Process for forming 312 phase materials and process for...

Details Process for forming 312 phase materials and process for... Process for forming 312 phase materials and process for...

1999-12-22

2002-10-08

Group, Karl (Department: 1755)

Compositions: ceramic

Ceramic compositions

Carbide or oxycarbide containing

C501S088000, C501S091000, C501S096300, C501S097100, C423S324000

Reexamination Certificate

active

06461989

ABSTRACT:

BACKGROUND OF THE INVENTION
Metals are generally easily machined but do not retain their machined form at high temperatures. Ceramics retain their shape at extremely high temperatures, but are brittle and very difficult to machine into a desired shape. Materials scientists have directed a great deal of effort towards finding compositions that are easily machined into a desired shape and are stable at extremely high temperatures.
One compound which is known to maintain its shape and form at high temperatures and possess machinability similar to metals is Ti
3
SiC
2
. Ti
3
SiC
2
is a type of compound known generically as a 312 phase material. Preparation of Ti
3
SiC
2
was reported as early as 1967, when powdered titanium hydride, silicon and graphite were combined in a self-contained graphite capsule at 2000° C. for about 20 minutes, and then quickly cooled down to 1200° C. The product was extracted to obtain a sample of Ti
3
SiC
2
, which was characterized in terms of its crystal structure. The structure was found to be hexagonal and comprised planar Si-layers linked together by TiC octahedra having a theoretical density of 4.51 g/cm
3
.
Chemical vapor deposition (CVD) approaches to Ti
3
SiC
2
have been reported wherein SiCl
4
, TiCl
4
, CCl
4
and H
2
were used as source gases at a deposition temperatures of 1573-1873° K. and total gas pressure in the CVD furnace fixed at 40 kPa. A monolithic plate of 40 mm by 12 mm by 0.4 mm was obtained at a deposition rate of 200 micrometers/hour.
Since then, other methods of synthesizing Ti
3
SiC
2
have been described. For example, the vacuum calcination of a compacted mixture of titanium, titanium carbide and silicon powders at temperatures from 1300° C. to 1600° C. to form Ti
3
SiC
2
has been reported. Such processes are thought to result in the vaporization, and consequent loss of silicon into the surrounding atmosphere. Attempts to adjust the initial silicon loading to anticipate silicon vaporization followed. However, such processes have not been able to prepare pure Ti
3
SiC
2
phase. It is generally regarded that the thermal decomposition of Ti3SiC2 proceeds via the following reaction, as described in Racault, C., et al., “Solid-state Synthesis and Characterization of the Ternary Phase Ti
3
SiC
2
”, J. Mat. Science
, Vol. 29, pp. 3384-92 (1994):
Ti
3
SiC
2
(s)→>3TiC
0.67
(s) +Si(g).
Accordingly, temperature is thought to be the only variable in the decomposition reaction of Ti
3
SiC
2
.
Researchers have also reported forming Ti
3
SiC
2
by igniting a stoichiometric mixture of titanium, silicon and carbon black which was either cold-pressed and the resulting pellet placed in a graphite crucible and heated extremely rapidly from 800° C. to 1020-1080° C., or was placed as a loose powder in a graphite-lined boat and the boat contacted with a heating coil at 1830° C. Under either set of conditions such mixtures ignited, causing a very rapid increase in temperature with concomitant formation of Ti
3
SiC
2
. The product as formed by either method was porous and contained titanium carbide (10-20%), as opposed to a more pure Ti
3
SiC
2
phase.
Other methods for producing purer 312 phases have involved multi-step procedures including subsequent treatments with aqueous hydrogen fluoride to remove TiSi
2
and leave a material consisting of 85% Ti
3
SiC
2
and 15% TiC, followed by steps of controlled oxidation at 450° C. in air for 10 hrs, which converts TiC to TiO
2
, and dissolution of the TiO
2
with a mixture of ammonium sulfate and sulfuric acid at about A method for the preparation of Ti
3
SiC
2
from a mixture of titanium, silicon and carbon powders has been reported. A powder mixture was compacted to pellets, optionally arc-melted in an argon atmosphere, and then heated in evacuated quartz tubes at either 900° C. for 24 hr. (no Ti
3
SiC
2
formed), 1400° C. for 5 hr. (Ti
3
SiC
2
with other phases formed but quartz tubes exploded) or 1200° C. for 100 hr (Ti
3
SiC
2
formed in addition to other phases). The arc melting process caused some loss of silicon and carbon, but yielded samples having superior homogeneity. Treatment of the product with hydrofluoric acid, to leach out titanium suicides, was necessary to prepare final powders with over 99% phase-pure Ti
3
SiC
2
.
Thus, there is a great deal of interest among materials scientists in preparing essentially pure 312 phase materials comprising Ti
3
SiC
2
. However, as discussed above, most efforts to maximize the percentage of 312 phase produced by such prior art methods have centered around the assumption that silicon evaporates as Si(g). Unfortunately, most prior art methods for preparing 312 phase materials have been unable to produce substantially pure 312 phase materials in an economic, one-step process not involving subsequent treatments which are time-consuming, expensive and inefficient.
U.S. Pat. No. 5,942,455 of Barsoum, et al. describes a one-step synthesis of 312 phases and composites thereof. Barsoum, et al. teach a process for the formation of 312 phases wherein relatively pure samples are prepared by heating powder mixtures under “non-oxidizing” atmospheres which are exemplified by and otherwise understood to be inert gases, optionally under applied pressure. However, the use of inert gas atmospheres at atmospheric pressure alone does not produce highly pure samples of 312 phases. Furthermore, the application of such external pressures can be expensive and may render production uneconomical.
Thus, there is a need in the art for a process by which highly pure 312 phase materials such as Ti
3
SiC
2
can be produced in a simple, one-step, economical manner.
BRIEF SUMMARY OF THE INVENTION
The present inventors have found that the high temperature thermal decomposition of Ti
3
SiC
2
proceeds by a different chemical reaction than that which was previously believed. As discussed below, the present inventors have found that the high temperature thermal decomposition of Ti
3
SiC
2
proceeds according to the following reaction:
Ti
3
SiC
2
+½O
2
→3TiC
0.67
+SiO(g)
and thus, have realized the importance of oxygen concentration in the formation of unwanted by-products such as TiC
0.67
. Additionally, during the initial formation of 312 phase materials, such as Ti
3
SiC
2
, oxygen may become involved in unwanted interactions with precursors and intermediates, resulting in a reduction in the purity of the 312 phase materials produced.
Thus, the present invention is directed to a process for forming highly pure 312 phase materials in a simple, one-step manner, by controlling oxygen partial pressure during formation of 312 phase materials.
The present invention includes a process for forming a material comprising an M
3
X
1
Z2 phase wherein M is at least one transition metal, X is at least one of Al, Ge and Si, and Z is at least one of B, C and N, said process comprising the steps of: (a) providing a mixture of (i) at least one transition metal species, (ii) at least one co-metal species selected from the group consisting of aluminum species, germanium species and silicon species and (iii) at least one non-metal species selected from the group consisting of boron species, carbon species and nitrogen species; and (b) heating said mixture to a temperature of about 1000° C. to about 1800° C., in an atmosphere within a substantially enclosed heating zone, for a time sufficient to form said M
3
X
1
Z
2
phase; wherein the atmosphere has an O
2
partial pressure of no greater than about 1×10
−6
atm.
According to the present invention the process for forming a material comprising an M
3
X
1
Z2 phase can be performed by heating said mixture in step (b) at substantially atmospheric pressure, or under an applied vacuum wherein the atmosphere preferably has an O
2
partial pressure of no greater than about 1×10
−8
atm, and preferably at a rate of no greater than about 25° C./min.
The present invention also includes a process for a forming dense, substantially single-phase M
3
X
1
Z
2
phase workpiece, said process comprising the st

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