US2018600A - Porcelain resistant to the passage of x-rays and method of making the same - Google Patents

Description

lit

Patented Oct. 22, 1935 PORCELAIN RESISTANT TO THE PASSAGE 0F X-RAYS AND METHOD OF MAKING THE SAME Leslie Brown, Trenton, N. J., assignor to Lenox Incorporated, Trenton, N. J., a corporation of New Jersey No Drawing. Application May 15, 1935,

Serial No. 21,696v I n Claims. ioi. 25-156) My invention relates to a novel porcelain for use with X-rays and to the method of making the porcelain, and more particularly it relates to a porcelain which is relatively resistant to the passage of X-rays and, therefore, is applicable for use as an X-ray tube shield, and, in the form of tiles onbricks, as a lining for X-ray booths or the like.

The principal object of the present invention is to provide a porcelain having a very high resistance towards X-rays and possessing at the same time high resistance to thermal and impact shock so that a material is provided which is particularly adapted for use in conjunction with X-ray apparatus.

Still another object of my invention is to furnish a porcelain for use as a shield for X-ray tubes which not only efiectively prevents the passage of X-rays therethrough, but 'also possesses no tendency to melt even though the tube be kept in service for a relatively long period of time.

Other objects, including the method of making porcelain of the novel characteristics, will be apparent from a consideration of the specification and claims.

it is necessary to isolate iX-ray apparatus as far as possible by surrounding it with a material which ofiers resistance to the passage of X-rays in order to focus the rays at the point desired and to protect the operator from the rays which are extremely dangerous unless carefully controlled.

Heretoiore, sheet lead or lead glass has been employed for isolating or absorbing X-rays but both of these materials possess inherent disade vantages. The lead glass shields tend to melt when employed with an X-ray tube which is in service any appreciable length of time and are not resistant to the thermal shock encountered in the use of the shield, breakage often occurring following the rapid heating and sudden cooling incident to the use of 'the tube. The impact strength of the lead glass is also low and losses often result, due to lack of sufilcientresistance to impact. The use of sheet lead is limited, due to its low melting point, to the lining of X-ray' booths and dimculties are encountered due to the creeping properties of the lead.

The porcelain product of the present invention possesses properties which makes it ideally suitable for use in isolating X-ray apparatus.

The porcelain is characterized by high X-ray absorption, by high resistance to both thermal and impact shock, and by sufficiently high melting points of all its components so that no tendency for the material to meltis encountered even though the X-ray tube is kept in service for long periods of time. As compared to lead glass shields previously employed, the porcelain of the 5 present invention possesses an X-ray absorption characteristic in the neighborhood of 25% greater, a resistance to thermal shock about five times greater, anda resistance to impact shock about seven times greater. The X-ray absorp- 30 tion characteristic of the porcelain of the present invention may be made as low as that of sheet lead without difliculty and without providing an undue thickness of porcelain.

The product of the present invention isa vltriiii fled porcelain mass containing lead held in combination therein, the combined lead ranging preferably from 32%-33% as a minimum up to %-66% as a maximum, the lead being figured at PM). In order that the lead may be held in 20 combination and not melt. or volatilize during the use of the product, it is preferably formed into a calcine or fritt prior to its manufacture into the porcelain. The calcine or iritt is made by combining a reactive lead compound such as one of 26 the lead salts or oxides, preferably lead oxide iPbO) with a suitable acidic oxide, preferably silicon dioxide, and a small amount of an alkali metal heat-decomposable salt such as a potassium or sodium salt. Other elements may also be present if desired, but in the manufacture of the calcine care must be taken to adjust the materials and the amounts thereof so that a relatively high melting point lead product is obtained. After the calcine has been manufactured by 55 firing the mixture of materials, it is combined in suitable proportions in a porcelain mix, preferably one containing a clay having a high percentage of alumina, such as a mix made up of English china clay, Florida clay, English ball clay to or Bentonite. A compound is also advantageously added which is characterized by its ability to increase the resistance of the product to thermal shock and to X-ray absorption and to increase the refractive propertiesthereof, such for example, as Q! zirconium oxide, zirconium silicate, bismuth oxide, thorium oxide, uranium oxide, and other compounds known to be compatible with clays in a porcelain mix. Thereafter, the final mix in a finely divided condition is pressed to suitable 50 form and heated to vitrification. In most cases, it will be desirable to place a glaze upon the surface of the porcelain, and if this is to be done the heating of the porcelain mass in the first instance is usually only suificient to bring about ll semi-vitrification. Thereafter. the materials forming the glaze are applied. to the surface of the semi-vitrified porcelain and the whole mass heated to a temperature where the glaze is fused and complete vitrification is accomplished.

As previously pointed out, the lead is preferably combined in the form of a calcine or fritt before it is mixed with the materials making up the porcelain. This step is advisable in order to provide a product in which no volatilization of the lead will occur during the use of the product and in which no tendency of the lead product to melt will be encountered. The calcine is prepared by mixing the materials together and thereafter heating them in a furnace to cause chemical combination and the subsequent fusion of the product. The materials employed in making the calcine comprise a lead compound, preferably lead oxide, an acidic oxideyand a small amount of a heat-decomposable alkali metal salt, the acidic oxide and alkali metal salt being capable of reacting with the lead compound to form a product with a relatively high melting point, Silicon dioxide is a typical acidic oxide and this may be displaced in part by other oxides such as bismuth oxide, zirconium oxide, uranium oxide and thorium oxide. The decomposable alkaline metal salt is either a potassium or sodium salt, although the former ispreferred. The carbonates, nitrates, and salts of organic acids and the like are typical. In most instances, potassium nitrate will be found to be the most desirable compound for use. The proportion of lead oxide to acidic oxide employed in forming the calcine will depend upon the amount of lead desired in the calcine and in the finished porcelain, and in general the acidic oxide is used in proportions necessary to make the mix up to 100% allowing for the desired amount of lead oxide and the small amount of decomposable a1- kali metal salt employed. As hereinbefore stated, the invention contemplates a porcelain containing preferably from 32% PbO to 66% PbO, and, therefore, the lead oxide content of the calcine will in general vary from 40% to 85%.

The following are examples of various mixtures which are applicable in forming the calcine:

Calc'ine mix No. 1

Per cent Lead oxide 85 Silicon dioxide 14 Potassium nitrate 1 Calcine mic: No. 2

Per cent Lead oxide 80 Silicon dioxide 19 Potassium nitrate 1 C'aZcine mix No. 3 4

Per cent Lead oxide '75 Silicon dioxide 24 Potassium nitrate l calcine min: No. 4

Per cent Lead oxide 70 Silicon dioxide 29 Potassium nitrate 1 Calcine mic: N0. 5

Per cent Lead oxide 65 Silicon dioxide 30 Potassium nitrate 5 Calcine mix No. 6

Per cent Lead oxide 55 Bismuth oxide- 20 Silicon dioxide- 24 Potassium nitrate 1 calcine mix No. 7

Per cent Lead oxide 45 Bismuth oxide 40 Silicon dioxide 14 Potassium nitrate 1 calcine mix N0. 8

Per cent 15 Lead oxide 40 Silicon dioxide 59 Potassium nitrate 1 calcine mic: No. 9

Per cent ,0 Lead oxide 40 Bismuth oxide Silicon dioxide Potassium nitrate 5 a In preparing the calcine, the ingredients in proper proportions are placed in a furnace having a lining resistant to the action of lead compounds, for example one made of mullite tile. The furnace is suitably heated and the temperature is raised to a point where the chemical reaction takes place and the resulting calcine melts. With the mixes given above, a temperature of 1640 F. has been found to be particularly applicabie, although other temperatures producing 85 the same results may be utilized. When the desired temperature is reached, the molten calcine is run from the furnace. As a matter of expediency, it is preferably nm in a thin stream into cold water which causes granulation of the product, 40.

and, therefore renders it in a condition which is easily handled. The calcine is thoroughly dried and may then be mixed with materials making up the main porcelain mix, but in order to insure complete combination of the lead and to bring about as complete a shrinkage of the product as possible, it is desirable to retreat the calcinein the furnace. In this instance, the calcine product is again heated until it is molten, preferably approximately to the same temperature as was previously employed. When the calcine is molten, it is again advantageously run in a thin stream into cold water, again resulting in granulation. Since the calcine is very finely ground with the porcelain-forming ingredients, the size of the calcine particles is immaterial although in most instances, particles that will pass through a twentymesh screen are desirable since these are easily handled and ground to the fineness desired.

The materials employed in the porcelain mix may be of a wide variety of compounds, provided that they are capable of reacting with the .lead calcine and of holding the lead in combination in use and of withstanding physical conditions encountered in the use of the product. The porcelain mix desirably contains one or more clays having relatively high alumina content. An oxide such as zirconium oxide, uranium oxide, bismuth oxide or thorium oxide may be employed to impart high thermal shock and refractory properties to the product, as well as to give a product with as high a resistivity to X-rays as possible. English ball clay, Florida clay, English china clay and Bentonites are examples of the clays that are particularly applicable. The 101- lowing analyses are more or less typical of these clays:

English china clay or. kaolin PORCELAIN CONTAINING 45.08% mo OXIDI (Formula IV) Parts Kaolin Florida clay 3 Zirconium oxide 2% No. 4 calcine 21% Porscsmm CONTAINING 34.1% ml) oxmn (Fonnula V) Parts Formula IV 16 No. a calcine... 2Y Zirconium oxide 6 Kaolin 2 Bentonite .i to .2

i Percent Silica 42.2 Alumina 34.6 Oxide of ir .33 Lime .32 Potash and soda .41 Combined water and organic matter 11.28 Moisture V 10179 Florida clay Per cent Silica 45.39 Alumina 39.19 Potash J .41 Soda .41 Magnesia .29 Oxide of ir .45 Combined water 14.01

English ball clay l Per cent Silica 57.77 Alumina 30.94 (lxlde of iron .15 Magnesia .58 Potash .41 Soda 1.65 Combined water 8.5

Bentortite Per cent Silica 59.57 Alumina, 19.67 Oxide of iron 2.91 Magnesia 2.46 Potash .29 Soda .209 ignition loss 14.73

. The amount or clay employed depends upon the amount of lead calcine to be incorporated therewith and the amount of oxide depends on the proportions of the other materials present and on the properties desired. In all cases, the proportions of materials are selected so as to produce a satisfactory porcelain. In typical cases where a porcelain containing a minimum and maximum amount of lead are desired, the following mixes have proved to be particularly desirable:

Poscumrn CONTAINING 33.75% LnAnoxron (Formula ll Per cent English china clay 311 Florida clay 8 Zirconium oxide 16 No. 3 Calcine d5 PORCELAIN CONTAINING 65.45% LEAD OXIDE (Formula II) Per cent English china clay 18 Florida clay 7 l Zirconium oxide 1 No. 1 calcine PORCELAIN cou'mrmuc 38% LEAD oxrnu (Formula III) Parts Kaolin Florida clay 16 Zirconium mlidc 80 No. 4 ca 45.7 Bentonlte V4 The materials to be used in the preparation of the porcelain are mixed and are thereafter treated in accordance with the usual practices en- .0 countered in the manufacture of porcelain products. Preferably the materials are placed in the desired proportions in a suitable grinding mill to render them in a finely divided condition. A wet-grinding ball mill has been found to be satisfactory for use and the wet-grinding ls preferably continued until the material is reduced to a fineness in the neighborhood of 200 mesh. The compound is then thoroughly dried and is placed in a suitable device to bring about the formation of a dust, a potters cutter being especially satisfactory. The dust is then sprayed and mixed with a gelatinous binder such as ammonium alginate dissolved in water in the proportions of w 30 pounds of dry compound to 311 pounds of porcelaindust. The dust associated with the binder is then placed in a steel die of a shape and size corresponding to those characteristics'desired in the finished product, allowance being made for w the shrinkage of the porcelain from the die size to the finished form oi about 1/20 theshrinkage, varying somewhat with the particular mix employed. Sufllcient pressure is applied to bring 45 about formation cf'the article and the die is removed. The rough edges formed during the pressing operation are then removed and the formed products are placed on fire clay slabs with supports for the product wherever necessary to prevent it from working out of shape during the firing operations. These fire clay slabs are placed in the kiln and suflicient heat is applied to cause vitrification if no glazing is to be applied, or sumcient to cause semi-vitrification ii the products are later to be glazed. The tem peratures employed depend on the proportions of the various constituents of the mix and in general if the products are to be vitrified in this step, a temperature of 1600 F. is usually sufficient. In the event that only semi-vitrification is to be brought about, a temperature somewhat lower is employed, for example, a temperature in 05 the neighborhood of 1540" F. After the semivitrification, the products are cooled and a product capable of forming a glaze is sprayed upon the products, preferably on all the surfaces thereof. Any suitable glaze may be employed but preferably it contains relatively large amount of lead oxide and, therefore, may be termed "a lead glaze". The following are examples of the glazes which have been found to give excellent results: 781

Black glaze calcine made up of 66 parts of u Albany slip clay, 46 parts of black manganese oxide, 68 parts of silicon dioxide, parts of cobalt dioxide .574

After the glaze is sprayed over the semi-vitrifled product, the articles are again placed in a kiln and are heated sufilciently to bring about a fiuxing of the glaze and to complete the vitrification of the article. The finished product is then cooled and is ready for use in conjunction with X-ray apparatus.

The vitrification of the mass involves a reaction between the lead calcine and the materials employed in the porcelain mix as is evidenced by a uniform, homogeneous texture found throughu out the finished product.

The final product, as previously pointed out, contains asubstantial proportion of lead, figured as lead oxide. The analyses of the porcelain made in accordance with Formula I, for example,

a is approximately as follows (exclusive of glaze) Per cent Silica 31.20

Alumina 18.02

40 Lead oxide 33.75

Zirconium oxide 16.00

Potash and soda .63

Iron oxide .20

Lime .10

45 Magnesia .10

while the analysis of the porcelain made in accordance with Formula II is approximately as follows (exclusive of glaze) 2-- The X-ray absorptive characteristic of the porcelain of the present invention depends upon the lead content and upon the thickness of the porcelain. For example, a porcelain one-half inch in thickness and containing approximately 33.75% lead oxide will correspond in absorptive 65 characteristic to a sheet of lead approximately .75 millimeter in thickness; while a porcelain five-sixteenth inch in thickness and containing approximately 65% lead oxide will correspond in 70 absorptive characteristic to a sheet of lead approximately 1.5 millimeters in thickness-a marked increase in absorption over the glass now in use. The porcelain may be used in the form of an X-ray tube shield, a tile or brick or other 1| form for the lining of X-ray booths and in general for all other uses in conjunction with the isolation of x-rays.

Determinationsoi' the coeilicients of expansion of porcelains made in accordance with the invention, and of the lead glass now in current use I shows that the porcelains uniformly over the temperature range have a much lower coeillcient. At five hundred degrees, for example. the porcelains have only an expansion of .26-28 per cent. linear expansion; while that of the lead 10 glass is .52 per cent. Furthermore, the softening points of the porcelain are above 750' 0., while the glass softens at a temperature in the neighborhood of 500 C.

Considerable modification is possible in the amount of lead employed in the porcelain within the ranges given, as well as in the choice of materials utilized in making the lead calcine and the porcelain product, and in thesteps used in making the product without departing from the essential features of the present invention.

I claim:

1. The process of preparing a porcelain, relatively resistant to the passage of X-rays, which comprises heating together a reactive lead compound, an acidic oxide, and a relatively small amount of a. heat-decomposable alkali metal salt to form a vitrified calcine of relatively high melting point, adding said calcine to a porcelain mix, and thereafter pressing the mixture in finely divided form to the desired shape and firing the shaped object to causeyitrification thereof.

2. The process of preparing a porcelain, relatively resistant to the passage of X-rays, which comprises heating together a reactive lead compound, an acidic oxide, and a relatively small amount of a heat-decomposable alkali metal salt to form a vitrified calcine of relatively high melting point, adding said calcine to a porcelain mix 40 in sufficient proportion to give to the final object a lead content of between 32% and 66% 118- ured as lead oxide, pressing the mixture in finely divided form to the desired shape, firing the shaped object to cause semi-vitrification thereof, applying a lead glaze to the surface of said object, and thereafter firing the object to complete its vitrification.

3. The process of preparing a porcelain, relatively resistant to the passage of X-rays, which comprises heating together a reactive lead compound, an acidic oxide, and a relatively small amount of a heat-decomposable alkali metal salt to form a vitrified calcine of relatively high melting point. adding said calcine to a porcelain mix containing zirconium oxide and a substantial amount of a clay having a high alumina content, and thereafter pressing the mixture in finely divided form to the desired shape and firing the shaped object to cause vitrification thereof.

4. The process of preparing a porcelain, relatively resistant to the passage of X-rays, which comprises heating together lead oxide, silicon dioxide, and a relatively small amount of a heatdecomposable alkali metal salt to form a vitrified 06 calcine of relatively high melting point, adding said calcine to a porcelain mix containing zirconium oxide and a substantial amount of a clay having a high alumina content, said calcine being added in sufiicient amount to give to the final object a lead content of between 32% and 66% figured as lead oxide, pressing the mixture in finely divided form to the desired shape, firing the shaped object to cause semi-vitrification thereof, applying a lead glaze to the surface of said ob- 15 aoraeoo iect, and thereafter firing the object to complete its vitrification.

5. A shaped vitrified porcelain product, relatively resistant to the passage of x-rays, having a a lead content from 32% to 66% figured as PhD and being coated with a vitrified lead glaze, said product being characterized by high resistance to both thermal and impact shock and by a relalain producing mix, said product being character-.

ized by high resistance to both thermal and impact shock and by a relatively high melting point.

7. A shaped vitrified porcelain product, relatively resistant to the passage of X-rays, comprising the reaction product of a vitrified lead calcine oi relatively high melting point and a porcelain producing mix having a vitrified lead glaze, said product being characterized by high resistance to both thermal and impact shock and by a relatively high melting point.

8. A shaped vitrified porcelain product, relatively resistant to the passage oi! X-rays, comprising the reaction product of a vitrified lead calcine of relatively high melting point and a porcelain producing mix, the content of lead in the finished product being between 32% and 66% figured as PbO, said product being char-' acterized by high resistance to both thermal and impact shock and by relatively high melting point.

9. A shaped vitrified porcelain product, rela tively resistant to the passage of X-rays, comprising the reaction product of a vitrified lead-silicaalkali-metal calcine of relatively high melting point and a porcelain producing mix containing a substantial amount of clay having a high alumina content, said product being characterized by high resistance to both thermal and impact shock and by a relatively high melting point.

10. A shaped vitrified porcelain product, relatively resistant to the passage of X-rays, comprising the reaction product of a vitrified lead-silicaalkali-metal calcine of relatively high melting point and a porcelain producing mix containing a substantial amount of clay having a high alumina content, said product being coated by a vitrified lead glaze and being characterized by 7 high resistance to both thermal and impact shock point and a porcelain producing mix containing zirconium oxide and a substantial amount of clay having a high alumina content, the content of lead in the finished product being between 32% and 66% figured as PbO, said product being chara) acterized by high resistance to both thermal and impact shock and by a relatively high melting point.

' LESLIE BROWN. I