54 %, teflon - 30 % and viton - 16 % has been patented. However, detailed information is not available in the open literature. Another composition for IR flare containing magnesium 48 powder - 55 %, teflon - 40 % and viton - 5 % has been studied by Clucas and Lowe for chemical analysis in which they found out that methyl ethyl ketone is a suitable solvent for viton extraction. A blend of ceramics containing Z1O2 - 50 %, S1O2 - 35 %, AI2O3 - 4 %,

49 Fe203 - 4 %, MgO - 2 %, BaO - 3 % and CaO - 2 %, studied by Takeo has indicated better IR radiation in the Far IR region at 1200 - 1300 ° C. Heat generating pyrotechnic carbon fiber web (1mm thick), coated with a layer of finely divided AI2O3, Si02 and/or Z1O2 has given better IR radiation in the 8 - 14 |a.m range as claimed by Baldi and Wynnewood in their patent. Coated webs have been found more effective in bent form against heat seeking missiles.

17 Table 1.4 :Major Emission Bands for Common Molecular Combustion Products Species Approx Band Centres in \im

H20 1.14, 1.38, 1.88, 2.66, 2.74, 3.17, 6.27. 2.01,2.69,2.77,4.26,4.82,15.0. C02 CO 1.57,2.35,4.66. NO 2.67, 5.30. OH 1.43,2.80. 4.50,6.17,15.4. N02

N20 2.87,4.54,7.78,17.0.

1.15 SENSITIVITY The purpose of a sensitivity test is manifold and depends on the parameters of design and the degree of safety that must be met. Primarily sensitivity is a measure of the response of the composition to such external stimuli as impact, friction, heat, static charge etc. Compositions that are extremely friction sensitive are usually considered too hazardous for use. Impact values greater than or similar to tetryl are desirable. If it is less than tetryl they are to be handled cautiously. Although delay compositions are initiated by heat sources, ignition temperature values that approximate storage temperatures are considered unsafe: Neither should the ignition temperature be so high as to make it difficult to ignite. Compositions containing finely divided fuels such as zirconium and magnesium appear to be quite sensitive to electrostatic discharges. Sensitivity tests are used to (a) determine the energy required to initiate the igniter, delay and terminal compositions, (b) indicate the effect of changes in the pressing procedures and (c) obtain information regarding the effect of storage under adverse conditions. Sensitivity is of utmost importance in the field of pyrotechnics. All the data on sensitivity, such as the sensitivity to impact, friction and flame etc., are obtained at special conditions via experiments and described by the percentage of explosion. Experimental data can only make us understand the difference of sensitivity for different explosives.

18 1.15.1 Sensitivity to Impact Definition. The minimum height at which a given impact weight will cause a sample of explosive to react; or the maximum height of drop that will cause a reaction in 50 percent of trials. One widely used criterion for sensitivity testing, when a limited number of samples are available, is the Bruceton staircase method. 1.15.2 Sensitivity to Friction Definition. The response of a sample to either a glancing blow from a weight or a sample kept between a porcelain pin and plate with a weight attached at the ends is expressed as the sensitivity to friction. 1.16 OBJECT OF THE PRESENT STUDY

A literature survey of the work done till date shows that a systematic and exhaustive study on the combustion behavior of the high performance fuels namely zirconium, titanium and nickel has not been carried out. Some references were available in literature but here a systematic study was undertaken to evaluate the combustion behavior of these three fuels especially zirconium and titanium have been studied with potassium perchlorates as the oxidizer and HTPB as the binder. Traditionally nitrocellulose has been used as a binder for pyrotechnic compositions, but in this study it was thought to use an alternative binder HTPB. In addition thermal conductivity, thermal diffusivity and laser initiation of a series of pyrotechnic composition has not been studied so far therefore in our study these parameters have been evaluated. 1.17 PLAN OF WORK

It was proposed to prepare pyrotechnic compositions based on zirconium, titanium and nickel as fuels and to use potassium perchlorate as the oxidizer. These pyrotechnic formulations were prepared by mixing the fuel and the oxidizer on the basis of stipulated mole fractions taking into consideration the stoichiometric ratio of the fuel and oxidizer. Three compositions above the stoichiometric ratio and three below were formulated. In all seven basic compositions from each series were mixed physically and ten parts HTPB binder was added to the mixture. The compositions thus prepared were tested for various parameters to evaluate their pyrotechnic performance, to find out the combustion behavior of all the three systems. In addition, for the zirconium series HTPB binder content was varied from 2 to 16 parts and the resulting compositions were also evaluated.

19 Following studies were used to evaluate the pyrotechnic compositions prepared. (i) Burning rate (ii) Heat of combustion (iii) Thermal analysis (iv) Thermal conductivity (v) Thermal diffusivity (vi) IR output at three different wavelengths (vii) Laser initiation (viii) Sensitivity (ix) End product analysis. (XRD / XPS ) (x) Theoretical modelling using REAL program (xi) Correlation of experimental data with theoretically obtained data

1.18 SCHEME OF THE THESIS The thesis is divided into four chapters. Chapter -1 Introduction This chapter deals with a general introduction to pyrotechnics for the reader, what pyrotechnics is all about, how it is used for military applications and what are the methods of characterization. The status of the pyrotechnic research in the world and the objective of the present study are also indicated. Relevant literature on the aspects such as burning rate, ignition temperature, thermal behavior, laser initiation, IR output and sensitivity are discussed in detail. Chapter - H Experimental This chapter gives a broad specification of the materials used in the present study, the method of preparation of the composition, the mixing techniques etc., the instruments and equipments used along with their importance and specific use and the methodology used for the determination of the pyrotechnic property of the composition. Chapter - III Results and Discussion This chapter contains the details of new technical information generated during the course of the present investigation. The chapter has been divided into sub - sections based on the three fuels studied. A comparison is made of the results with the three fuels and their behavior to various parameters is discussed. This chapter also correlates experimental data with theoretically obtained data, and suggestions for future research are also given.

20 Chapter - IV Summary and Conclusions This chapter summarizes the findingso f the present study and highlights important observations and their importance to the field of pyrotechnics. References are included at the end of each chapter and the thesis concludes with a list of publications of the author.

21 1.19 REFERENCES

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