US3615899A

US3615899A - Method of producing materials having a high strength a high electrical conductivity and a high heat resistance

Info

Publication number: US3615899A
Application number: US774356A
Authority: US
Inventors: Takao Kimura; Toyoaki Ishibachi; Yasushi Watanabe
Original assignee: Furukawa Electric Co Ltd
Current assignee: Furukawa Electric Co Ltd
Priority date: 1968-01-27
Filing date: 1968-11-08
Publication date: 1971-10-26
Anticipated expiration: 1988-10-26
Also published as: DE1807906A1; GB1223325A; DE1807906B2

Abstract

Materials having a high strength, a high electrical conductivity and a high heat resistance are produced by directly internally oxidizing a wire or plate of dilute copper alloy consisting of less than 0.2% Be, less than 0.5% mg. less than 1.0% Ti, less than 0.2% Zr, either single-componentwise or multicomponentwise, or therewith less than 0.9% Al and/or less than 0.9% Si with the total addition amount not in excess of 1%, the remainder being copper, at a temperature above 600* C. and at an internal oxidation velocity no smaller than a critical value. Such a production is carried preferably out by coating the wire or plate, with a slurry composed of cuprous oxide, a heat-stable sinter inhibitor, and water or an organic solvent, completely drying said slurry-coated wire or plate and then embedding said slurry-coated wire or plate in a protective agent and then effecting the internal oxidation.

Description

United States Patent Takao Kimura Yokohama;

Toyoaki Ishibachi, Fujisawa; Yasushi Watanabe, Nikko, all of Japan Nov. 8, 1968 Oct. 26, l 971 The Furukawa Electric Company Limited [72] Inventors [21 Appl. No. [22] Filed [45] Patented [73] Assignee Tokyo, Japan [32] Priorities Jan. 27, 1968 [33] Japan Aug. 31, 1968, Japan, No. 43/55152; Aug. 29, 1968, Japan, No. 43/61978 [54] METHOD OF PRODUCING MATERIALS HAVING A HIGH STRENGTH, A HIGH ELECTRICAL CONDUCTIVITY, AND A HIGH HEAT [50] Field of Search 148/13.2, 1 1.5, 32 56] References Cited UNITED STATES PATENTS 3,399,086 8/1968 Das et a1. 148/13.2

Primary Examiner-Richard 0. Dean Attorney-Stevens, Davis, Miller & Mosher ABSTRACT: Materials having a high strength, a high electrical conductivity and a high heat resistance are produced by directly internally oxidizing a wire or plate of dilute copper alloy consisting of less than 0.2% Be, less than 0.5% mg. less than 1.0% Ti, less than 0.2% Zr, either single-componentwise or multicomponentwise, or therewith less than 0.9% Al and/or less than 0.9% Si with the total addition amount not in excess of 1%, the remainder being copper, at a temperature above 600 C. and at an internal oxidation velocity no smaller than a critical value. Such a production is carried preferably out by coating the wire or plate, with a slurry composed of cuprous oxide, a heat-stable sinter inhibitor, and water or an organic solvent, completely drying said slurry-coated wire or plate and then embedding said slurry-coated wire or plate in a protective agent and then efiecting the internal oxidation.

internal oxidation veloclty or interparticle spacing Center of specimen Internal oxidation velocity lnterparticle spacing Distance from specimen surface PATENTEDnm 2s IQYI 3,615,899

swan 1 BF 4 Fig. l

g Center of speclmen c 9, Internal oxidation .2 a velocity B m time spacing Distance from specimen surface INVENTORS TAKAO KIMURA, TOYOAKI ISHIBACHI, YASUSHI WATANABE ATTORNEYS PATENTElJnm 2s m: 3,615,899

sum 2 [1F 4 Cier 0f scimmu Infernal pxldufion T55 velocity 2 8 lnferporficle spacing Distance from specimen surface INVENTORS TAKAO KIMURA, TOYOAKI ISHlBACHI, YASUSHI WATANABE BY My MM/ w ATTORNEYS PATENTEDUET 2619?! 3,615,899

SHEET 3 OF 4 Fig.

3.64XIO" (0.4)

Yield strength (kg/mm 6 8 Pure copper Annealing fempcb) INVENTORS TAKAO KIMURA, TOYOAKI ISHIBACHI, YASUSHI WATANABE ATTORNEYS PATENTEUUCT 2619?! 3,615,899

sum u or 4 Fig. 4

Yield strength (kg/mm m on 9 Pure copper 260 400 soo e00 :000

Annealing fempCC) INVENTORS TAKAO KIMURA, TOYOAKI ISHIBACHI, YASUSHI WATANABE BY MflWf/MMM ATTORNEYS METHOD OF PRODUCING MATERIALS HAVING A HIGH STRENGTH, A HIGH ELECTRICAL CONDUCTIVITY, AND A HIGH HEAT RESISTANCE The present invention relates to a method of producing materials having a high strength, a high electrical conductivity, and a high heat resistance.

The object of the present invention is to provide materials having more excellent strength, electrical conductivity, and heat resistance than ordinary materials, by means of direct internal oxidation of a wire or plate of a copper alloy containing a small amount of metal elements which have a higher negative free energy than the matrix and are selectively oxidized under well-defined conditions to be explained below, said wire or plate thus treated acquiring a high strength and a high conductivity, being recrystallization-resistant, and maintaining these properties even when annealed at high temperatures after working.

Ordinary high-strength, high-conductivity, high-heat-resistance materials such as Cu-Ti-Ni, Cu-Ti-Si, Cu-Ag, Cu-Ni- Si, Cu-Cr alloys and the like, are all a precipitation-hardened type of Cu-base alloys in which high strength and high conductivity are achieved firstly by quenching the alloys from a temperature higher than the precipitation temperature, whereby the solutes being forcedly dissolved in the matrix, and subsequently by annealing them at relatively low temperatures, whereby the solutes being precipitated from the matrix. These alloys are incapable of retaining the strength and conductivity thus obtained against annealing at a temperature high enough for the precipitates to redissolve into the matrix.

The recent remarkable development of electric and electronic devices such as motors, generators, computers, and electronic switching systems, clearly indicates a trend toward increased capacity, miniaturization, compactization of components, and further progress is possible only with the appearance of materials of better quality.

Among the typical materials widely used in this field, Cu-Ti- Ni alloys, the most heat resistant of all the precipitation hardened Cu-base alloys, lose considerably their strength when annealed above 600 C., and Cu-Ag alloys of the highest conductivity have a value of only about 83 percent IACS.

In these circumstances, dispersion strengthening has recently been proposed as a promising method for obtaining a material having a high strength, a high conductivity, and a high heat resistance. Conventional alloys of this type are produced either by internally oxidizing a powder of Cu-Al, Cu-Sj, Cu-Be alloys, etc., or by mechanically mixing copper powder with AI,O SiO,, or BeO powder, and subjecting the oxidized powder or the mixture to compression and sintering with or without further extrusion. The dispersion of fine oxides in the matrix is responsible for the enhancement of the concerned properties of these alloys. However, the highest electrical conductivity reached by these methods is only about 80 percent IACS. Moreover, these dispersion-strengthened al loys are too brittle to withstand subsequent drawing or rolling.

Former attempts at direct internal oxidation of a wire or plate of copper alloy did not yield a satisfactory result because of the failure in meeting various requirements for solute concentration, specimen size, atmosphere, oxidation temperature, etc.

The present invention aims at removing the difficulties connected with the conventional direct internal oxidation processes. The inventors have made extensive studies on the factors governing internal oxidation, and succeeded in controlling them and producing in an economical way materials having desired properties.

The present invention consists of a wire or plate of dilute copper alloy, said wire or plate being internally oxidized at a temperature above 600 C. and at an oxidation velocity higher than a critical value, and if necessary, said oxidized wire or plate being worked and heat-treated.

After extensive studies on the process of internal oxidation, the inventors have found that the final properties of internally oxidized materials are critically dependent on the internal oxidation velocity, or the interparticle spacings of dispersed oxides.

For a better understanding of the invention, reference is made to the accompanying drawings, wherein FIGS. I and 2 show the internal oxidation velocity and interparticle spacings as a function of depth for a wire and plate respectively, while FIGS. 3 and 4 show the yield strength versus annealing temperature for internally oxidized and cold-worked wires and plates respectively, with internal oxidation velocity (cm./hr.) and specimen size (mm.) (in parentheses) as parameters.

In FIGS. 1 and 2, point A is where the internal oxidation velocity becomes minimum, and point B where the interparticle spacings become maximum.

In general, the minimum internal oxidation velocity or the maximum interparticle spacings depend on solute concentration, form and size of specimen, internal oxidation temperature, oxygen pressure, pressures, etc. Alloys internally oxidized at a low velocity acquire a better electrical conductivity but no better strength than by conventional methods, and are susceptible to recrystallization around the position of the maximum interparticle spacings on a subsequent working and annealing at relatively low temperatures. The ascent of strength may be effected by increasing the minimum internal oxidation velocity, by increasing the solute concentration, or by lowering the internal oxidation temperature. The recrystallization temperature also can be raised in the same manner.

From the studies of the internal oxidation processes of dilute copper alloys containing, in addition to not more than 0.2% Zr (hereinafter percent means weight percent unless otherwise stated), at least one element selected from the group consisting of Be, Mg, Ti, Al and Si in the percentage of not more than 0.2% Be, not more than 0.5% Mg, not more than 1.0% Ti, not more than 0.9% Al and not more than 0.9% Si, the total addition amount including Zr not exceeding 1% of the dilute copper alloy, the inventors have reached the idea of a critical internal oxidation velocity. As long as the minimum internal oxidation velocity equals or exceeds this critical velocity, the oxidized alloys acquire a high strength, a high conductivity, and a high resistance to recrystallization, and these desirable properties can be further improved by subsequent working and heat treatment. Electron microscopic examination of said worked and annealed alloys showed no appreciable rearrangement of dislocations, which may account for the retention of as-worked strength against annealing. Further, the improvement in conductivity of said worked alloys upon annealing may then be ascribed to the annihilation of positive and negative dislocations lying within a short range, or to that of point defects, their clusters, and stacking faults, all contributing to the reduction of scattering areas for conduction electrons.

In general, the minimum internal oxidation velocity decreases with increase in specimen size, other conditions being equal. Therefore, a specimen of a size large enough for its minimum internal oxidation velocity to become smaller than the critical value acquires a relatively high electrical conductivity but only at the cost of strength, and is still more susceptible to recrystallization on cold working and annealing at high temperatures.

The present invention makes use of a series of dilute copper alloys containing, in addition to not more than 0.2% Zr, at least one element selected from the group consisting of Be, Mg, Ti, Al and Si in the percentage of not more than 0.2% Be, not more than 0.5% Mg, not more than 1.0% Ti, not more than 0.9% Al and not more than 0.9% Si, the total addition amount including Zr not exceeding l% of the dilute copper alloy.

The limitations to the kind and concentration of solutes are based on the investigations by the inventors into the influence of the solutes on the mechanical, electrical, structural properties of an oxidized wire or plate. The size of dispersed oxide particles increases in the order of beryllium, magnesium, zirconium, aluminum, titanium, and silicon, namely, in the decreasing order of negative free energy oxide formation. Conversely, in order to obtain given interparticle spacings for the same volume content of oxides, the addition amount may be in the reversed order. Moreover, the electrical conductivity of an internally oxidized alloy containing a given volume contact of oxide decreases in the order of silicon, titanium, aluminum, zirconium, magnesium, and beryllium, namely, in the increasing order of negative free energy of oxides decreases in the order of silicon, titanium, have a sufficient solubility in copper, and the workability of an oxidized wire or plate decreases in decreasing order of solubility, or increasing degree of segregation of solutes along grain boundaries. Zirconium is meant as a grain finer as well as an oxide former. Its presence makes the working of an internally oxidized alloy easier than otherwise.

In the presence of more than 0.2% Zr, or more than 0.2% Be, or more than 0.5% Mg, or more than 1.0% Ti, or more than 0.9% A1, or more than 0.9% Si, or in the presence of these elements to a total of over 1% of the alloy, the optimum conductivity is no longer attainable, and the segregation of oxides along grain boundaries makes subsequent working of oxidized alloys extremely difficult or impossible. 4

In order to obtain materials of superior quality, it is preferable to use copper alloys containing, in addition to 0.01 to 0.15% Zr, at least one element selected from the group con sisting of Be, Mg, Ti, Al, and Si in the percentage of 0.01 td 0.1% Be, 0.05 to 0.4% Mg, 0.05 to 0.8% Ti, 0.05 to 0.8% A1; and 0.05 to 0.8% Si, the total addition amount including Zr not exceeding 1% of the dilute copper alloy. The most desira ble compositions are 0.01 to 0.15% Zr, 0.01 to 0.1% Be, and the remainder Cu; 0.01 to 0.15% Zr, 0.05 to 0.8% Al and the remainder Cu; and 0.01 to 0.15% Zr, 0.01 to 0.1% Be, 0.05 to 0.8% A1 and the remainder Cu.

1n the present invention, the internal oxidation is carried out by heating the solid alloy in the presence of cuprous oxide (Cu,0) to a temperature and for a length of time sufficient to insure the complete oxidation of the solute of the alloy. B taking advantage of the dissociation of oxygen from th cuprous oxide, all of the complications arising from the control of oxygen pressure may be easily avoided.

The novel aspects of the internal oxidation processes of thd invention consist in, prior to heating, coating the solid alloy, in the form of wire or plate, with a slurry composed of Cu O, ari inhibitor, and a solvent, completely drying the slurry-coated alloy, protecting the alloy in a pack of shroud powder front ambient air, and then heating the alloy to a temperature abov 600C. and at an internal oxidation velocity not smaller than critical value.

In the conventional procedure of internal oxidation, the alloy is heated in a pack of a shroud powder consisting of Cu,() or a mixture of Cu O and Cu powder. It has now been found by experience that heating the solid alloy, particularly in the form of coils, be it of a wire or plate, in a pack of conventional shrouds, is inevitably accompanied by the sintering of the coils at the positions of mutual contact, the agglomeration of shroud powder particles, and the sticking of the particles to the surface of the alloy, which causes great trouble to the packing and unpacking of the alloy.

Now according to the present invention, it has been found that, if the alloy coils, usually in a bundle, are pervasively im-.

mersed in a slurry composed of Cu,0 powder, another oxide powder called inhibitor, and a solvent, dried, embedded in a pack of shroud powder, and then heated, one can completely suppress the sintering between coils. Examples of the inhibitor are A1 MgO, MgzrO CuO, SiO and a mixture thereof. it has been further found by experience that the desired effects of the invention are produced not in the presence of an inhibitor alone but only in the simultaneous presence of an inhibitor and Cu,0 in the slurry. Examples of the slurry are a mixture of Cu,0 and Ago, powder at a weight ratio of 3:7; a mixture of Cu,0 and MgO at a weight ratio of 5:5; a mixture of Cu O and SiO at a weight ratio of 3:7. Examples of the solvent are water, methanol, ethanol.

In the conventional internal oxidation procedure, Cu O was a major constituent of the shroud. 1n the present invention, however, the major constituent of the shroud is any one of the inhibitor group oxides, and whether or not the shroud contains Cu O is immaterial, for it is already present in the slurry. In this way the agglomeration of the shroud particles due to sintering, or the surface roughening of the alloy due to the sintering or the shroud particles thereon may be easily avoided. Further, considerable economies may be achieved in the use of CLlgO, which is another advantage of the invention.

Both the inhibitor and the shroud must be thermally stable and inactive against the materials to be oxidized. They may be alumina, magnesia, magnesium zirconate, silica, cupric oxide, or a mixture thereof. The solvent to be used for the slurry may be water or highly volatile organic compounds having a boiling point lower than 200 C. and being inactive against the material to be oxidized and slurry components.

The limitation of internal oxidation temperature to above 600 C. is made by the following reason. Internal oxidation can be effected at any temperature within this range. At temperatures below 600 C., the time necessary to complete internal oxidation becomes so long that there is no industrial benefit.

However, below 800 C., grain boundaries tend to be selectively oxidized, which results in the segregation of oxides there, while above l,000 C., the oxides tend to make harmful growth, both to a varying degree depending on the kind of alloy to be oxidized. For this reason, it is preferable to carry out internal oxidation in the temperature range of 800 C. to 1 ,000 C.

The following examples are given in illustration of the present invention and are not intended as limitations thereof.

EXAMPLE 1 Cu-0.038% Be alloy wires having various diameters were internally oxidized at 1,000 C., cold drawn to 60 percent of the original cross-sectional area, and then annealed at various temperatures for 1 hour. FIG. 3 shows the yield strengths of these annealed wires, with internal oxidation velocity (cm/hr.) and specimen size (mm.) (in parentheses) as parameters. As seen from this figure, increasing internal oxidation velocity has the effect of suppressing the decreasing tendency of yield strength with temperature. Electron microscopic examination revealed that the decrease of yield strength with temperature is mainly due to the occurrence of recrystallization. To be more specific, for a minimum internal oxidation velocity of 8.06-9.l 1X10 cm./hr., recrystallization sets in at a temperature as low as 300 C. and strength begins to fall down there, and for a velocity of l.04 l0", the recrystallization temperature rises as high as 600 C., and finally for a velocity higher than 1.21X10 even an annealing at 1,000 C. cannot induce recrystallization. In other words, this Cu-0.038% Be wire has a critical internal oxidation velocity of 1.21X l0" cm./hr.

Plates of Ctr-0.083% Be alloy having various thicknesses were internally oxidized at 1,000 C., cold rolled to 50 percent of the original thickness, and annealed at various temperatures for 1 hour. FIG. 4 shows the yield strengths of these plates against annealing temperatures. As in the case of wires, the recrystallization temperature rises and the decreasing degree of strength with temperature is suppressed, with increasing minimum internal oxidation velocity. A plate internally oxidized at a minimum velocity larger than 4.90 l0 cm./hr. resists recrystallization even on an annealing at a temperature as high as l,000 C. That is, the critical internal oxidation velocity of this Cu-0.083% Be alloy plate is 4.90Xl0 cm./hr.

EXAMPLE 2 Wires of various diameters having the compositions as shown in table 1 were treated with a slurry made of methyl alcohol, 3 parts of cuprous oxide and 7 parts of silica, and after complete drying, embedded in a protective agent composed of 1 part of alumina and 9 parts of cupric oxide powder, and completely internally oxidized at l,000 C. The oxidized wires were cold drawn to about 90 percent of the original cross-sec- Table 3 Alloy composition Critical internal (Z by weight) oxidation velocity tional area, and then annealed at either 800 C. or l,000 C. 5 (cm/hr) for 1 hour. The critical internal oxidation velocities measured at 1,000 C. are listed in the right-hand column of table 1. 01 995; Be 7 490x10" Cir-0.13 AI-0.04 Zr 157x10" Table 1 l0 Table 4 shows the mechanical and electrical properties of 0.4 mm. thick plates which were completely internally ox- Auuy composmon Cmicay internal idized and subsequently subjected to various thermal and 91, by weight) oxidation velocity mechanical treatments as shown at the headings of the same table. The size 0.4 mm. thick was chosen so that the minimum internal oxidation velocity be greater than the critical velocity. i -0. 3 B 134x10" As tables 2 and 4 show, the materials internally oxidized by 2:32; 2 the present method possess an excellent strength and conducr r r o CHHO AH); 1, tivity even in its as-oxidized state, and are capable of Cu-0.20Al-0.02 zr 497x10" withstanding subsequent severe working which further ims 913x10 proves the strength with very little sacrifice of conductivity,

II n o r r e cu'om aeom 105x") 1 and the worked materials are highly recrystallization resistant Cll0.l0 $141.08 AI0.02 zr 954x10" i 850m 930x105 and maintains most of their as-deformed strength even after annealing at very high temperatures.

v TABLE 4 J Cold rolled and an- Cold rolled and an- As internally oxidized As cold rolled healed one hour at nenled one hour at (kgJmmfl) (kg/mm!) 800 C. (kg/mm?) 1,000 C. (kg/mm!) Yield Tensile Yield Tensile Yield Tensile Yield Tensile Alloy composition (percent by strength strength strength strength strength strength strength strength weight) Gil-0.083 Be 26. 4 1 32. 1 47. 0 47. 2 41. 0 43. 8 39. 8 42. 7 Cu-0.13 A10.04 z 18.9 27. 5 38.6 38. 7 28.8 33. s 27. 7 33. 5 Table 2 shows the mechanical and electrical properties of EXAMPLE 3 .4 i w l l g 2 2 g z ig zig In 2 :2": 2222 2 312; is Wires and a plate having the size and compositions as shown t m th n h I t t ts in table 5 were coated with slurries as shown in table 6, and an Jec z g g an g it. g l after complete drying, embedded in protective agent as shown 2. L 6 ea 5: i a t e 2'"- 40 in table 7, and internally oxidized at 1,000 c. The size 0.4 t ,i c i i m m ema ox] a on mm. thick was chosen so that the minimum internal oxidation ve oclty e greater an t e cmlca ye velocity be greater than the critical velocity.

TABLE 2 As internally oxidized As 90% cold drawn Conduc- Conduc- Yield Tensile tivity Yield Tensile tivity Alloy composition (percent strength strength (IACS strength strength (IACS by weight) (kg/mm!) (kg/mm?) percent) 1 (kgJrnmfl) (kg/111m!) percent) Oil-0.03 Be 26. 7 39. 4 94. 9 62. 6 62. 6 91. 1 01141.05 Al-0.02 Be- 24.1 36. 9 96. 0 57. 6 57. 6 92. 2 Cu-0.03 Be 0.02 Zr- 26. 9 40.1 93. 6 6.30 63. 5 89. 8 (Du-0.10 A1002 Zr 16. 8 29. 5 97.1 56.6 58.0 91. 8 0110.20 Al-0.02 Zr 23. 3 34. 2 96. 4 58. 6 59. 5 92. 2 Cu005 A1010 M 16. 5 30. 4 93.9 52. 3 52. 3 s9. 9 (Du-0.05 A1002, Be-0.02 24.6 38.5 90.9 58.2 58.5 117.4 Cu-0.10 81-0 08, A1002 Zr. 17. 0 29. 3 95.0 55. 0 56.1 91.3 Gil-0.10 Ti-0.02, Be-0.02 Zr- 24. 0 35. 9 92. 2 57. 3 57. 5 83. 3

Cold drawn and annealed one hour Cold drawn and annelaed one Table 5 at 800 0. hour at 1,000 0. Yield Tensile Oonduc- Yield Tensile Conduc- AHOY shape of strength strength tivity strength strength tivity composition material g/ e/ (IACS g g-l A mm!) mm?) percent) mm.- mm?) percent) 41 5 45 s 92 9 3s 2 44.0 93 0 (k by weight) 39: 1 44' 9 94' 1 36: 5 43. 6 9410 3 41.7 46.3 91.5 37.4 43.8 91.4 a 3 8 4&6 M 3 8 4a 8 6 Cu-O.l3 Be-0.04 Zr 0.4 mm. plate 40. 3 47. 2 93. 7 35. l 42. 2 93. 6 28.? g 5 32. 5 3g. 6 91. 5 .9 7.2 4 .5 88.9 36. 6 43. 9 92. 0 31. 4 38. 9 93. 0 Table 6 38. 7 45. 3 90. 2 35. 7 41. 8 90. 3

Slurry composition Mixing ratio Plates of various thicknesses having the compositions as i s w 501m" shown in table 3 were internally oxidized and mechanically as well as thermally treated in the same manner as with the above Cuprous oxide-alumina powder 3:7 methyl wires except that the reduction applied is 50 percent of the C I s S i e c c n uprous OX] e-magnesia I ace one original thickness. The critical internal oxidation velocities Cupmusoxide smm Oxide 3:7 mm

found are listed at the right-hand column of the same table.

Claims

2. The method as claimed in claim 1, wherein said alloy contains, besides 0.01 to 0.15% Zr, at least one element of the group consisting of 0.01 to 0.1% Be, 0.05 to 0.4% Mg, 0.05 to 0.8% Ti, 0.05 to 0.8% Al, and 0.05 to 0.8% Si, with the total addition amount not exceeding 1 percent, and the remainder copper.
3. The method as claimed in claim 1, wherein said alloy consists of 0.01 to 0.1% Be, 0.01 to 0.15% Zr, and the remainder copper.
4. The method as claimed in claim 1, wherein said alloy consists of 0.05 to 0.8% Al, 0.01 to 0.15% Zr, and the remainder copper.
5. The method as claimed in claim 1, wherein said alloy consists of 0.05 to 0.8% Al, 0.01 to 0.1% Be, 0.01 to 0.15% Zr, with the total addition amount not in excess of 1 percent, and the remainder copper.
6. The method as claimed in claim 1, wherein said internally oxidized wire or plate is further worked.
7. The method as claimed in claim 1, wherein said internally oxidized wire or plate is further worked and heat treated.
8. The method as claimed in claim 1, wherein said inhibitor is from the group consisting of alumina, magnesia, magnesium zirconate, cupric oxide, silica, and a mixture thereof.
9. The method as claimed in claim 1, wherein said shroud is from the group consisting of alumina, magnesia, magnesium zirconate, cupric oxide, silica, and a mixture thereof.