Investigation of the Effect of Aluminum Powder on the Combustion Rate of the Composite

Abstract

This article discusses the combustion of high-energy systems based on the oxidizers ammonium nitrate, potassium nitrate, and a mixture of highly active aluminum grade PAP-1 under conditions of a deficiency or excess oxidant α, under a pressure reduction in the range of 0.1–3 MPa. The objective of this study is to develop materials for the production of high-energy compositions based on oxidizers of ammonium and potassium nitrate, fuel binder, and metallic fuel in the form of aluminum powders ASD-6 and PAP-1. The influence of the amount of excess oxidizer, the amount of metallic fuel, and the grade of aluminum on the rate and completeness of combustion of a high-energy composition have been studied. Experiments were carried out on the studied high-energy systems, depending on the grade of aluminum used and the excess of the oxidizing agent α, and the influence of pressure on the burning rate. The compositions of high-energy compositions based on highly active aluminum with the highest combustion rate of mixtures and combustion completeness were determined.

1. Introduction

Optimization of the energy and environmental characteristics of high-energy materials is possible by varying their component composition. Prescription-based regulation of the burning rate of high-energy materials is mainly achieved by introducing into the composition of combustion catalysts, partial or complete replacement of oxidizer particles and metallic fuel, increased coefficients of the oxidizer subject, and the use of active, high-energy oxidizers and combustible binders. It is known that the introduction of metallic fuel (mainly aluminum) leads to a significant increase in the burning rate of solid fuels.

In these experiments, it was shown that the introduction of aluminum above 15–20 wt % of the fuel mass leads to a decrease in the burning rate and an increase in the content of condensed combustion products, and incomplete burnout of aluminum.

One of the most important problems in the use of metals in fuels is to ensure the complete combustion of the metal.

The combustion of metals, in which the boiling point of the oxide is significantly higher than the boiling point of the metal, occurs predominantly in the vapor phase. Therefore, particles of metals such as magnesium and aluminum burn in the diffusion mode, and if the ignition of the particles and mixing with an oxidizing agent (oxygen) is ensured, then the time required for the complete burnout of the particles is proportional to the square of their diameter.

One of the ways to control the burning rate of a high-energy condensed system is to replace standard ASD-6 aluminum powders with highly active aluminum powder.

To develop new optimal components of high-energy materials in the research study being carried out, PAP-1 brand aluminum powder and a mixture of powder with aluminum ASD-6 were used as a metal fuel instead of ASD-6 grade aluminum. The paper experimentally shows the possibility of increasing the burning rate by 1.5–3 times and increasing the combustion temperature of the fuel when using aluminum powder.

2. Materials and Methods

One of the ways to improve the energy characteristics of high-energy condensed systems is the use of metal in their compositions in the form of powders of various dispersions (aluminum, beryllium, boron, lithium, magnesium, etc.), as well as their hydrides. Metal fuel in its pure form cannot be a source of reactive energy, but if it is mixed with an oxidizing fuel, then the heat generated during the combustion of the metal fuel significantly increases the temperature of the resulting gas and, consequently, its outflow rate increases, which increases the specific jet thrust [1,2].

The possibility and expediency of using one or another metallic fuel are determined by the complex requirements for high-energy systems (its chemical compatibility with other fuel components, environmental friendliness, the availability of a raw material base, durability, etc.) [3].

Powdered aluminum of two grades, ASD-6, and aluminum powder PAP-1, were chosen as the metallic fuel [4,5].

ASD-6 and PAP-1 are distinguished by the following characteristics. ASD-6 has a spherical shape. Particle size 0–125 micrometers. Specific surface 0.9–0.15 m²/g. The main components are Al—99.0; Ti—0.35–0.60. PAP-1 has a plate shape. Particle size 0–80 micrometers. It has as an impurity fat add −3.8; Fe—0.5; Si—0.4; Cu—0.05; Mn—0.01.

The proportions of the oxidizer and fuel in the fuel composition are determined by a value called the ratio of components; the theoretical (stoichiometric) ratio of the components is the minimum amount of oxidizer that is necessary for the total consumption of the oxidizer and fuel, at which point the oxidizer completely oxidizes the fuel, for the oxidation of 1 kg of fuel. In other words, the theoretical ratio of components is such a ratio while remaining in excess. The actual ratio of components is the actual ratio of the costs of the oxidizer and fuel and may differ from the theoretical one.

The coefficient α, at which the maximum value of the specific impulse is obtained, is determined by the following formula.

$$\alpha =\frac{\sum (number of atoms of oxidizing elements·valency)}{\sum (number of atoms of reducing elements·valency)}$$

(1)

Knowing the coefficient allows us to solve the problem of determining the percentage composition of a high-energy condensed system.

When researching the effect of pressure on the burning rate of a high-energy composition, experiments were carried out in parallel for two types of aluminum powders, ASD-6 and PAP-1, depending on α systems in the pressure range of 0.03–3 MPa, the oxidizer excess coefficient was varied at intervals of 0.4, 0.5, 0.6.

The samples used in this study were obtained by the mixed method, the loose bulk density of which was 1.79–1.82 g/cm³. Figure 1 shows the dependence of the burning rate on pressure.

To measure the temperature difference of zones, none of which contains a secondary converter (thermo-EMF meter), it is convenient to use a differential thermocouple: two identical thermocouples connected electrically towards each other. Each of them measures the temperature difference between its working junction and the conditional junction formed by the ends of the thermocouples connected to the terminals of the secondary converter.

Experimental data on the effect of pressure on the combustion rate of high-energy condensed systems make it possible to obtain empirical formulas for the laws of combustion rate in the pressure range of 0.03–3.0 MPa. The burning rate is a function of only the pressure in the combustion chamber. Such a dependence is called the “law of combustion,” which is different for each fuel.

Attaining a point on a graph is not a difficult task. The fuel samples are burned in special chambers (“constant or variable pressure bombs”). They burn a sample of fuel under certain conditions, measure all the parameters, and plot the combustion law according to the pressure and combustion rate.

Table 1. The composition of the studied high-energy condensed systems and the dispersion of the components.

High Energy Condensed Systems	α	Content of Components, wt %
		Ammonium Nitrate	Potassium Nitrate	Fuel Binding	ASD-6	PAP-1
1a	0.4	64	-	21	15	-
2b	0.4	32	32	21	-	15
3c	0.5	72	-	13	7.5	7.5
4b	0.5	36	36	13	-	15
Dispersity, microns		100–125	90–100	-	10–20	-

shows the composition of the studied high-energy condensed systems and the dispersity of the components.

The burning rate was measured using the thermocouple method. The samples were ignited with a nichrome coil. The completeness of combustion was estimated by the mass of slag after combustion. Three experiments were carried out for each point.

Replacing micron aluminum grade ASD-6 with PAP-1 leads to a decrease in the flammability threshold. It can be seen that the use of aluminum powder PAP-1 increases the burning rate by more than four times. A mixture of aluminum PAP-1 and ASD-6 in a ratio of 1:1 also increases the burning rate. With an increase in the value of α in PAP-1 systems, an increase in the burning rate is observed [6,7].

Replacing micron aluminum ASD-6 with PAP-1 leads to a decrease in the value of n and an increase in the value of V (combustion rate) in the combustion rate law, which indicates that the replacement of micron aluminum by PAP-1 shifts the reactions taking place between the components of the systems from gas to k-phase.

Replacing micron aluminum grade ASD-6 with PAP-1 leads to a decrease in the flammability threshold. It can be seen that the use of aluminum powder PAP-1 increases the burning rate by more than four times. A mixture of aluminum PAP-1 and ASD-6 in a ratio of 1:1 also increases the burning rate. With an increase in the value of α in PAP-1 systems, an increase in the burning rate is observed.

Replacing micron aluminum ASD-6 with PAP-1 leads to a decrease in the value of n and an increase in the value of V in the combustion rate law, which indicates that the replacement of micron aluminum by PAP-1 shifts the reactions taking place between the components of the systems from gas to k-phase.

The laws of the burning rate of the considered mixtures in the form $V=V_1{\left(\frac{P}{atm}\right)}^n$ in the pressure range of 0.1–3 MPa.

Knowing the degree of combustion in the combustion rate law allows you to choose the pressure that provides the optimal combustion rate for a particular fuel composition [8,9].

The laws of the burning rate of the studied mixtures in the form in the pressure range of 0.1–3 MPa.

The effect of pressure on the reduction in the mass of slags has been studied. When using aluminum grade ASD-6, the amount of slag decreased at 0.4 MPa, and for compositions using PAP-1, the decrease in the amount of slag, in comparison with the calculated one, occurs at atmospheric pressure. This is probably due to the high thermal process and sublimation of aluminum suboxides.

Table 2. Mass of slags of the initial mixtures of masses. % of the original sample.

Pressure, MPa	1a	1b	2a	2b	3c	3b	4a	4b
atm.	52	23.07	51.4	22.8	50	21	51.8	22.0
0.2	47	17.4	46.3	16.5	46.0	18	45.2	16.8
0.4	41.2	15.7	39	14	36.4	9.4	43.4	10.9
0.8	30.4	12.1	30.3	12.54	30.8	9	42	10.4

gives the masses of slags of initial mixtures with different values of α.

The effect of pressure on the reduction in the mass of slags has been studied. When using aluminum grade ASD-6, the amount of slag is reduced at 0.4 MPa, and for compositions using PAP-1, the amount of slag is reduced compared to the calculated value already at atmospheric pressure, which is probably due to the high thermal nature of the process and sublimation of aluminum suboxides. Table 2 gives the masses of slags of initial mixtures.

In order to increase the energy properties of powder materials (ammonium nitrate, potassium nitrate, aluminum powders) and give them the required dispersion, the raw material is subjected to mechanical activation. Mechanical activation in centrifugal planetary mills contributes to an increase in the internal energy of materials due to the formation of various crystal defects (shear, cleavage, etc.). These factors provide a higher process temperature, reduce the ignition temperature threshold and the ignition induction period of the mixtures, and, as a result, increase the fuel charge burning rate [10].

The preparation of raw materials should be carried out in separate rooms.

The raw materials used are subject to incoming control—quality control of raw materials and auxiliary materials prevents the production of raw materials that do not meet the requirements. Raw materials are checked for moisture, chemical composition, and phase composition. Powdered materials are characterized based on particle size distribution.

The preparation of initial components for high-energy systems consists of the main operations: pre-drying, re- or final drying, and sieving [11].

Preliminary drying of the components is carried out in order to facilitate grinding; since the substance contains less moisture, it is crushed more easily.

To facilitate the mixing of components of high-energy systems and uniform combustion of the compositions, the components are crushed. For grinding components, ball mills, runners, and disintegrators are used.

After grinding, the components are dried, and their moisture content is 0.1–0.5%.

In order to obtain the required size of particles and to separate accidentally trapped mechanical impurities, the components are sieved through a sieve with certain hole sizes before preparing high-energy mixtures.

The oxidizing agents used in the form of saltpeter and perchlorates are hygroscopic and prone to caking. The process of preparation of oxidants consists of preliminary loosening on a vibrating screen followed by drying.

Ammonium nitrate drying parameters: temperature—100–105 °C for two hours. Grinding occurs after drying; if the moisture content is above 0.2%, re-drying to a moisture content of 0.2% is performed [12,13].

Potassium nitrate drying parameters: temperature—100–110 °C, time—until the content of the mass fraction of water is 0.08–0.1%. The saltpeter is sieved on a sieve with a hole size of 90–100 microns.

The preparation of aluminum ASD-6 consists of drying at a temperature of 130–140 °C. The moisture content should be no more than 0.2%. Subsequent sieving is used for the dispersion of aluminum powder—20–30 microns.

Preparation of aluminum powder: grinding—during caking, drying temperature—80–85 °C, drying time—2 h. The residual moisture content should be 0.1–0.15 wt %. If the quantity exceeds this value, then re-drying is carried out. After drying, crushing and dispersion are carried out on a sieve with mesh number 1 in accordance with GOST 5494-95.

GOST 5494-95 is the standard that applies to aluminum powder (powder), which is an especially finely divided aluminum particle of a lamellar shape and is used as aluminum pigments for a wide range of purposes.

The preparation of additives—combustion modifiers (catalysts, inhibitors, stabilizers) consists of mechanical activation in order to increase their efficiency in planetary centrifugal mills [14].

The preparation of the fuel binder consists of its evacuation and mixing of liquid elements—plasticization with low molecular weight liquids (transformer oil, dibutyl phthalate).

Preparation modes are individualized for each type of raw material.

3. Results

The combustion of high-energy systems after the initiation of the combustion process occurs in a spontaneous mode and depends on the composition of the fuel composition. For the combustion of the system to pass in a stationary mode at a constant speed and temperature, careful selection of the combustion mode of the fuel charge and its composition is necessary.

As high-energy condensed systems, compositions based on oxidizers, fuel binders, metallic fuels, and additives were chosen.

Calculation of the coefficient of excess oxidizer. The proportions of oxidizer and fuel in the fuel composition are determined by a value called the ratio of components; the theoretical (stoichiometric) ratio of components is the minimum amount of oxidizer that is necessary for the full consumption of the oxidizer and fuel at which the oxidizer completely oxidizes the fuel, for oxidation 1 kg of fuel. The actual ratio of components is the actual ratio of the costs of the oxidizer and fuel and may differ from the theoretical one [15,16].

The oxidizer excess coefficient is determined by Formula (1). The coefficient at which the maximum value of the specific impulse is obtained is called optimal.

The oxidizing atoms of fuel compositions include oxygen, chlorine, and fluorine. Reducing ones include carbon, hydrogen, sulfur, as well, as metal fuel atoms [17].

To select the optimal combustion parameters for high-energy systems, the compositions of fuels based on an inactive fuel binder were calculated: SKDM-80 butadiene rubber, oxidizers: their mixtures, and reducing agent in the form of aluminum powder. Oxidizer excess coefficient α = 0.5.

Table 3. Composition and equivalent formulas of mixed oxidizers.

N	Composition of Oxidizers, %
	NH4NO3	KNO3
1	95	5
2	90	10
3	85	15
4	100
5		100

shows the component composition and equivalent formulas of the mixed oxidizers used in this study.

Table 4. Compositions of the fuel mixture, α = 0.5.

N	Component Content, wt %
	KNO3	NH4NO3	SKDM-80	Al
1	2	3	4	5
1	-	80.68	19.32	-
2	-	64	21	15
3	-	-	23.2	15
4	-	-	23.2	-
5	-	7.2	13	15
6	-	36.2	13	15
7	-	36.2	27.6	-
8	-	7.2	13	-
9	-	-	18.8	-
10	-	-	18.8	15
11	36.2	36.2	13	15
12	80.64	-	21	-
13	64	-	21	15

shows the composition of a high-energy fuel mixture containing single and mixed oxidizers with and without aluminum additives. α = 0.5.

Method for manufacturing samples of fuel charges.

The technological process of preparation includes the preparation of mixtures of powdered components, a binder (vacuum, mixing with aluminum), fuel mass, and the formation and polymerization of a charge with subsequent armoring [17,18].

In this study, samples were made by mechanical mixing of the initial components by hand, with no more than 15 g of fuel mass per bag. To obtain high-quality samples, the oxidizer sample was divided into three parts, one of which was preliminarily mixed with technological additives (hardener and catalyst).

The entire SKDM-80 sample was placed in a porcelain cup. Two portions of the oxidizer were mixed with a part of the metallic fuel before being added to the fuel mass. The mixing time of each portion is 10–15 min.

Then, bulk fuel components were gradually added to the bundle, and after each addition, the mixture was thoroughly mixed. The hardener was introduced into the finished fuel mass with subsequent mixing [19,20,21].

In the manufacture of the fuel mixture, the dispersity of the initial components was strictly controlled.

ASD-6 aluminum powder (d = 10 µm) and PAP-1 aluminum powder were used as the metallic fuel. The hardener di-N-oxide-1,3-dinitrile-2,4,6-triethylenebenzene was introduced into the system in an amount of 0.3 wt % of the weight of the fuel binder.

The resulting fuel mass was molded using a fluoroplastic assembly in the form of cylindrical specimens 10 mm in diameter and 30–40 mm high. A day later, polymerized (polymerization temperature 25–30 °C) samples were pressed out. Then the samples were weighed, the height and diameter were measured (with an accuracy of 0.05 mm), and the loose bulk density was determined. The loose bulk density of samples of the same composition should differ by no more than 0.02 g/cm³. The samples were stored in a desiccator [21,22].

Preparation of fuel samples for experiments. Samples on the end and side cuts should not have shells and cracks. Ensuring the necessary mechanical strength is achieved by armoring samples. In this study, epoxy resin was used as a booking agent.

Table 5. Characteristics of samples.

№	Component Name	Compound, %	Mass, g	Height, mm	ρ, g/cm3	α	Burning Speed, mm/s
1	2	3	4	5	6	7	8
1	NH4NO3	80	4.2	3.02	1.75	0.5	0.6
	SKDM-80	20
2	NH4NO3	64	4.3	3.06	1.78	0.5	1.15 ± 0.15
	SKDM-80	21
	ASD-6	15
3	NH4NO3	64	4.5	3.20	1.78	0.5	2.01 ± 0.57
	SKDM-80	21
	ASD-6	7.5
	PAP-1	7.5
4	NH4NO3	32	4.4	3.12	1.79	0.5	2.16 ± 0.5
	SKDM-80	21
	ASD-6	7.5
	PAP-1	7.5
	KNO3	32
5	NH4NO3	64	4.8	3.40	1.8	0.5	2.6 ± 0.3
	SKDM-80	21
	PAP-1	15
6	KNO3	32	4.5	3.19	1.72	0.5	2.8 ± 0.61
	NH4NO3	32
	SKDM-80	21
	PAP-1	15
7	KNO3	64	4.5	3.1	1.79	0.5	3.0 ± 0.5
	SKDM-80	21
	PAP-1	15
8	NH4NO3	87	4.7	3.31	1.79	0.5	0.8
	SKDM-80	13
9	NH4NO3	72	4.7	3.30	1.81	0.5	1.67
	SKDM-80	13
	ASD-6	15
10	NH4NO3	72	4.6	3.2	1.80	0.5	3.78 ± 0.28
	SKDM-80	13
	PAP-1	15
11	NH4NO3	72	4.7	3.3	1.79	0.5	2.74
	SKDM-80	13
	ASD-6	7.5
	PAP-1	7.5
12	NH4NO3	36	4.65	3.2	1.81	0.5	2.98
	KNO3	36
	SKDM-80	13
	ASD-6	15
13	KNO3	72	4.7	3.28	1.79	0.5	3.7 ± 0.39
	SKDM-80	13
	PAP-1	15
14	KNO3	72	4.69	3.25	1.80	0.5	3.01
	SKDM-80	13
	ASD-6	7.5
	PAP-1	7.5

gives the characteristics of the obtained fuel samples and their burning rate.

To obtain the characteristics of conductive ignition, we used the method of igniting heterogeneous systems in air at atmospheric pressure in a muffle furnace. These experiments were carried out in the temperature range of 643–713 K.

The introduction of metallic fuel increases the burning rate of a high-energy system at α = 0.5 from 0.6 mm/s without aluminum to 3.0 mm/s with aluminum in the composition, both for mixed fuel and for fuel from a monoxide caster.

Based on the equivalent formulas of the selected fuel compositions, the oxidizer excess coefficients α were calculated. Depending on the value of α, the required amount of weight of the fuel binder is calculated. In these experiments, the excess of the oxidizing agent was 0.5 and 0.9.

To study the effect of the value of α on the combustion rate of the fuel mixture, experiments were carried out with the value of α—0.3; 0.4; 0.5; 0.9. The fuel system consisted of an inert fuel, ammonium nitrate and aluminum ASD-6, and highly active aluminum PAP-1. Figure 2 shows the dependence of the burning rate on the oxidizer excess coefficient.

An increase in the oxidizer excess coefficient contributes to an increase in the burning rate of the system and a decrease in smoke emission, but it leads to a decrease in the strength of fuel samples, a decrease in ductility, and it can collapse during extrusion (chips, cracks form) [23,24,25].

When using PAP-1 instead of ASD-6, the burning rate at α = 0.5 increases by more than twice (2.26), at α = 0.9–2.1 times, when mixing metal fuel 1:1, the speed increased at α = 0.5 by 1.92 times, and at α = 0.9 it increased by 2.1 times, which indicates the effectiveness of partial or complete replacement of aluminum ASD-6 with highly active aluminum grade PAP-1.

One of the factors affecting the burning rate of a high-energy system is the content of metallic fuel in the composition.

The influence of the amount of ASD-6 on the combustion rate of a system based on ammonium nitrate and an inert fuel has been studied.

The maximum burning rate can be traced at an aluminum content of 15%; an increase to 20% leads to a decrease in the process rate and temperature. The likely reason is that excess aluminum in the system acts as a ballast.

In all experiments, the total weight of the prepared fuel mixture was taken to be 15 g. Three samples weighing 4.2–4.7 g were prepared from this amount. The loose bulk density of the sample was selected empirically and ranged from 1.69 to 1.8 g/cm³. Samples with a lower loose bulk density have a higher burning rate but lower strength. The analysis of the cinders showed that the cinders of samples with low loose bulk density contain unreacted components of the mixture [26,27,28].

To study the composition of cinders, a series of samples of various compositions with α = 0.5 and α = 0.9 was prepared.

A volumetric analysis of the cinder sample was carried out. It was established that the maximum amount of unreacted aluminum remained in the cinder of the composition containing ASD-6.

These experiments were carried out in air. Ignition was carried out with a heated metal plate to a temperature of 570 °C. The combustion of samples was uniform [29,30,31].

A comparative analysis of the combustion of systems containing micro- and highly active aluminum when used without chlorine oxidizers and inert fuel showed that highly active aluminum increases heat release in the condensed phase due to the intensification of exothermic transformations in a narrow reaction layer, the stronger, the higher the value of the excess oxidant coefficient [32,33].

Analysis of fuel samples after combustion using ASD and highly active aluminum showed that when using highly active aluminum in fuel samples, complete combustion of the initial components occurs, both at α = 0.9 and α = 0.5. Only compounds of aluminum with calcium present in rubber remain in the cinder.

4. Conclusions

Based on the analysis, components for high-energy compositions were selected that are environmentally friendly, affordable, and provide a high burning rate in a self-propagating mode. Such a fuel component is PAP-1 grade aluminum powder, which has a low ignition and combustion temperature in a self-propagating mode. The excess coefficients of the oxidizing agent from among the selected ones—ammonium nitrate and potassium nitrate were calculated. The equivalent formulas of all components of the mixture are calculated, and the equivalent formulas of high-energy mixtures and the composition of the fuel compositions are calculated. The influence of the excess amount of oxidizing agent on the burning rate has been studied. It has been established that with an increase in α, the burning rate of the system increases. The influence of sample loose bulk density on the burning rate was determined. The optimal loose bulk density of fuel samples was chosen to be 1.75–1.8 g/cm³. The influence of the amount of metallic fuel on the burning rate was studied. The optimal amount of aluminum in the high-energy mixture was 15%. Experiments were carried out with metal fuel of the ASD-6 brand and aluminum powder of the PAP-1 brand. It has been established that replacing ASD-6 aluminum with PAP-1 grade aluminum increases the burning rate of the composition by 2.5–3 times. When using mixed oxidizers using PAP-1, the burning rate also increases, which is associated with a higher energy of potassium nitrate and a decrease in polymorphic transformations of ammonium nitrate when potassium nitrate is added to the fuel mixture. The effect of pressure on the combustion rate of high-energy systems in the pressure range of 0.1–3.0 MPa has been studied. It has been established that with an increase in pressure, the burning rate of the composition and the temperature of the process increase, which helps to reduce the amount of cinder. It has been established that the use of PAP-1 aluminum instead of ASD-6 increases the process temperature. The measured combustion temperature of samples with ASD-6 was 920 °C and with aluminum PAP-1 of the same composition—1970–1980 °C. The modes of preparation of feedstock for high-energy compositions have been worked out.