Coaxially Conducting Element And Process For Manufacture

Abstract

Disclosed is a method for forming an array of conductive crystalline dendrites of reduced rutile in a glass-ceramic insulating matrix by crystallizing certain compositions containing titania and silica in a non-oxidizing atmosphere under the influence of a thermal gradient to form a parallel array of conductive reduced rutile dendrites.

Description

There is a need in the electronics industry for a device comprising an insulating plate having embedded therein and passing therethrough an array of mutually insulated conductors. Such devices are used in the faceplates of cathode ray tubes and other electronic transmission systems where an interaction between an electronic charge generated in vacuum and processing equipment located in air is desired. General background for such applications is provided in U. S. Pat. Nos. 3,321,657; 3,193,364; 3,220,012; 3,424,932; 2,952,796; 3,140,528; and 3,366,817.

For such applications the device must be vacuum tight and this requirement has resulted in severe fabrication difficulties when conventional manufacturing techniques are employed. For instance when metal filaments are embedded in a glassy matrix the devices often have structural defects due to the difference in thermal expansion coefficients between the glassy matrix and the metal filaments. Moreover it is often difficult to achieve a vacuum-tight seal between the glassy matrix and the individual conductors.

One particularly important application of the present invention is an electron image transfer device as in the face plate of a cathode-ray tube. In such a device the coaxially conducting element is sealed in the faceplate of a cathode-ray tube so that the ends of the conductors present a mosaic pattern upon which electronic information is imposed by means of the electron gun within the tube. The conductor ends which are in the cathode-ray tube each receive an electronic charge which is then transmitted outside the face plate and can be used for reproduction or display purposes.

An image transfer device of this type must incorporate a very large number of relatively small diameter conductors which are spaced and insulated from one another, in order to provide adequate optical resolution for electron charge information thus transmitted. Furthermore, the device must have sufficient strength so that a relatively thin section can serve as a cathode-ray tube faceplate and the individual conductors must be vacuum tight within the insulating matrix to provide for the maintenance of a prolonged vacuum.

To accomplish these objective the prior art has proposed various methods of binding as assemblage of short wires or other conductors together with an insulating matrix. This has often proven to be unreliable or economically impractical for many commercial applications.

The present invention provides a unique and novel solution to the problem of preparing such coaxially conducting element by the controlled crystallization of conducting crystalline dendrites orientated along an axis of the element and growing through an insulating glassy or glass-ceramic matrix. This approach obviates the problems associated with handling and physically installing several thousand conductors into a single insulating matrix.

Accordingly it is a primary object of the present invention to provide an efficient and practical coaxially conductive element comprising an array of mutually insulated electrical conductors passing through an insulating matrix.

In attaining the objects of this invention one feature resides in forming a molten mass of thermally crystallizable glass composition comprising silica, at least one alkaline earth oxide, and titania, removing gaseous materials from said molten mass under non-oxidizing conditions (i.e., reducing or neutral conditions) cooling a first cross-sectional portion of said molten mass to establish a temperature gradient in said molten mass, and selectively crystallize an array of discrete, conductive needle-like dendrites of titanium oxide or dendrites of stuffed titanium oxide represented by the structural formula Ti.sub.x O.sub.2x.sub.+1 wherein x is an integer of at least one, cooling cross-sectional portions of said molten mass adjoining said first cross-sectional portion to advance the temperature gradient throughout said mass thereby crystallizing said dendrites in a substantially parallel coaxial array with said dendrites being axially aligned in the direction of said temperature gradient, and cooling the resulting mass to form an insulating matrix around said array of conductive dendrites. The term dendrites of stuffed titanium oxide has been used above and refers to dendrites having a crystalline structure which is stabilized with inclusions of matrix constituents. The resulting body is then formed into the desired configuration by conventional glass and ceramic forming techniques such as cutting, drawing, grinding and so on, to form the desired coaxially conducting element. The terminal points of individual conductive dendrites are exposed on surfaces of the element to establish electrical conductivity through the dendrites.

For convenience in reference the titanium oxides represented by the formula Ti.sub.x O.sub.2x.sub.+1 wherein x is an integer of at least one will be hereinafter called "reduced rutile."

In the drawings, which will be discussed in relation to the examples,

FIG. 1 illustrates an idealized time-temperature profile for crystallizing conductive reduced rutile in a preferred composition range;

FIG. 2 illustrates an actual time-temperature profile employed in example 2;

FIG. 3 is a partial sectional view of a coaxially conductive element of invention; and

FIG. 4 is an enlarged view of the cross section of FIG. 3.

U. S. Pat. No. 3,065,091 to Russell discloses a process for growing crystalline fibers of titania, zirconia or zircon in a sodium borosilicate flux. According to this patent, titania, zircon or zirconia is melted in the borosilicate flux at a sufficiently high temperature to cause all of the crystal-forming materials to go into solution and form a homogeneous melt. This resulting melt is then cooled to cause the crystals to precipitate and form a reinforced ceramic structure. In the Russell Patent the crystals are shown to be growing as a ball of needles from a point nucleation source (see FIGS. 2 and 3) and a parallel array of needles is not disclosed. Conduction in reduced rutile is mentioned by Russell at column 6 although there is no mention of coaxially conducting elements or methods for manufacture of same.

In commonly assigned U.S. Pat. No. 2,484,248 the disclosure of which is incorporated by reference, are disclosed crystalline glass ceramics having a high dielectric constant which are formed from a melt of thermally crystallizable glass compositions containing silica, and titania under reducing or neutral conditions. The glass composition is crystallized to obtain a conductive phase of reduced rutile by a random nucleation and crystallization process. The crystallized glass-ceramic is thereafter surface-oxidized at an elevated temperature under oxidizing conditions to obtain a non-conductive surface on the desired dielectric body.

The present invention utilizes a specific range of thermally crystallizable glass compositions within the broad range of 3,484,258 which, when subjected to a specifically defined thermal treatment, will form a parallel array of conducting dendrites of reduced rutile in an insulating matrix.

Compositions suitable for practicing the present invention consist essentially of alkaline earth-titania-silicate within the weight range of about:

Broad Preferred Preferred CaO-TiO .sub.2 -SiO.sMgO-TiO .sub.2 iO.sub.2 Compositions Compositions % % % SiO 25 .sub.2 -60 25 -50 30 -60 TiO 10 .sub.2 -40 20 -40 10 -35 0aO -30 10 -25 -- 0gO -30 -- 10 -30 Wherein MgO 10 +CaO -30 Al 0 .sub.2 O.sub.3 -30 0-30 0-30 B 0.sub.2 O.sub.3 -15 0-10 0-10

other conventional glass forming ingredients such as Na.sub.2 O, K.sub.2 O, P.sub.2 O.sub.5, ZnO, PbO, and BaO can be added if desired in combined proportion of up to about 10 percent by weight of the above composition so long as such addition does not prevent the formation of the reduced rutile phase.

In the preferred CaO-TiO.sub.2 -SiO.sub.2 system, reduced rutile and sphene can exist as the crystalline phases. Due to the mechanics of crystallization, reduced rutile will always form as conductive needle-like dendrites while the non-conductive sphene (if it crystallizes at all) will crystallize in the matrix. Whether or not sphene crystallizes in the matrix is of no importance to the present invention because the matrix is non-conductive in either case. For some applications it may be desirable to have a glassy matrix and for these applications the crystallization conditions will be selected to avoid the formation of sphene. When a glass-ceramic matrix is desired, the crystallization conditions will be selected to promote this formation of sphene in the matrix.

The batch compositions can be selected from conventional fritted or unfritted glass making materials such as feldspar, oxides, carbonates, aluminates and so forth. Impurities can also enter the compositions, depending on the source of starting materials provided they do not adversely affect the desired properties of the final element.

In preparing the melt, the batch material are placed in a refractor container and brought to a temperature where the molten state is achieved. For most of the compositions described above this temperature is about 1400.degree.C - 1600.degree.C. When the conductive element to be formed must be vacuum tight, the prevention of the formation of bubbles during crystallization of the reduced rutile phase is of great importance. The source of these bubbles appears to be the release of gases dissolved or occluded in the melt during the nromal process of melting the glass. This results in the formation of an elongated bubbles in the vicinity of the reduced rutile dendrite.

The present invention provides for minimizing the formation of such elongated bubbles by out-gassing the melt. One of these outgassing methods is vacuum melting wherein the entire melt is processed under a total pressure of less than 1mm of Hg or less and often as low as 10.sup.-.sup.3 mm of Hg. While this method is efficient, it requires specialized vacuuming melting equipment. Accordingly, other methods such as purging or sparging the melt with an inert gas such as nitrogen, argon, neon or carbon dioxide can be employed. This sparging can be accomplished by bubbling the purging gas through this melt or by employing a batch material which releases a purging gas upon decomposition during melting. Carbonates as raw materials release carbon dioxide during melting which has the effect of purging the melt and sweeping away dissolved and occluded gaseous components. The amount of purging required varies from application to application. In most applications the melt should be purged so that no visible bubbles are observed by visually examining the finished element with the naked eye.

In the inert gas sparging technique the inert gas is bubbled through the melt at a temperature sufficiently far above the melting temperature that the glass is fluid enough so that a reasonable rate of gas flow through the melt can be achieved, while at the same time the bubbles formed are sufficiently small to have a high ratio of surface to volume. Both of these factors are functions of melt viscosity. It has been found that sufficient outgassing to practically eliminate the formation of bubbles and voids from the finish element can be achieved by bubbling argon gas through the melt at the rate of about 0.1 to 0.5 SCFH at a temperature of about 1400.degree.-1500.degree.C for a period of 3 and 1/2 hours for melts having a volume of about 10 cubic inches.

In the preparation of electrically conducting elements according to the present invention a glass composition as described above is melted in an essentially neutral atmosphere or a reducing atmosphere. Thereafter the desired article is shaped and crystallized while still in the same atmosphere.

The effect of the neutral or reducing atmosphere is to reduce some of the potentially conductive titania present in the composition to the lower member of the homologous series Ti.sub.x O.sub.2x.sub.]1 where x is an integer with a value of at least one (i.e., reduced rutile). This provides the mixture of valence states in the titanium which is necessary to achieve electrical conductivity.

Many crystalline species other than the reduced rutile species can be present in the resultant element in addition to the reduced rutile without materially effecting the conductivity characteristics.

Example of neutral and reducing atmospheres for use in this invention are argon, argon-hydrogen, nitrogen, nitrogen-hydrogen, carbon monoxide, and nitrogen-oxygen gas mixtures. These atmospheres function to form reduced rutile phase by the exclusion of the required amount of oxygen necessary to convert all of the titanium compounds present to TiO.sub.2.

In another embodiment invention a metal or reducing agent is added to the melt in an amount sufficient to reduce the titanium oxide present to the conductive reduced rutile state. Suitable reducing agents for this purpose include carbon and titanium titanium-oxide. Carbon (i.e., graphite) melting vessels are often employed in which case the reducing agent is available by reaction of the melt with its containing vessel.

The heat treatment required to crystallize the reduced rutile dendrites is more complicated than is usually encountered in crystallization processes. A first cross-sectional portion of the melt, after it has been purged to remove dissolved and occluded gases and while still in a neutral or reducing atmosphere, is cooled so as to initiate the crystallization of reduced rutile dendrites therein while maintaining the balance of the melt at a temperature above the crystallization temperature of reduced rutile. The cross section portion so cooled is essentially planar in cross sectional area so that reduced rutile dendrites are randomly crystallized throughout the cross sectional portion rather that at a point as a "ball of dendrites." For most compositions described above, this crystallization temperature is in the range of about 1050.degree.C to 1150.degree.C.

Once the reduced rutile dendrites have been randomly crystallized throughout the first cross sectional portion, cross sectional portions adjoining the first cross sectional portions are cooled to within the crystallizing temperature range to cause the dendrites to grow through such adjoining cross sectional portions. This process is repeated until the dendrites have achieved the desired length at which time the resulting mass is cooled to form an insulating glass or glass-ceramic matrix around the array of conductive dendrites.

The technique of advancing a planar temperature gradient through the melt usually forms a substantially parallel array of conductive dendrites of reduced rutile is an insulating matrix having the following characteristics:

1. a conductive dendrite distribution of at least about 50,000 per sq. in. although conductive dendrites of 200,000 to 3,000,000 per square inch are not uncommon, with about 1,000,000 per square inch being typical;

2. a conductive dendrite diameter in the range of about 0.1 to 1.5 mil;

3. a conductive dendrite resistance of about 300 to 1,000 ohms per linear inch;

4. essentially all of the dendrites in parallel alignment;

5. matrix resistivity of at least about 10.sup.10 ohm-cm;

6. essentially void free elements;

7. high mechanical strength.

The present invention will be illustrated in the follwoing examples wherein all parts are by weight, all percentages are weight percentages and all temperatures are in .degree.C unless stated otherwise.

EXAMPLE 1

The following batch materials are placed in a refractor crucible:

Titania 23 parts Silica 37.2 parts Alumina 7.5 parts Calcium Carbonate 32 parts Aluminum 0.5 parts (reducing agent)

The charged crucible is placed in a furnace and the temperature is raised to 1,350.degree.C while the contents of the crucible are melted and stirred. During this melting procedure, a forming gas (10 percent hydrogen--90 percent nitrogen) atmosphere is maintained in the furnace. After melting for 4 hours under the above conditions, a homogeneous molten mass of approximately 1 inch in thickness is achieved. The prolonged heating effectively removes the gaseous materials from the melt. The composition of the molten mass is:

Mole % Weight % SiO .sub.2 47.5 43.2 TiO .sub.2 22.0 26.8 CaO 24.5 20.8 Al .sub.2 O.sub.3 6 9.2

at the end of this 4 hour period, a stream of forming gas at room temperature is directed against the bottom of the crucible to establish a thermal gradient of about 60.degree.C from top to bottom of the molten mass. Thus, the temperature at the top of the molten mass is about 1,350.degree.C while the temperature at the bottom of the molten mass is about 1,290.degree.C. The cooling with forming gas is continued over a 5 hour period to maintain the 60.degree.C temperature gradient from top to bottom while gradually lowering the bottom temperature of the mass to about 980.degree.C and the top temperature of the melt to about 1,040.degree.C. This thermal treatment results in the nucleation and growth of an array of axially aligned, conductive dendrites of reduced rutile in a glass-ceramic matrix containing sphene as the crystalline phase.

The element thus formed is then held at 1,250.degree.F for about 10 hours to anneal and remove strains. After this annealing period sample is cooled to room temperature over a 24 hour period while the forming gas atmosphere is maintained in the furnace.

The element thus is removed from the crucible and the top and bottom faces are ground and polished to clearly expose the dendrites. The ground and polished faces are observed to contain conductive, black, reduced rutile dendrites in the proportion of about 700,000 to 1,000,000 dendrites per square inch.

About 80-90 percent of the dendrites are in a parallel array and axially aligned from bottom to top of the element. The dendrites are about one mil in diameter and are spaced at about 1.5 mils center line to center line. About 30 percent of the element comprises conductive dendrites and the remaining 70 percent comprised the insulating glass-ceramic matrix. The dendrites have a resistance of about 500 to 1,000 ohms as determined by placing the leads of the ohmmeter on terminal points of the individual dendrites on opposing faces of the element.

The element thus formed is designated generally as reference numbered 10 in FIGS. 3 and 4. In these figures the insulating matrix is designated by reference numeral 11 and the conductive dendrites of reduced rutile are designated by number 12. 10a represents that portion of the element derived from the bottom of melt and 10b represents that portion of the element derived from near to top of the melts so that the dendrites 12 grew in the direction from 10a to 10b.

The element is suitable for use in transmitting electronic information.

EXAMPLE 2

In this example glass frits of the following weight percent are used as the batch materials:

Frit A Frit B % % SiO .sub.2 46.4 38 TiO .sub.2 23.6 35 CaO 19.9 18 A1 .sub.2 O.sub.3 9.5 9

a melt is prepared by melting 200 parts of Frit A, 200 parts of Frit B, together with one part of silicon, 1 part of titanium, 0.3 parts of aluminum as reducing agents in a refractory crucible at about 1,450.degree.C to 1,500.degree.C for 4 hours. Because of the furnace construction the temperature at the bottom of the melt is 1,520.degree.C while the temperature at the top of the melt is measured to be 1,430.degree.C. During this melting period a flow rate of 0.2 SCFH of argon gas is bubbled through the molten mass, and argon is maintained in the furnace atmosphere. The composition of the molten mass is:

Mole % Weight % SiO .sub.2 47.0 42.2 TiO .sub.2 24.3 29.3 CaO 22.0 19.0 Al .sub.2 O.sub.3 6.0 9.3

at the end of this melting period the mass is subjected to a time temperature crystallization profile as illustrated in FIG. 2. To achieve this profile the temperature at the bottom of the mass is cooled over a 20 minute period to 1,030.degree.C by directing a stream of argon gas at room temperature against the bottom of the crucible. The temperature at the top of the mass is maintained at about 1,430.degree.C. The top of the melt is then cooled with a stream of argon gas to a temperature of 1,030.degree.C over a period of about three-quarters of an hour while the temperature at the bottom is maintained at 1,030.degree.C. Conductive coaxially aligned dendrites of reduced rutile grow through the mass during this three-quarters of an hour period as described in Example 1. The entire mass was then held at 1,030.degree.C for an additional hour to relieve thermal stresses and then slowly cooled to room temperature.

When the element thus formed has cooled, it is removed from the mold and the top and bottom faces are ground and polished. The element is observed to be an array of substantially parallel, substantially 100 percent axially aligned conductive dendrites of reduced rutile of about 1 1/2 inch to 2 inches in length and passing through a glass-ceramic matrix containing sphene as the crystalline phase. The dendrites extend in the bottom to top direction of the original melt. The ends of the conductive dendrites are identified on either face of the sample with the leads of an ohmmeter. The resistance of the dendrites is measured to be 300to 1,000 ohms. The dendrites are about 1 mil in diameter and are spaced at about one-half to 2 mils center line to center line. The dendrites are present in the proportion of 700,000 to 1,000,000 dendrites per square inch. The element is suitable for use in transmitting electronic information. The element is substantially as illustrated in FIG. 3 and 4 and is suitable for transmission of electronic information.

EXAMPLE 3

To further demonstrate the principles of the present invention in the MgO-SiO.sub.2 system, 0.74 parts of titania, 0.67 parts of silica, 0.68 parts of magnesium carbonate and 0.34 parts of alumina (all material being minus 40 mesh screen size) are melted under neutral conditions in a refractory crucible at 1,450.degree.C for a 10 hour period. The resulting molten mass has the weight composition 36.3 percent TiO.sub.2 ; 32.8 percent SiO.sub.2 ; 16.7 percent Al.sub.2 O.sub.3 ; and 14.1 percent MgO.

After the 10 hour melting period a thermal gradient is established across the molten mass as in Example 1 with a flow of argon gas. The mass is then cooled as in Example 1 and an array of coaxial, conductive dendrites of reduced rutile is observed in the resulting solidified mass. The composition of the residual matrix is not determined although it appears to be glass-ceramic in nature. This conductivity of the dendrites is established by means of an ohmmeter. The resulting element is substantially as shown in FIGS. 3 and 4.

Substantially similar results are obtained when the compositions listed below are employed in the procedures of Example 3. ##SPC1##

Claims

Having thus described the invention, what is claimed is:

1. The method for forming an electrically conductive element comprising an array of coaxial, mutually insulated, crystalline conductors through an insulating matrix, comprising the steps of:

forming a molten mass of a thermally crystallizable glass composition consisting essentially of:

under reducing or neutral conditions,

removing gaseous materials from said molten mass,

cooling a first cross-sectional portion of said molten mass to establish a temperature gradient within the melt to nucleate and initiate crystallization of an array of discrete, conductive, dendrites of reduced rutile within said first cross sectional portion, said reduced rutile being represented by the structural formula Ti.sub.x O.sub.2x.sub.+1 wherein x is an integer of at least one,

cooling cross sectional portions of said molten mass adjoining said first cross sectional portion to advance the temperature gradient throughout said mass thereby growing said dendrites within said adjoining cross sectional portions in a substantially parallel array with said dendrites being axially aligned in the direction of said temperature gradient,

further cooling the resulting mass to terminate dendrite growth and form an insulating matrix around said array of conductive dendrites.

2. The method of claim 1 further including the steps of exposing terminal points of individual conductive dendrites on the surface of said matrix to establish electrical conductivity through said dendrites.

3. The method of claim 1 wherein a reducing agent is present in said molten mass.

4. The method of claim 3 wherein said reducing agent is carbon or a metal.

5. The method of claim 1 wherein said crystallizable composition consists essentially of:

6. The method of claim 1 wherein said crystallizable composition consists essentially of:

7. The method of claim 1 wherein the resulting matrix is a glass ceramic.

8. The method of claim 1 wherein the resulting matrix is glassy.

9. An electrically conductive element comprising a coaxial array of discrete, conductive, crystalline dendrites of reduced rutile in an insulating matrix, said reduced rutile being represented by the structural formula Ti.sub.x O.sub.2x.sub.+1 wherein x is an integer of at least one, said dendrites having been formed by in-situ crystallization and growth in said insulating matrix.

10. The element of claim 9 wherein the distribution of dendrites is at least about 50,000 per square inch.

11. The element of claim 9 wherein the diameter of said dendrites are in the range of about 0.1 to about 1.5 mil.

12. The element of claim 9 wherein the resistance of said dendrites is in the range of about 300 to about 1,000 ohms per linear inch.

13. The element of claim 9 wherein the resistivity of the matrix is at least about 10.sup.10 ohm-cm.