WO2009102259A1

WO2009102259A1 - Method of increasing the burn rate, ignitability and chemical stability of an energetic fuel, and an energetic fuel

Info

Publication number: WO2009102259A1
Application number: PCT/SE2009/000085
Authority: WO
Inventors: Arno Hahma
Original assignee: Totalförsvarets Forskningsinstitut
Priority date: 2008-02-14
Filing date: 2009-02-13
Publication date: 2009-08-20
Also published as: EP2247557A1; EP2247557A4; SE0800328L; SE532026C2

Abstract

An energetic fuel comprising particles with a base of metal or semimetal selected among Al, Mg, B, Ti, Zr, Hf, Si, Be, Ca and alloys of two or more of the same and a coating applied to the base particles and containing an oxide, hydroxide, oxide hydroxide or carbonate of another, more noble metal than the base and which coating reacts exothermally with the base particle in ignition of the energetic fuel. The invention also relates to a method of improving the burn rate, ignitability and chemical stability of an energetic fuel based on particles of metal or semimetal by applying said coating to the particles.

Description

Method of increasing the burn rate, ignitability and chemical

fuel, and an energetic fuel ,

The invention relates to a method of increasing the burn rate, ignitability and chemical stability of an energetic fuel which contains particles selected among Al, Mg, B, Ti, Zr, Hf, Be, Si, Ca and alloys of two or more of the same. The invention also relates to a modified energetic fuel for use in propellant and explosive compositions.

Solid energetic fuels in finely divided form, e.g. powder, fibres or flakes, are used on a large scale in propellants and explosives to provide increased energy. A drawback of energetic fuels is that as a rule they do not burn completely within the time scale in which it is desirable to utilise their energy. One way of improving the burn properties is to grind the metals or semimetals concerned to a very fine powder. Grinding of metals/semimetals, however, is expensive, and it is difficult in fine grinding to check the quality of the powder as to surface structure, particle size, specific surface etc. In addition, the explosive or propellant will be more sensitive to impact and friction the finer the powder. Additional improvements may be achieved with nanometre-sized powder, although this requires special manufacturing processes which make the energetic fuel significantly more expensive. It is therefore desirable to be able to control the burn properties of the metal/semimetal in some other way than by particle size and shape.

Aluminium fuel requires a very high ignition temperature. This is due to the natural aluminium oxide layer on the surface of the metal, which prevents the oxidising agent used from entering into contact with the fuel. The advantage of an oxide layer is that it allows aluminium powder to mix with oxidisers and explosives in propellant and explosive compositions without any great risks, in spite of the fact that the element in itself is highly reactive. For the particle to burn, the surface of the aluminium particle usually must be heated until the oxide layer evaporates, which requires a temperature above 2000°C. Such a high temperature must also be maintained during the entire combustion process since otherwise the aluminium particle is extinguished by a new oxide layer forming.

WO 2004/048295 and WO 2005/121055 disclose different ways of improving the burn rate and ignitability of energetic fuels by providing the individual fuel particles with a surface coating. In WO 2004/048295 use is made of a fluoride complex which, upon ignition, dissolves the oxide layer occurring naturally on aluminium fuel particles. The fluoride complex is applied to the surface of the aluminium fuel particles by treating the fuel with a solution of hydrofluoric acid and a fluoride salt and/or complex fluoride salt of an alkaline metal or an alkaline earth metal. The fluoride and/or complex fluoride salt reacts with the normal oxide layer of the particle and causes the formation of a surface layer of a fluoride complex on the fuel particle. In 2005/121055 use is made of an alloying material which, upon ignition, causes an exothermal alloying reaction with the base metal.

An object of the present invention is to provide an alternative method of improving the ignitability and burn rate by means of components reacting exothermally in intimate contact with each other on each individual fuel particle.

Another object is to provide more chemically stable fuel particles, which may be handled and used in the same way as the currently used energetic fuels and replace these in prior-art propellant and explosive compositions in order to improve the performance of the compositions.

This is achieved by a method and an energetic fuel as defined in the claims.

Base metal is defined as the metal or semimetal onto which the surface coating described below is to adhere.

According to the invention, particles of a base metal are provided with a coating selected among oxide, hydroxide, oxide hydroxide or carbonate of a more noble metal than the base metal. The coating reacts exothermally with the base metal in the ignition of the fuel particles. The base metal is selected among Al, Mg, B, Ti, Zr, Hf, Be, Si, Ca and alloys of two or more of the same or mixed with metals outside this group. The coating is dimensioned so as to react with a surface portion of the base metal bringing about rapid heat generation to accelerate the ignition of the base metal. The continued combustion of the base metal is achieved by means of another oxidiser, which preferably may be part of the composition in which the energetic fuel is used. Thus, the aim is not to completely oxidise the base metal with the coating, but merely to initiate the combustion. The coating preferably constitutes not more than 10% of the weight of the base particles and, most preferred, constitute from 0.5% to 5% of the weight of the base particles. The coating is selected among oxides, hydroxides, oxide hydroxides or carbonates of nickel, iron, manganese, chromium, cobalt, copper, zinc, molybdenum, niobium, tungsten, lead, tin, antimony, bismuth and vanadium so that the coating metal is more noble than the base metal already selected. The exothermal reaction between the base metal and the coating starts when the fuel particle (i.e. base metal powder with coating) is heated to a relatively high temperature, which occurs when the powder is ignited in a propellant or explosive compo- sition. However, at normal temperatures the coating serves as extra protection against oxidation of the base metal. As a result, coated fuel particles according to the invention can be made chemically more stable than untreated particles of the base metal. Since the entire surface of the base metal is coated by the coating material, the coating will determine the chemical appearance of the fuel particles at normal temperatures. Many of the conceivable coating materials are highly corrosion-resistant and inert materials, which make it possible to use the coated particles in compositions where the base metal would normally not be fit.

The coating is applied to particles of the base metal by a wet-chemical method.

The metal particles are suspended in a sufficient amount of distilled water to obtain a low- viscosity suspension, which can be vigorously agitated without difficulty. A concentrated solution of a soluble salt of the metal selected for coating of the base metal is added to the suspension. In addition, a powdered salt of the metal selected for coating may be added, but this requires checking that the added powdered salt is dissolved in the amount of water used. Particularly suitable salts are nitrates, perchlorates and fluoroborates of the selected coating metal. These are highly soluble and do not cause corrosion of the base metal. Chlorides and bromides are also highly soluble salts, but are less appropriate since they cause corrosion of the base metal.

After the metal salt has been added and dissolved in the water, a precipitation chemical is added which may be a soluble carbonate or a hydroxide. Particularly suitable are potassium carbonate or sodium carbonate as well as potassium hydroxide or sodium hydroxide. They precipitate the coating metal as a carbonate or a hydroxide, whereby the suspension becomes highly viscous and gelatinous. Agitation is continued to enable conversion of the metal carbonate or metal hydroxide on the surface of the base metal for forming a metal oxide or metal oxide hydroxide on the surface of the base metal. An increased temperature is often required for the conversion of the metal carbonate or metal hydroxide into metal oxide or metal oxide hydroxide to take place. If the conversion into metal oxide or metal oxide hydroxide does not occur spontaneously in the water suspension despite an increased temperature, the suspension can be filtered under pressure, thereby triggering the conversion. The temperature for forming metal hydroxide or metal carbonate on the surface of the base metal may be from 18°C to HO⁰C depending on which metal hydroxide or metal carbonate that is to be precipitated on the surface of the base metal. The best way for most metals is to start at room temperature, from 18°C to 22⁰C, and then heat the solution, once the precipitation chemical has been added under agitation, until the suspension reaches the boiling point, whereupon the suspension is rapidly cooled by adding ice to the suspension to bring the temperature down to room temperature. The increased temperature serves to break down the gelatinous structure of the suspension, which is formed in the precipitation step, and enables the final filtering. It is also possible to work close to the boiling point right from the start, but often not all of the hydroxide or carbonate will adhere to the surface of the base metal and the outcome is instead a finely divided, loose precipitate in the suspension.

The suspension is filtrated and the precipitate washed with clean, cold water. The coated metal powder is dried at room temperature to avoid corrosion and when the powder is dry it is placed in an oven at a temperature from 250⁰C to 300⁰C for at least four hours. This step ensures that the remaining hydroxides or carbonates decompose into metal oxide, that the metal oxide adheres to the surface of the base metal particles and that the remaining water rests adsorbed by the material are evaporated.

Also other methods of coating the base metal with a surface layer consisting of metal oxide, metal carbonate, metal hydroxide or metal oxide hydroxide are conceivable. Metal complexes with a high oxidation state, in particular potassium permanganate and potassium ferrate, can be reduced to a lower oxidation state by adding a reducing agent to the suspension of base metal powder. The reducing agent may be aldehydes such as formaldehyde, sugar (saccharose), glucose, sulphites, dithionites. The amount of reducing agent is calculated to be just enough to reduce the metal complex to metal oxide and form a coating on the surface of the base particle. Once the reaction is completed, which can often be judged by the colour of the suspension, the suspension is heated until it boils. It is then rapidly cooled by adding ice to the suspension. The precipitate is filtered off, dried first at room temperature and then placed in an oven at a temperature from 250⁰C to 300⁰C for at least four hours.

The invention will in the following be illustrated by some typical examples of methods and by the appended drawings.

Fig. 1 shows TG analysis curves for weight increase as a function of temperature for a. untreated aluminium powder; b. aluminium powder with an iron oxide coating; c. aluminium powder with a nickel oxide coating; and d. aluminium powder with a manganese oxide coating.

Fig. 2 shows DSC analysis curves for heating capacity as a function of temperature for a. aluminium powder with an iron oxide coating; b. aluminium powder with a nickel oxide coating; and c. aluminium powder with a manganese oxide coating.

The rate at which the coated fuel particles were reacted with air was measured by thermogravimetric (TG) analysis and compared with untreated fuel particles of the same particle size and particle shape, see Fig. 1. At the same time the heating capacity of the coated fuel particles was measured by calorimetric (DSC) analysis, see Fig. 2. A sample quantity of (1.8±0.05) mg was placed in an aluminium oxide pot and measured in a thermobalance in a dry air gas flow, 70 ml/min, at a heating rate of 207min in the range of 100-1200°C. The weight increase and the heating capacity, respectively, owing to oxidation were registered as a function of the temperature to 1200⁰C and this temperature was then kept constant for another 15 minutes in order to complete the oxidation of the sample to a stationary level. These data are used as a base for the following assessments of how much quicker the coated powder burns as compared with untreated aluminium powder.

Example 1 Coating of Base Metal Particles of AL Mg. B. Ti. Zr, Hf, Be, Si, Ca with Iron Oxide Hydroxide, Iron Oxide, Iron Hydroxide or Iron Carbonate

The base metal powder is suspended in clean water, which has been made slightly basic. Preferably, the pH is above 8 but below 10. The temperature may be from 0°C to 100°C. The preferred temperature is from 10⁰C to 60⁰C and the most preferred temperature is from 20⁰C to 40⁰C. Iron(III) sulphate in powder form or iron(II) sulphate in powder form is added to the water. The amount of iron sulphate may be from 0.01 mole % to 10 mole % of the amount of base metal. Preferably, from 0.1 mole % to 5 mole % of iron sulphate is used, most preferred from 0.5 mole % to 2 mole % of iron sulphate. A solution of sodium hydroxide, NaOH, or sodium carbonate, Na₂CO₃, is then added under agitation. The amount of hydroxide or, alternatively, carbonate is selected so as to be sufficient for precipitating all the added iron ions as oxide hydroxide, FeO(OH), or carbonate, Fe₂(COs)₃, if iron(III) sulphate is used, and as hydroxide, Fe(OH)₂, or carbonate, FeCO₃, if iron(II) sulphate is used.

The alkaline hydroxide solution or carbonate solution is added at a low rate under smooth agitation so that a coating of FeO(OH), Fe(OH)₂, Fe₂(CO₃)₃ or FeCO₃ is applied to the surface of the base metal particles. The powder is then filtered off and dried at room temperature, whereupon the coated fuel particles are dried in an oven at a temperature from 200°C to 300°C for at least four hours in order to convert the coating into FeO and Fe₂O₃, respectively.

Aluminium powder treated in this way burns 5-10 times quicker than untreated powder of the same particle size and particle shape.

Example 2 Coating of Base Metal Particles of Al. Mg, B. Ti, Zr. Hf. Be. Si. Ca with Nickel Oxide. Nickel Hydroxide or Nickel Carbonate

The coating method is the same as in Example 1 but instead nickel(II) sulphate is added to the suspension. A coating OfNi(OH)₂ and NiCO₃, respectively, is applied to the surface of the base metal particles. After filtering and drying at room temperature and in an oven, a coating of NiO is obtained on the base metal particles.

Aluminium powder treated in this way burns 10-20 times quicker than untreated powder of the same particle size and particle shape.

Example 3

Coating of Aluminium Powder with Manganese Dioxide

Aluminium powder (Carlfors Bruk AlOO, 50-100 μm) is suspended in clean water, which has been made slightly basic. Preferably, the pH is above 8 but below 10. The temperature may be from 0°C to 100°C. A preferred suitable temperature is from 1O⁰C to 60°C and the most preferred temperature is from 2O⁰C to 4O⁰C. A concentrated solution of potassium permanganate, KMnO₄, is added to the water. The amount of potassium permanganate may be from 0.01 mole % to 10 mole % of the amount of base metal. Preferably, from 0.1 mole % to 5 mole % of KMnO₄ is used and most preferred from 0.5 mole % to 2 mole % OfKMnO₄. A solution of a reducing agent is then added under agitation, which reducing agent may be any arbitrarily selected compound, which is oxidised by permanganate. Preferably, ordinary sugar (saccharose), aldehydes such as formaldehyde and sulphites are used. The amount of reducing agent is selected to be just enough to reduce the permanganate ions to manganese oxide, MnO₂. The manganese oxide is precipitated in colloidal form and forms a coating on the surface of the base metal particles.

When the solution no longer has the violet colour of permanganate, agitation is stopped and the powder is filtered off, washed in clean water and dried at room temperature. The powder is then dried in an oven at a temperature from 250⁰C to 300°C for at least four hours, so that any adsorbed water evaporates and any hydroxides formed in the process are decomposed.

Aluminium powder treated in this manner burns 50-100 times quicker than untreated powder of the same particle size and particle shape. Aluminium powder coated with manganese oxide exhibits an increased heating capacity, i.e. an activation of the powder, at the melting point of pure aluminium (660°C) according to Fig. 2. To obtain an increased heating capacity with a powder of pure aluminium at the same temperature the particles must be of nanometre size (Jones et al., Thermal Characterisation of Passivated Nanometer Size Aluminium Powders. J. Therm. Anal. CaL, 61 (200) 805-818).

Claims

Claims:

1. An energetic fuel suitable for use in propellant and explosive compositions, comprising base particles of metal or semimetal selected among Al, Mg, B, Ti, Zr, Hf, Si, Be, Ca and alloys of two or more of the same, c h ar a c t e r i s e d in that the base particles have a coating of an oxide, a hydroxide, an oxide hydroxide or a carbonate of another, more noble metal than the base particles, which coating reacts exothermally with the base particles in the ignition of the energetic fuel, and that the coating affords improved chemical stability to the energetic fuel .

2. An energetic fuel as claimed in claim 1, c h ar a c t e r i s e d in that the base particles consist of Al.

3. An energetic fuel as claimed in claim 1, c h ar ac t e r i s e d in that the base particles consist of an alloy of Al and one or more of Mg, B, Ti, Zr, Hf, Si, Be and Ca.

4. An energetic fuel as claimed in claim 1, c h ar a c t e r i s e d in that the base particles consist of Mg.

5. An energetic fuel as claimed in claim 1, c h ar a c t e r i s e d in that the base particles consist of B.

6. An energetic fuel as claimed in claim 1, c h ar a c t e r i s e d in that the base particles consist of Ti, Zr or Hf.

7. An energetic fuel as claimed in claim 1, c h ar a c t e r i s e d in that the coating is selected from the group consisting of oxides, hydroxides, oxide hydroxides or carbonates of nickel, iron, manganese, chromium, cobalt, copper, zinc, molybdenum, niobium, tungsten, lead, tin, antimony, bismuth and vanadium.

8. An energetic fuel as claimed in claim 1, c h arac te r i s e d in that the coating constitutes not more than 10% of the weight of the base particles.

9. An energetic fuel as claimed in claim 1, c h ar a c t e r i s e d in that the coating constitutes from 0.5% to 5% of the weight of the base particles.

10. A method of improving the burn rate, ignitability and chemical stability of an energetic fuel based on base particles of metal or semimetal selected among Al, Mg, B, Ti, Zr, Hf, Si, Be, Ca and alloys of two or more of the same, c h ar ac t e r i s e d in that a coating containing oxide, hydroxide, oxide hydroxide or carbonate of another, more noble metal than the base particles is applied to the base particles, which coating is capable of reacting exothermally with the base particles in ignition of the fuel, and that the coating affords improved chemical stability to the energetic fuel.

11. A method as claimed in claim 10, c h ar ac t e r i s e d in that the base particles consist of Al.

12. A method as claimed in claim 10, c h ar a c t e r i s e d in that the base particles consist of an alloy of Al and one or more of Mg, B, Ti, Zr, Hf, Si, Be and Ca.

13. A method as claimed in claim 10, c h ar ac t e r i s e d in that the base particles consist of Mg.

14. A method as claimed in claim 10, c h ar a c t e r i s e d in that the base particles consist of B.

15. A method as claimed in claim 10, c h ar a c t e r i s e d in that the base particles consist of Ti, Zr or Hf.

16. A method as claimed in claim 10, c h ar ac t e r i s e d in that the coating is selected from the group consisting of oxides, hydroxides, oxide hydroxides or carbonates of nickel, iron, manganese, chromium, cobalt, copper, zinc, molybdenum, niobium, tungsten, lead, tin, antimony, bismuth and vanadium.

17. A method as claimed in claim 10, c har ac teri s e d in that the coating constitutes not more than 10% of the weight of the base particles.

18. A method as claimed in claim 10, c h ar a c t e r i s e d in that the coating constitutes from 0.5% to 5% of the weight of the base particles.