US3109762A - Hafnium-potassium perchlorate pyrotechnic composition - Google Patents

Description

United States Patent Ofifice 3 %762 HAFNIUM-PGTASSHJM PERCHLORATE PYRO- TECHNIC CGMPOSITION Robert E. Betts, Huntsviile, Ala, assignor to the United States of America as represented by the Secretary of the Arm No nawing. Filed Aug. 1, 1962, Ser. No. 214,147 1 Claim. (Cl. 149-42) (Granted under Title 35, U.S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

This invention relates to pyrotechnic compositions and more specifically to the use of hafnium metal as a fuel in pyrotechnic compositions.

Recent developments have produced many new propellant compositions for use as fuels in rocket engines. However, many of these fuels present ignition problems since they have relatively high ignition temperatures. Many of the pyrotechnic igniters, or igniter squibs as they are sometimes referred to, now in use do not provide a sulficiently high flame temperature or produce adequate heat of reaction to consistently ignite the propellants.

Metals have been employed as fuels for these pyrotechnic igniters as well as in the propellant compositions themselves. Among the metals which have been utilized are aluminum, titanium, and zirconium. While increasing the performance of the igniters, the metals previously employed have not proven to be the ultimate solution in the perfection of ignition systems.

The pyrotechnic compositions of the present invention which employ hafnium metal as a fuel represent a further improvement in the development of pyrotechnic igniters. This metal affords unexpectedly superior results in the Way of high flame temperature combined with the evolution of large amounts of heat per unit volume and per unit weight of the composition. This combination renders the compositions very suitable for use in pyrotechnic igniters as the initial combustible, as the booster, or as the igniting material for the rocket fuel. Because of the high flame temperature the compositions are especially useful as igniters for use with the previously mentioned propellants which have higher ignition temperatures.

It is, therefore, an object of the invention to provide a pyrotechnic composition utilizing hafnium as a fuel which burns with a high flame temperature.

It is a further object of this invention to provide pyrotechnic compositions containing hafnium metal which produce a very high flame temperature and which also furnish large quantities of heat per unit volume and per unit weight of composition.

It is a still further object of the instant invention to provide pyrotechnic compositions based on hafnium metal as a fuel which are especially suitable for employment in the ignition system of rocket motors utilizing propellants having high ignition temperatures.

The manner in which these and other objects may be accomplished will become apparent from the description presented below.

The compositions of this invention vary from 68%- 76% by weight hafnium, the remaining portion of the composition being the oxidizer. Compositions comprising 68%-70% by weight hafnium yield more heat per unit weight of composition with those compositions having about 69% by weight hafnium producing the maximum quantity of heat per unit weight. If hafnium comprises 72%-76% by weight of the composition with the remainder of the composition composed of an oxidizer, the compositions produce more heat per unit volume of composition. The compositions that comprise 73 by weight give maximum heatevolution per unit volume with those compositions comprising about 74% by weight hafnium constituting the optimum for achieving high heat evolution per unit volume of composition.

The conventional inorganic oxidizers are satisfactory in the composition of the invention. Among these oxidizers are ammonium nitrate, sodium nitrate, potassium nitrate, barium nitrate, sodium perchlorate, ammonium perchlorate, and potassium perchlorate. The latter oxidizer provides the maximum performance with the hafnium fuel and is, thus, preferred.

t is sometimes desirable to employ a binder in the compositions of the invention. Binders can comprise from 0.05%10% by weight of the total composition. Depending on the particular binder, flame temperature and other properties will vary somewhat from the compositions not incorporating binders. However, these variations resulting from the binders are to be expected, being similar to the variations experienced with other igniters containing a binder and employing zirconium, aluminum, titanium, etc. Ordinarily, the ratio of the weight of hafnium to the weight of oxidizing agent is within the percent by weight proportions set forth above, the binder being in excess to these components. A particular binder may serve as an oxidizer, however, and in such an instance it would be beneficial to increase the hafnium fuel content accordingly.

Very simple techniques are involved in the preparation of the compositions. The requisite amount of powdered hafnium is mixed with the powdered oxidizing agent until a uniform distribution of the particles of the two ingredients is achieved. As a safety precaution, if the size of the hafnium particles is ten microns ,or less, the hafnium should be wet at all times prior to and during mixing. Water, ethyl acetate, ethyl alcohol, for example, are suitable for this purpose. A slurry of the metal in the liquid is mixed with the oxidizing agent. After thorough mixing, the mixture is placed in an ovenat a temperature that will vaporize the particular liquid used in wetting the hafnium. Temperatuers of 150 F. will be adequate for drying the compositions employing the above mentioned liquids in the hafnium slurry. The

ingredients may be mixed in the dry state when the particle size of hafnium exceeds ten microns.

If it is desired to prepare compositions containing hinders, the following technique can be employed. The

required amounts of hafnium and oxidizer are mixed as' described above allowing the particle size of hafnium to determine whether the components will be mixed in the dry or wet state. To the wet or dry mixture of hafnium metal and oxidizer the binder, dissolved in a suitable solvent, is added while stirring. Air is then passed through the mixture with continued stirring until the mixture separates into small, damp, rounded particles. These particles are placed in an oven and dried. The dried particles can be ground into a powder and used as such or the powder can be pressed into pellets of any desired shape and size according to known methods.

With the above method, a composition was prepared comprising 68.6% by weight hafnium, 26.7% by weight potassium perchlorate, and 4.7% by weight nylon binder. Dry hafnium powder and potassium perchlorate were mixed in a glass container. The nylon binder dissolved in methanol was added to the mixture. Thereafter, the ingredients were thoroughly mixed. By means of a tube, compressed air was passed through the mixture while maintaining the stirring. As the methanol evaporated from the mixture, damp globules of the mixture formed. The globules were placed in an oven at F. and dried. This dry mixture was ground into a powder and pelleted.

Patented Nov. 5, 1963 snoavea Other conventional binders can be introduced simply by dissolving them in suitable solvents and adding the resulting solution to the mixture of hafnium metal and oxidizer. In this manner tetrafluoroethylene resins, shellac, and various rubbers can be incorporated into the composition as binders.

The preparation of the compositions discussed above fall within the general techniques of the art for preparing similar compositions. It is readily apparent, therefore, that all that is required to produce any given composition within the specified percent by weight composition previously mentioned is to alter the weight of hafnium, oxidizer, and binder accordingly.

To determine the optimum ratio of hafnium fuel to oxidizing agent, compositions containing from 60%80% by weight hafnium were prepared and tested to evaluate the quantity of heat produced in terms of calories per gram of composition and kilocalories per cubic centimeter of composition.

The heat of reaction was measured by placing a measured amount of the composition in a Parr oxygen bomb. The bomb was then filled with helium to a pressure of twenty atmospheres. An electrical discharge through a wire tipped with a zirconium pyrotechnic composition ignited the composition. By means of an adiabatic calorimeter, the heat of reaction was ascertained for each composition. Two or more determinations were made for each composition tested and the results averaged.

By plotting this data, it was possible to determine the preferred ratios of hafnium to oxidizer. In this manner it was found that compositions of 68%-70% by weight hafnium produce maximum heat per unit weight of the composition while compositions of 72%-76% by weight produce maximum heat per unit volume of the composition.

The influence of hafnium particle size was evaluated by determining the heat of reaction for compositions wherein the ratio of the percent by weight hafnium to the percent by weight of the oxidizer was constant. The same oxidizer was employed for each evaluation. Hafnium of various particle sizes was obtained from commercial sources. An example of the results of these determinations is tabulated below. Sample A consisted of particles of hafnium ranging in size from 200 to +325 mesh. Sample B consisted of 2 micron size particles of hafnium.

From results such as these, it is seen that, generally, a decrease in the particle size of hafnium increases the heat of reaction. This is probably due in part, at least,

air-

to a faster rate of burning and more efiicient oxidation of the fuel.

Flame temperature characteristics of the hafnium compositions were ascertained through standard techniques based on the heat of reaction and assumed combustion products. In this manner, the hafnium compositions of this invention were found to provide flame temperatures of 4835 K. while analogous zirconium compositions yielded flame temperatures of only 4160 K. Thus, the hafnium composition flame temperature is over 1200 F. higher than corresponding zirconium composition.

A difference in flame temperature of this size was totally unexpected and completely unpredictable in view of the knowledge of similar prior-art compositions, especially the compositions of zirconium. The higher flame temperature of the compositions of the invention represent a substantial achievement in the search for materials which will function reliably in propellant ignition systems, especially those wherein ignition of the propellant has been a problem due to a high ignition temperature.

The compositions of the invention can be employed in rocket propellant ignition systems merely by substituting them for the prior-art pyrotechnic compositions now being used. If the composition is to be used as the initial combustible material on the squib, it is converted into a slurry and applied to the squib bridge by dipping, spraying or painting. When employed as the squib booster or the actual propellant igniting material, the compositions are used as powders or pellets.

It should be apparent to those skilled in the art that the compositions will find other applications in addition to their use in propellant ignition systems. For example, the high flame temperature renders the compositions particularly suitable for use in thermal batteries and other areas where it is desirable to achieve high temperatures.

It is apparent from the detained description given hereinabove that many different embodiments of the invention may be made without departing from the spirit and scope thereof. Therefore, the invention is limited only by the appended claim.

I claim:

A pyrotechnic composition comprising about 69% by weight powdered hafnium metal, the remainder of the composition being potassium perchlorate.

References Cited in the file of this patent UNITED STATES PATENTS 1,974,015 Decker et al Mar. 22, 1932 2,008,366 Pickett July 16, 1935 2,309,978 Pratt Feb. 2, 1943 2,798,368 Anderson July 9, 1957 3,068,129 Schaffel Dec. 11, 1962 OTHER REFERENCES Rare Metals Handbook, Second Edition, 1961, Reinhold, by Hampel, pp. 198, 212.