US3118798A - Composition and method of forming - Google Patents

Description

United States Patent f 3,118,798 COMPOSITION AND METHGD F FORMING Gunnar A. F. Winckier, Orange, Conn, assignor to Olin Mathieson Chemical Corporation, a corporation of Virginia No Drawing. Filed Oct. 26, 1961, Ser. No. 147,718 5 Claims. (Cl. 14930) The present invention relates to a method of decreasing the thermal sensitivity of initiator compositions and to compositions formed by this method.

Initiators are a class of compositions capable of selfsustained rapid combustion. A self-sustained combustion is one which requires no separate supply or source of combustible material or of combustion supporting element.

Initiators are generally of two types depending on the rate of combustion of their ingredients. Initiators capable of self-sustained combustion to produce a flame are termed igniters. Where the rate of combustion of an initiator attains a very high level, it is known as a detonator. The initiators which are capable of undergoing combustion as a result of the application of mechanical force are called percussion initiators.

Substances capable of self-sustained combustion are generally subject to destruction as a result of heating above a given ambient temperature level. Substances known heretofore to be useful as initiators have been subject to destruction as a result of reaction or combustion of their ingredients in the range of 300 F.

The property of initiator composition being subject to reaction when the whole body of the initiator is raised to a given temperature is referred to herein as ambient thermal sensitivit and such heating is referred to as ambient heating. This must be distinguished from the property exhibited by an initiator composition of being capable of undergoing self-sustaining combustion when a localized portion of the composition is raised to relatively high temperature. As is Well-known, the reaction of initiator compositions can be started by supplying an activating heat energy at any region of a body of the composition and this activation will bring about the combustion of the entire body.

As used herein, the term ambient ignition temperature is that to which the entire body of initiator composition must be brought to cause ignition. The local ignition temperature is that to which any localized portion of the composition must be brought in order to initiate combination of the fuel and oxidizing portions of the ingredients.

One of the problems which has limited the use to which initiator compositions could be put in the past is the tendency of these materials to detonate or burn when the ambient temperature is raised above a given value. This value depends on the composition of the material itself. As indicated above, it has not heretofore been possible to \maintain these compositions at ambient temperatures in excess of about 300 F.

"Accordingly, one of the objects of the subject invention is to provide initiator compositions capable of being maintained at ambient temperatures in excess of 300 F.

Another object is to provide a method of forming these compositions.

A further object is to provide a unit capable of being activated to generate a flame at temperatures above 300 F.

A further object is to provide an initiator composition capable of being ignited or detonated at relatively high temperatures.

Other objects will be in part apparent and in part pointed out hereinafter.

3,118,798 Patented Jan. 21, 1964 In one of its broader aspects the objects of the invention are achieved by providing a composition containing a first ingredient capable of undergoing rapid oxidation, and a second ingredient capable of sustaining this oxidation, heat treating the composition at a temperature below its ambient ignition temperature for a time suiticient to raise the ambient ignition temperature.

In another of its aspects the objects of the invention are achieved by providing a composition having a first ingredient capable of undergoing rapid oxidation, a second ingredient capable of sustaining this oxidation and being subject to ignition on attaining an initial ambient ignition temperature, heating the composition at a rate below'that at which ignition of the composition commences and continuing said heating to a temperature above the initial ambient ignition temperature of the com position.

The method of the invention may be practiced with particularly effective results by the heat treatment of igniter compositions containing finely divided solid metal particles in intimate admixture with finely divided solid oxidizer compositions in proportions to support the rapid oxidation or" said metal.

The finely divided metals useful in practice of this invention are all those metals which are solid at room temperature and which form oxides which have free energies of formation, per gram atom of oxygen, more negative than minus 40 kilocalories. Metals such as aluminum, magnesium, zirconium, calcium, iron, nickel, titanium, and alloys and mixtures of these and other metals which alloys and mixtures in a fine state of subdivision form oxides which have free energies of formation, per gram atom of oxygen, more negative than minus 40 kilocalories, or any other metals which are solid at room temperature, which are in a fine state of subdivision to permit rapid oxidation thereof, and which have the needed high free energy of formation, are included within the scope of metals useful in the practice of this invention.

The finely divided oxidizer must be included in intimate admixture with the finely divided metal in proportions which support the rapid combustion of the igniter composition to produce flame therefrom. For particular combinations of finely divided metals and oxidizers the limits of the proportions which will give desirable flame production can be readily worked out by a few scoping experiments.

The oxidizer component of the intimate admixture is any of the conventional oxidizer compositions used in initiator compositions. Numerous such conventional oxidizer compositions are disclosed in the patent literature and are illustrated by such oxidizer compositions as BaO cupric oxide, cuprous oxide, lead dioxide, calcium peroxide, barium chromate, lead chromate, manganese dioxide, potassium chlorate, and combinations of these and other conventional oxidizers.

Inert or active ingredients may be included in the compositions in minor proportions without deleteriously affecting the results obtained, as is more fully illustrated in the examples which follow.

The following examples are given to illustrate the method, composition and article of the subject invention although it will be understood that the concept of the invention is not limited thereto.

EXAMPLE I A composition was formed by mixing together a finely divided wet packed zirconium powder and a number of other ingredients as follows:

12. grams inert inorganic diluent 53 grams barium chromate 17 grams zirconium powder (by dry weight) The foregoing ingredients were introduced into one quart of water in an agitator type of mixer. The fluid composition was agitated for approximately 30 seconds after which the ingredients were poured into a Biichner funnel and filtered. The precipitate on the supporting filter paper was removed to a vacuum oven and heated at a temperature of 90 C. for a sufficient time to dry the cake. When dry the paper was removed from the dried filter cake. The cake was then ground to a powder in mortar and passed through a 40 mesh screen to break up any large pieces.

A sample of approximately one gram of the above powder was placed on a metal plate, having a temperature measuring thermocouple attached thereto, and introduced into a mufiie furnace which had been preheated at a temperature of 500 F. The temperature of the powdered material rose rapidly and the powder ignited.

EXAMPLE II One gram of the powder prepared as described in Example I was introduced into an initiator case. This case was essentially in the form of a short section of a metal tube having a diameter of about 7 inch and a length of about inch. One end of the case was sealed closed with an insulating ceramic or glass plug, through which extended an electrically conductive rod. An electrical resistance filament was contained in the tube and was electrically connected between the insulated rod and the metal tube wall. The casing and rod were both made of brass.

The other end of the tube, through which the powder was introduced into the tube interior, remains open.

The powder was then compressed and packed into contact with the closed end of the tube and the electrical filament by applying a pressure of about 2 tons per square inch to the powder through the open end of the case.

The resulting article is an initiator unit capable of being ignited by localized activation of the contained initiator composition. In this case the localized activation is caused by electrical heating of the resistance filament.

The resistance filament was then electrically heated to an instantaneous white heat by connecting a dry cell voltage source to the rod and easing. Rapid combustion of the initiator composition in the casing occurred. This caused a combination of sparks and flame to be emitted from the open end of the case and a distinctly audible report.

Aninitiator unit prepared as described above but not heat treated at all, ignited when introduced directly into a furnace preheated to a temperature of 500 F.

EXAMPLE III A one gram sample of the powder prepared as described in Example I was introduced into an initiator case as described in Example II. The initiator unit was introduced into a mufiie furnace at room temperature. The temperature of the furnace was then raised at a rate of approximately 500 F. per hour to a final temperature of 800 F. The heat treated initiator unit was then re moved from the furnace and allowed to cool.

The filament was then heated electrically to bring it to an immediate white heat. This caused a rapid combus tion of the powder in the initiator case and indicated that the material had not been inactivated by the heat treatment accorded to it.

EXAMPLE IV A sample of the powder prepared as described in Example I, weighing approximately one gram, was loaded into an initiator case as described in Example II.

The initiator case and contents were introduced in the oven at room temperature. The oven was then heated to 500 F. at a heating rate of 500 F. per hour and the unit was then heated at 500 F. for about one hour. It was removed from the oven while hot.

EXAMPLE V The procedure described in Example IV was repeated except that the oven temperature was 600 F. The initiator unit fired normally as described in Example II.

EXAMPLE VI A number of samples of initiator composition prepared as described in Example I were pressed into a wafer or strip form. These strips are formed to permit test of burning rate using conventional apparatus which indicates the rate at which burning progresses from one end to the other. These strips can be approximately two inches in length, about one half inch wide and of uniform thickness.

These strips were introduced into a mufiie furnace at room temperature and heated to 500 F. within about 15 minutes.

After the furnace reached 500 F. one strip was removed at time intervals as shown below. The burning rate of these test samples was compared to control samples and the following results were obtained.

The control samples used in determining burning rates were substantially identical to the test samples except that they had received no heat treatmens.

EXAMPLE V I I A number of initiator units were prepared as described in Example II.

They were introduced into a mufile furnace preheated at a temperature of 300 F. The temperature was raised to 500 F. at a relatively slow rate and maintained for 5 minutes at 500 F. after which the intiators were removed from the furnace and cooled. One sample thus prepared was introduced directly into an oven preheated to 600 F. and it was observed it did not ignite although it attained the 600 F. temperature rapidly.

After the oven had been raised to 700 F. a second initiator unit was introduced into the oven in the same manner and it did not ignite.

The oven was then raised to 800 F. and a third initiator sample was introduced into the furnace without causing ignition.

Upon later ignition by electrical heating of the filament there was no noticeable slowing down of the burning speeds of the initiators heated rapidly to the 600 and 700 F. temperatures. The sample introduced into the oven preheated to 800 F. was not later ignited.

This example demonstrates that a change in the initiator composition has been effected by the lower temperature heat treatment which protected the initiator against ignition resulting from rapid ambient temperature rise.

EXAMPLE VIII An initiator composition was formed in substantially the same manner as that described in Example I but containing the foliowing ingredients in place of those given in Example i.

Ingredient: Weight in grams Inert inorganic diluent 4 lBarium peroxide 24 Finely divided magnesium powder 2.5

After screening the powder, a number of small samples were introduced into a like number of initiator units prepared as described in Example II. The magnesium-containing powder was compressed into place within each casing in contact with the composition prepared as described in Example I.

One of the units thus formed was introduced into a furnace preheated to a temperature of about 500 F. It was observed to ignite and to produce a larger flame than those produced by the units ignited in the same manner but containing none of the above composition.

The tests performed in Example VII on initiator units prepared as described in Example H were repeated using initiator units prepared as described above. The results obtained at the various temperatures used were substantially the same as those obtained in Example VII with the exception that in each case where the initiator was fired a larger flame was produced.

EXAMPLE IX A sample of approximately 0.1 gram of composition having the ingredients listed below, was wet-loaded into a conventional metal prime-r cup.

Ingredient: Percentage by weight Red phosphorous 30 Potassium chlorate 4O Antimony sulphide, Sb S 30 A conventional metal anvil was mounted in the primer cup to provide a surface against which the composition could be rapidly compressed or deformed on striking or deforming the primer cup by a sharp impact on the cup.

When the cup was subjected to the impact of a conventional firing pin, such as is used to fire the primer of small arms ammunition, the composition fired and produced a distinctly audible report.

EXAMPLE X A sample of the composition described in Example IX was contained in a vessel open to the air, such as an evapcrating dish, and placed in a mufiie furnace preheated to a temperature of 250 F. It was heated at this temperature for 12 hours and then heated at 500 F. for 2 hours.

The composition was then cooled, wet-loaded into a standard primer cup and anvil as described in Example IX and fired by subjecting the primer to the impact of a firing pin. The composition fired in substantially the same manner as that in Example IX.

EXAMPLE XI A second primer cup was prepared containing a composition prepared and heat treated as described in Example X. This cup and contained composition were heated to a temperature of 600 F. without causing ignition. The primer was then subjected to the impact of a firing pin and was observed to fire in a manner similar to .a primer containing untreated material.

From the foregoing, it is evident that the present invention provides efiective initiator compositions which are capable of being fired or initiated at elevated temperatures above 300 F. It will be appreciated that a number of changes and modifications can be made in the individual ingredients of the composition and in the individual steps used in forming the composition without departing from the scope of the subject invention.

With regard first to the combustible ingredient of the initiator composition, a finely divided inorganic material capable of undergoing pyrophoric reactions can be used in forming compositions in accordance with this invention. Metals such as magnesium, zirconium, and other combustible metals and compounds of metals such as for example, sulfides, phosphides and the like which are known to be cap-able of undergoing very rapid oxidation and which may be mixed by wet or dry mixing procedures with substances capable of supporting rapid oxidation, as well as other additives, to form compositions which are stable in air, are useful in practicing the present invention. Inorganic combustible ingredients which are stable at room temperature and which are capable of fueling trap-id combustion at elevated temperatures. can be included in the composition of the present invention. Preferably the fuel component of the initiator composition must be inorganic in nature and capable of being converted r apid- 1y to a higher state of oxidation.

The initiator ingredients which supports the rapid combustion must be capable of maintaining its chemical potential for supporting the combustion at elevated temperatures for an extended period of time. These materials must be inorganic in nature and must contain oxygen in combined form. The compound should be substantially nonvolatile at elevated temperatures and particularly above the initial ambient ignition temperature of the initiator composition. It is preferred that the combustion supporting ingredient contain a metal in a combined form and that this metal be in a relatively high oxidation state.

Alternatively, the metal oxide may be in a lower state of oxidation where the free energy of formation of the oxidizer metal oxide is very substantially lower than that of the oxide of the metal to be oxidized.

Other materials may be included in the composition without detracting from the essential initiator properties thereof and these may be used for various purposes such as controlling speed of reaction, controlling flame propagation, controlling heat content, and physical characteristics such as strength. Examples of materials which are used for these and other purposes are given in the above illustration. Thus, numerous inorganic, fibrous or particulate substance may be included. These materials should be selected to add the needed physical or reaction characteristics to the initiator material depending on the use which is to be made of the initiator.

The compositions and treatments described above may be used in connection with formation of initiator units and similar articles in many different forms. For example, a primer cup prepared as described in Example IX may be inserted into an initiator casing as described in Example II in place of the ceramic or glass plug. The casing may then be loaded with an initiator composition such as that described in Examples I or VII-I to obtain desired fi-ame characteristics. The unit formed thereby may be heat-treated at temperatures below the initial ambient ignition temperature, or it may be heat treated by slowly raising the temperature thereof above the initial ambient ignition temperature, to impart a decreased thermal sensitivity to the unit. Units formed to include both percussion and thermally sensitive initiator compositions have been fired successfully at elevated temperatures above the initial ambient ignition temperatures after having received a heat treatment as prescribed in the present invention. Similarly, other articles containing initiator compositions may be formed with the compositions as described herein either by loading the composition prior to heat treatment, or by heat treating the composition first, and then loading the composition into the article enclosures. Alternatively, the compositions may be formed into self supporting shapes such as the strips described in Example VI. Other shapes such as a string or fuse could be similarly formed from these compositions.

The useful life of an initiator composition at high temperatures depends on the heat treatment which is given to it before it is to be subjected to high temperature conditions, and upon the selection and combination of the ingredients and additives which are used in the initiator composition. In general a composition will have a longer useful life expectancy after a particular heat treatment if it is maintained at high temperature conditions of about F 400 F. than if it is subjected to an upper range of elevated temperature in the neighborhood of 700 F.

Although specific preparation procedures are given for the formation of initiator compositions useful under high temperature conditions, it will be appreciated that other procedures may be employed. In general, it is desirable to use wet handling techniques for safety reasons. Although Water is indicated for this purpose, other liquids such as relatively inert liquids of the hydrocarbon and halocarbon series may be used. The choice of liquids and the necessity of use of liquids depends very largely on the chemical constituents which are being combined or mixed. In the case of the composition given in Example VIII, this material was successfully mixed in the dry state and used successfully as a fiame booster, i.e., it improved the flame which occurred when the initiator was fired.

Where the material is sensitive to, or aifected by a particular liquid, others may be substituted. As for example, if the material is sensitive to kerosene, water may be used in place of kerosene. Although it is possible to make these compositions using dry processing techniques, wet processing, using one of the inert liquids, is preferred, as indicated, for safety reasons.

The composition after being thoroughly mixed in the liquid medium may be dried either by low temperature drying in the range of 90 or 100 F. or may be inserted into the container in the wet state and then dried by similar low temperature heating.

When the initiator units are formed, it is sometimes desirable to seal the open end of the initiator in order to prevent any contamination from entering the initiator and to avoid loss of the initiator composition therefrom. A metal foil may be placed over the open end of the initiator and sealed in place to give a protection against undesirable contamination or loss of initiator composition from the unit. This is also desirable where the units are to be stored under adverse conditions such as under long term high humidity conditions. Other conventional enclosure means may also be employed depending on the temperature under which the unit is to be stored.

As indicated above, the ambient ignition temperature of initiator compositions can be raised from an initial low value by a heat treatment. The length of heat treatment which is necessary to effect a significant increase for a particular composition may be readily determined by a few scoping experiments which involve heating the composition to a temperature either entirely below its initial ambient ignition temperature, or both below and above the initial ambient ignition temperature as indicated in the above examples, and subsequently, testing the thus heated initiators at various temperature conditions. The extent of heating to which a heat treated initiator composition is subjected above the initial ambient ignition temperature will largely determine the useful life of the initiator at high temperatures. Accordingly, the heat treatment either below or above the initial ambient ignition temperature should not be in excess of that which is suflicient to impart the requisite decrease in ambient thermal sensitivity of the composition.

As is evident from the illustrative examples given above, the time during which a material is treated, and the temperature of this treatment, will determine the extent of the reduction of herrnal sensitivity of the initiator composition. Generally, after the composition is treated at a lower temperature for a longer time, the decrease in thermal sensitivity will be approximately equivalent to that achieved as a result of a higher temperature treatment for a shorter time.

In general also the formulation of the composition will determine the upper temperature to which the material may be heated without initiating the reaction. Temperatures up to about 800 F. have been achieved for the compositions as indicated.

With regard to the rate of heating for heat treatment to impart reduced thermal sensitivity, the heating rate must be slow and, as indicated, may be at the order of 500 F. an hour. However, this rate will vary for different compositions and it is evident from the results given that a determination may be made for a particular composition as to the rate of heating which will impart a reduced thermal sensitivity rather than initiating the reaction of the composition.

In general the ingredients used for these compositions should preferably be in the form of finely divided powder as this permits an intimate mixture to be made. The process is preferably carried out by a mechanic mixing operation using ingredients in finely divided state and in a particulate form.

The initiation of reaction of the composition may be carried out by imparting to the localized portion thereof an activation energy sufiicient to cause a combination 5 the fuel and oxidizing components of the mixture. A localized activation or heating may be produced for example by electrical, chemical, or mechanical means.

Special abrasive ingredients such as glass, chalk or stone fragments may be included in the initiator composition to assist in the activation of the composition as by percussion.

Although the subject method has been described with reference to compositions formed into particular shapes and containments, it will be appreciated that the initiator treatment may be accorded to initiator compositions which are in still other forms. In general it is preferred that the heat treatment be carried out in an atmosphere containing oxygen. Heat treating in air has been shown to be highly satisfactory in reducing the thermal sensitivity of initiator compositions.

The foilowing is given as an explanation of the mechanism which governs the formation of high temperature initiator compositions from low temperature compositions although it will be understood that the validity of the method is not dependent on the accuracy of this explanation.

in general, it is thought that where a first inorganic material, capable of undergoing rapid combustion, is present in an intimate mixture with a second inorganic material which is capabie of supporting the rapid combustion of said first material, the initiation of the rapid combustion reaction is brought about by the introduction into any portion of the composition of a suificient activating energy to cause a localized reaction to proceed with the release of energy at an appreciable rate. The released energy is then responsible for the continuation of the reaction in proximate portions of the composition and this energy release and initiation are repeated until the entire composition is consumed.

The thermal stability of such compositions has heretofore been limited because the ambient heating to a given temperature provides the necessary localized activation to cause the reaction to proceed.

However, it has now been discovered that although two ingredients, normally subject to combining in a combustion reaction, are present in an intimate mixture and heated above their ambient ignition temperature, if this heating has been preceded by a particular type of heat treatment, a protection is afforded to the composition which prevents the ambient temperature from initiating the reaction. This is based on a surprising finding that compositions norrnaliy subject to a very rapid reaction may undergo a relatively slow conversion as a result of a particular heat treatment to a point where their thermal sensitivity is significantly lowered.

It has now been discovered in this connection that it is possible to obtain a slow modification of one or more of th ingredients by prolonged heating at temperatures below the initial ambient ignition temperature. As indicated from Example VI, an initiator composition can also be rendered completely inactive after 5 hours of heating at 500 F. However, prior to the time when the com- 9 position loses its activity as an initiator, it may be ignited and rapidly reacted by imparting a necessary localized activation energy to start the chemical combustion reaction.

t is further a surprising discovery that until the material has been substantially inactivated, it retains its property of serving as an initiator with substantially no decreased elfectiveness. Accordingly, some of the improvements made possible from this invention depend on the finding that an inorganic initiator composition can be rendered thermally stable at temperatures above its initial ambient ignition temperature by a treatment which causes a partial modification of the ingredients of the initiator composition, and that this partial modification is not sufiicient to seriously interfere with the functioning of the initiator composition for a substantial period of time during which the material is maintained at'a high temperature above its initial ambient ignition temperature.

Furthermore, some of the improvements made possi ble depend on the further finding that the modification produced by the heat treatment may be interrupted and that the modified composition, as well as articles containing the modified composition, may be stored for extended periods of time without losing their properties as initiators. A number of initiators prepared as described above were stored for more than a year and were fired after this time with no noticeable change in their ignition or firing characteristics. From the results obtained it is evident that the storage properties extend for considerably longer periods and that the initiators exhibit substantially the same properties as they do within a short period following their heat treatment.

Since many examples of the foregoing procedures, compositions and articles may be carried out and made, and since many modifications can be made in the procedures, compositions and articles described without departing from the scope of the subject invention, the foregoing is to be interpreted as illustrative only, and not as defining or limiting the scope of the invention.

What is claimed is:

1. A method for decreasing the ambient thermal sensitivity of a finely divided solid initiator composition which comprises heating said initiator composition to a temperature in the range between 300 and 800 F. for suificient time to raise the ambient ignition temperature of said initiator composition without inactivating and without igniting said initiator compostion, said initiator composition consisting essentially of a mixture of a powdered material and an oxidizing proportion of a fineiy divided oxidizing compound, said powdered material being selected from the group consisting of red phosphorus and a finely divided solid metal which forms an oxide having a free energy of formation, per gram atom of oxygen, more negative than minus 40 kilocalories, said finely divided inorganic oxidizing compound having a free energy of formation, per gram atom of oxygen, more positive than minus 40 kilocalories, and recovering the resulting heat-treated finely divided solid initiator composition produced thereby.

2. A finely divided solid initiator composition formed by heating a mixture of finely divided solid initiator ingredients to a temperature in the range between 300 and 800 F. for suflicient time to raise the ambient ignition temperature without inactivating and without igniting the mixture, said mixture consisting essentially of a powdered material and an oxidizing proportion of a finely divided oxidizing compound, said powdered material being selected from the group consisting of red phosphorus and a finely divided metal which forms an oxide having a free energy of formation, per gram atom of oxygen, more negative than minus 40 kilocalories, said finely divided solid inorganic oxidizing compound having a free energy of formation, per gram atom of oxygen, more positive than minus 40 kilocalories.

3. The composition of claim 2 wherein said metal is zirconium and said oxidizer is barium chromate.

4. The composition of claim 2 wherein said metal is magnesium and said oxidizer is barium peroxide.

5. The composition of claim 2 wherein said mixture is a mixture of red phosphorus, potassium perchlorate, and antimony sulfide.

References Cited in the file of this patent UNITED STATES PATENTS 2,027,825 Jacobs Jan. 14, 1936 2,115,047 Stevenson et al Apr. 26, 1938 2,457,860 Bennett et a1 J an. 4, 1949 2,696,429 Hart Dec. 7, 1954 3,025,795 Sutton et a1 Mar. 20, 1962 FOREIGN PATENTS 555,360 Canada Apr. 1, 1958

Claims (1)

1. 2. A FINELY DIVIDED SOLID INITIATOR COMPOSITION FORMED BY HEATING A MIXTURE OF FINELY DIVIDED SOLID INITIATOR INGREDIENTS TO A TEMPERATURE IN THE RANGE BETWEEN 300 AND 800*F. FOR SUFFICIENT TIME TO RAISE THE AMBIENT IGNITION TEMPERATURE WITHOUT INACTIVATING AND WITHOUT IGNITING THE MIXTURE, SAID MIXTURE CONSISTING ESSENTIALLY OF A POWDERED MATERIAL AND AN OXIDIZING PROPORTION OF A FINELY DIVIDED OXIDIZING COMPOUND, SAID POWDERED MATERIAL BEING SELECTED FROM THE GROUP CONSISTING OF RED PHOSPHORUS AND A FINELY DIVIDED METAL WHICH FORMS AN OXIDE HAVING A FREE ENERGY OF FORMATION, PER GRAM ATOM OF OXYGEN, MORE NEGATIVE THAN MINUS 40 KILOCALORIES, SAID FINELY DIVIDED SOLID INORGANIC OXIDIZING COMPOUND HAVING A FREE ENERGY OF FORMATION, PER GRAM ATOM OF OXYGEN, MORE POSITIVE THAN MINUS 40 KILOCALORIES.