US 3116137 A
Description (OCR text may contain errors)
Dec- 31, 1953 T. vAslLos ETAI. 3,116,137
HOT PRESSED MATERIAL AND METHOD OF' PRODUCING THE SAME u Filed July 1, 195s s sheets-sheet 1 20% a a e @\w@ @UVM/Hwy, l (C f, au ,v\\\\\\\\\ \\\Avm X /z/ 8 THOMAS VASILOS RICHARD B.WAGNER INVENTORS Byag, mm,
ATTORNEYS Dec. 31, 1963 1'. vAslLQs ETAL HOT PRESSED MATERIAL AND METHOD QF PRODUCING THE SAME Y Fied July l, 1958 5 Sheets-Sheet 2 THOMAS VASILOS RICHARD B.WAGNER INVENToRs TTORNEYS Dec. 3l, 1963 T. vAslLos l-:TAL 3,116,137
HOT PRESSED MATERIAL AND METHOD OF' PRODUCING THE SAME Filed July l, 1958 3 Sheets-Sheet 5 THOMAS VASILOS RICHARD B.WAGNER INVENTORS liga Bfi l xT oRNEYs The invention also comprehends a process for making articles of fused silica having controlled transmissivity to radiant energy.
lt is also within the purview of the invention to provide a process for producing articles of fused silica in which are embedded a variety of reinforcements and operative structures, such as resistance-type heating elements and associated connectors.
Another important object of the invention is the provision of a process for pressing articles of fused silica characterized by fineness of detail and dimensional accuracy.
The novel features that are characteristic of the invention are set forth in the appended claims. The invention itself, however, together with additional objects and advantages thereof, wiii best be understood from the following description of specific processes and embodiments when read in conjunction with the accompanying drawings, in which:
FIGURE l is a vertical sectional View through a furnace and press that may be used for hot pressing materials;
FEGURE 2 is a cross sectional View through a hot pressed article which demonstrates the versatility of the process;
FIGURE 3 is a longitudinal sectional view through a hot pressed article having zones of different porosity;
FIGURE 4 is a plan view of a hot pressed article having a heating element embedded in it; and,
FIGURE 5 is a cross sectional View through the article of FIGURE 4 taken on plane 5 5 of that figure.
It has been found that dense articles exhibiting many of the desirable characteristics of glassy, vitreous silica can be made by hot pressing powdered fused silica at temperatures in the range of 1050 C.-l450 C. and at pressures above 250 p.s.i. and usually in the range of SGO-3000 p.s.i. The pressing operation can be accomplished in a relatively few minutes or over a period of several hours depending upon the particular characteristics of the materials that are desired. The process can be carried out in an ordinary atmosphere, or in a vacuum, or any other atmosphere, such as a hydrogen atmosphere, that may be desired. Carried on below l200 C., the process yields a finished material that is entirely free of cristobalite. Even at temperatures close to l450 C., very little cristobalite is formed. The initial material being fused silica, there is little or no tendency to form tridymite, another crystalline form of silica.
The fused silica used may be finely ground vitreous silica. The particle size is not critical and may be between one micron and 1000 microns. Particle size can be adjusted for the particular characteristics which are desired in the finished material. Generally speaking, the use of an admixture of fine and coarse particles will favor the formation of a denser material since the fine particles tend to fill the voids between the coarse ones. Increased density of material is favored by:
(a) High processing pressure (b) Long pressing time (c) High pressing temperature (within the recommended range) To illustrate, a material that is so dense that it is translucent, and almost transparent, can be formed by hot pressing fused silica at 125()o C. at 2000 p.s.i. for a period of 3 hours. For such processing, the fused silica may be a half-and-half mixture of particles of --2l0-}-l05 micron size and -l05i45 micron size. Such material has significant strength, having a modulus of rupture of 700() p.s.i.
Turning attention now the FlGURE l, there is shown a typical press for hot pressing articles of fused silica. The ported by a graphite die bodv 3. A pair of graphite powdered fused silica is indicated generally at 1, confined Within a graphite sleeve or die 2, which in turn is suppunches d and 5 are siidably supported by the sleeve and serve to compress the powdered silica when pressure is applied to the punches by the rams of a hydraulic press (not shown), as suggested by arrows 6 and 7.
r[he graphite die body and fused silica are surrounded by fire brick S which in turn is held in position by quartz insulation 9. The insulation and fire brick form a furnace surrounding the die which may be heated in any suitable manner, as by electrical resistance heater l@ or by an induction heater. A thermocouple is shown at l1 to measure the temperature of the carbon sleeve adjacent the fused silica.
ln FGURE l graphite partitions f2 and i3 are shown Within the confines of the graphite sleeve. hrough use of such partitions, it is possible to make more than one, for example, three, in the case iliustrated, articles simultaneously.
in practice, the fused silica is confined within the graphite sleeve by the punches and the furnace is assembled as iilustrated. Pressure is then applied to the fused silica and the temperature is maintained for the desired period by the heating element. Because of the thermal shock resistance of fused silica, the articles may be removed from the dies as soon as the forming process is completed.
It is important to note that articles formed in this manner have great dimensional accuracy and stability. They will assume and hold exactly the dimensions of the graphite sleeve or other die that may be used.
For pressing operations carried out in the lower portion of the temperature range, dies may be made from stainless steel. ln die upper portion of the range, dies may be made from graphite, aluminum oxide, steel, cermets, refractory metals, etc. Since graphite dies are more easily fabricated than aluminum oxide dies, they are preferred. Even with the use of graphite, die deterioration is not a significant problem since it does not oxidize readily. For pressing operations carried on at the top of the temperature range, it may be desirable to exclude oxygen from the graphite dies by conventional methods.
Reinforcement of hot pressed materials has been found to be particularly effective. Novel forms of reinforcement are described in detail in co-pending application Serial No. 740,987, filed on .Tune 2, 1958, entitled Honeycomb Reinforced Material and Method of Making the Same by S. Motta, B. W. Rosen and T. Vasilos.
FIGURE 2 illustrates a hot pressed object which demonstrates some of the various types of reinforcements that may be used. In this figure, hot pressed silica is shown at lil-i within the cells of a metallic honeycomb reinforcing structure defined by walls 15. The entire honeycomb may be filled, as suggested at 16, or portions of the honeycomb cells may be unfilled, as indicated at f7. Voids, such as f7, may be formed during the process by filling the space to be left void by zirconium dioxide or similar materials which will not bond or harden during the forming process. Afterwards, the zirconium dioxide may be easily removed to form the voids.
The composite structure may be hot formed or coined at temperatures and pressures similar to those of the powder consolidation process.
Other types of reinforcements that have been found to be effective are metallic screens, such as shown at 1S.
To facilitate attachment of an object, such as shown in FIGURE 2, to a substructure, fasteners, such as bolt i9, may be directly embedded in the silica. ln the case illustrated, the threaded portion of the bolt projects from the silica while the head is securely anchored within the body of the silica. The threaded portion of the bolt obviously may be passed through an opening in the substructure and secured thereto.
For some purposes it is desirable to provide a metallic back on the object. This is illustrated at 2@ where a sheet of metal, which may be the same as that of the honeycomb, is secured to the terminating edges Z1 of a few cells of the honeycomb structure. T he backing sheet may be attached to the honeycomb by welding prior to hot pressing of the silica. It is possible, however, to form the object with the honeycomb projecting slightly from the silica, as explained in the before-mentioned application. In this case, the backing sheet may be brazed to the honeycomb after formation of the silica. For certain applications, the backing sheet may be cemented to the object.
Both FIGURES 2 and 3 illustrate that hot pressed fuse silica may be made with different degrees of porosity. Porosity can be controlled both through forming pressure and temperature, as has been indicated, or by inclusion of a foreign substance which can be burned away after the forming process to leave voids in the final structure. Not only can the porosity of a given article be controlled over wide limits, but a single article can be made having Zones of different porosity. Two illustrations are given.
ln FIGURE 2, the fused silica within zones A, B, and C have different porosities and may be separately formed at different times. The material of one zone may be separated from material of the adjacent zone by the honeycomb reinforcement itself. Within anyone Zone, however, such as Zone B, the porosity may also vary. rl`his is most easily explained by reference to FGURE 3, which shows a bar of hot pressed fused silica 22 having one zone Z3 at its left end having less than 5% porosity; zone 2d having 5 to 15% porosity; zone 25 having l5 to 25% porosity; and zone 25 having 25 to 50% porosity. These various zones may be produced in a single article as follows:
Zone 23: Pihis may be formed by hot pressing fused silica at 123C" C. and 3G60 psi. for a period of 2 hours. Zone 245;: Pl`his may be formed by compression at 120()D C.
and 2G09 p.s.i. for a period of one hour.
Zones 25 and 26: These zones may be formed at i260 C. and 2G00 p.s.i. for a period of one hour, a quantity of nely powdered graphite being included in die material of each zone prior to compression. Obviously, about twice as much graphite would be included in Zone 25 as in zone It will be noted that zones 24, 25, and 26 are all formed at the same temperature and pressure during an identical compression time. Thus, after formation of Zone 23, fused silica may be added to the die to form zone 2d, and fused silica bearing graphite may be added to form zones 25 and 2e, and the composite may then be co npressed for a period of one hour at 1200 C. and 2066 psi. After compression, the resulting article is a rigi consolidated composite mass of fused silica and graphite (or any other combustible material). During the compression process, the graphite and fused silica particles are contained within the dies and accordingly, in an environment which is substantially free of air or oxygen. Although the hot pressing takes place at 120i)o C. and graphite may be burned 883 C., no graphite is oxidized or burned away during compression.
After the compression process, the resulting rigid composite article may be fired at about 800 C. for a period of 48 hours to burn away the graphite inclusions, resulting in the desired graduated porosity in the nal article. The products of the combustion are not capable of deforming or otherwise changing the shape of the composite material since the fused silica, at the firing temperature of 860 C., is solid. It will be obvious that the size of the graphite particles may be chosen at will to provide a networlrL of interconnected voids of any desired size. Actual experiments have shown, however, that porosity is easily controlled by using graphite having a particle size of 80 to 20() microns.
The same principles may be employed in forming the hot pressed silica of diilerent porosity indicated at 27 and 2S in FGURE 2.
Instead of graphite inclusions to produce porosity, particles of other substances, such as metals (iron, for eX- ample), may be incorporated in the silica and removed after consolidation by leaching, such as by dilute hydrochloric acid.
Attention should now be directed to FIGURES 4 and 5 which show a disc 2@ of hot pressed fused silica within which is embedded an electrical resistance heating element 3?. The ends of the heating element are secured to terminals 31 and 3.?. to which conductors 53 and 34, leading to a power supply (not shown) are connected by bolts 3S.
It has been found by actual reduction to practice that the coefcient of expansion of hot pressed fused silica is so low that a heating element, such as 3d, can be fully ernbedded within the silica and operated at red heat without in any way damaging the surrounding siliceous material. ln fact, the structure is reasonably resistant to mechanical shock and is extremely resistant to thermal shoclr and will not crack even if chilled by cold water immediately after being operated at red heat'.
A heating element that is well adapted for use in fused silica is made of a chromium, nickel, iron alloy. This alloy contracts as it cools from the silica forming ten perature creating air spaces adjacent the heating element which prevent formation of internal stresses as the element heats up and cools down.
Obviously a structure such as shown in FGURES 4 and 5, is well adapted for use as a surface heating unit for a domestic electric range. An important. character'- istic of the structure, when used for such purposes, is its ability to transmit infrared radiation with practically no absorption since fused silica will transmit radiant energy having a wave length less than 5 microns with very little energy loss.
Thus, if the structure of FIGURES 4 and 5 were used as a heating unit, a cooking utensil placed upon it would be heated not only by direct conduction from the unit to the utensil but also by infrared radiation to the utensil. This is of great importance since it eliminates the need for conformity between the utensil and unit and is highly conducive to rapid heating of the utensil.
Some applications, instead of requiring high transmissivity, require a reduction of transmissivity. 1t has been found that this may be readily accomplished by including finely `divided carbon, such as lamp black, in the powdered fused silica. As little as 1% carbon by weight added to the powdered fused silica before hot pressing Will reduce radiant heat transfer through the silica by 97%.
This invention has been described with particular reference to powdered substances which :are formed into consolidated masses. .lt should be understood, however, that the invention may be broadly applied not only to powder (usually 1 micron to 1G00 micron size) but also to larger particles and fragments of the substance being consolidated. For instance, the invention may be applied in consolidating fragments, having dimensions `as large as one centimeter or more. The invention may also be applied to silica and other vitreous substances in fibrous form, such as silica wool.
Having :described our invention, we claim:
l. "the method of producing a translucent article of fused silica, a non-crystalline phase of sil-icon dioxide, comprising hot pressing powdered fused silica, a noncrystalline phase of silicon dioxide, at 1200 C. and at 20G() psi. for a period of at least three hours.
2. The process of producing `an object of fused silica, a non-crystalline phase of silicon dioxide, comprising subjecting powdered fused silica, a non-crystalline phase of silicon dioxide, to pressure in excess of 25S p.s.i. while maintaining the silica at a temperature in the range of 1050 C.-l450 C. until a consolidated mass is formed.
3. rlhe method of producing an article of fused silica, la non-crystalline phase of silicon dioxide, `which comprises placing powdered fused silica, a non-crystalline phase of silicon dioxide, between for-ming dies and maintaining the powdered fused silica, a non-crystalline phase of Asirens? 7 silicon dioxide, under pressure and at a temperature in the range of 1050o C.-1450 C. until a consolidated article is formed.
4. The process lof producing an article of fused silica, a non-crystalline phase of silicon dioxide, which is substantially free from cristobalite inclusions comprising subjecting powdered fused silica, a non-crystalline phase of silicon dioxide, to pressure at a temperature less than 1200 C. for a time period suiicieni; to `cause consolidation of the particles of silica.
5. The process Vfor hot pressing powdered fused silica, a non-crystalline phase of silicon dioxide, which comprises compressing the powder at a pressure in excess of 250 p.s.i. and at a temperature in the range of 1050 C. to 1450 C. below the range where cristobalite forms and maintaining the temperature and pressure until a rig-id product of said fused silica, a non-crystalline phase of silicon dioxide, is formed.
6. The process for producing a porous article of fused silica, a non-crystalline phase of silicon dioxide, which 2 comprises placing an adrnixture of powdered fused silica, a non-crystalline phase of silicon dioxide, and graphite within forming dies, subjecting the powdered admixture to pressure in excess of 250 psi. and heat in the temperature range of 1050 C. to 1450 C. until a solid article of said fused silica and graphite is formed having the shape of the dies, and subsequently subjecting the article to 800 C. temperature for a period yof time sullicient to burn the graphite out of the article to develop porosity therein.
7. The process of producing a fused silica, a non-crystalline phase of silicon dioxide, `article having zones of different porosity comprising subjecting powdered fused silica, a non-crystalline phase of silicon dioxide, to pressure in excess yof 250 psi. and temperature in the range of 1050 C. to 12150 C. sufcient to cause consolidation of the particles of silica thereby forming a first solid Zone within the article, placing `an admixture of powdered fused silica, a non-:crystalline phase of silicon dioxide, and graphite adjacent the first Zone and subjecting it to ternperature in the ran-ge of 1050 C. to 1450 C. and pressure in excess of 250 psi. suiicient to form a second zone of solidification comprising said fused silica and graphite 4within the article, and subsequently subjecting the article to 800 C. temper-rature for a time sufficient to burn the graphite out of the second zone whereby an rticle having zones of diierent porosity is produced.
8. The process for producing a porous article of fused silica, a non-crystalline phase of silicon dioxide, which comprises placing an admixture of powdered `fused sil-ica, `a non-crystalline phase of silicon dioxide, and a combustible substance within forming dies, subjecting the powdered admixture to pressure in excess of 250 p.s.i. and heat in the temperature range of 1050 C. to` 1450" C. until a consolidated article of said fused silica and said combustible substance is formed having the shape of the dies :and subjecting the consolidated article to the combustib'le temperature for a period of time sufficient to burn the combustible substance .out of and to develop poro-sity Within the article.
References Cited in the le of this patent UNTED STATES PATENTS 829,427 Potter Aug. 28, 1906 1,051,638 Rod-man lan. 28, 1913 1,465,545 Demongeot Aug. 21, 1923 1,536,821 Bevers May 5, 1925 1,610,182 Thomson Dec. 7, 1926 1,738,663 Morse Dec. 10, 1929 1,838,680 Hudson Dec. 29, 1931 2,270,718 Skaupy Jan. 20, 1942 2,310,432 Haux Feb. 9, 1943 2,459,209 Zagwyn lan. 18, 1949 2,762,487 Cummins et al Dec. 13, 1955 2,775,524 DEustaehio Dec. 25, 1956 2,837,873 Lunsavage lune 10, 1958 2,890,126 Ford Jurre 9, 1959 2,890,127 Ford lune 9, 1959 3,010,839 Drrunheller etal Nov. 28, 1961 FOREIGN PATENTS 425,413 GreatBritain Mar. 11, 1935 510,081 Great Brita-in July 20, 1939 `622,218 Great Britain Apr. 28, 1949 549,723 Canada Dec. 3, 1957 OTHER REFERENCES Science, vol. 118, July 31, 1953, pages 131, 132, article titled, A New Dense Crystalline Silica. (Copy in Scientic Library.)
UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent Ne, 116,137 DecemberV 31, 1963 Thomas Vasilos et al.,
It is hereby certified that error appears in the above numbered patent requiring correction and that the seid Letters Patent shouldread as corrected below.
Column 2q line 29 for "strinkage" read e shrinkage column 3, line 7l, for "the" read M to m-; column 3, line 73, strike out "ported by a graphite die body 3 A pair of graphite" and insert the same after "is supe" in line same column 3; column 5, line 22 for "anyone" readv any one Signed and sealed this 26th day of -May l94 (SEAL) Attest:
ERNEST W, SWIDER EDWARD J. BRENNER Attesting Officer Y Commissioner of Patents