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(11) | EP 0 156 454 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Thermionic electron emitter |
| (57) A thermionic electron emitter includes an electron emissive layer (5) formed solely
of an alloy containing osmium in the proportion, expressed in atomic percent, in the
range from 32 percent to 34 percent inclusive, the balance being of tungsten, and
a porous refractory medium (4) impregnated with an alkaline earth activator (FIG.2).
In another example the activator is confined on the side of the medium remote from
the layer (FIG.3). Alternatively (FIG.4) the layer is porous and is impregnated with
the activator, or the activator is confined on the side of the porous layer remote
from the electron emissive surface thereof (FIG.5). |
[0002] A known form of thermionic cathode, commonly referred to as "M" type, is disclosed in U.S. Patent 3,373,307. The cathode comprises a matrix of tungsten or tungsten-molybdenum in reactive relationship with an alkaline earth activator which supplies free barium or barium oxide to the-emitting surface of the matrix. A thin porous coating of a refractory metal having a work function higher than that of tungsten is applied to the emitting surface by sputtering, for example. The coating is selected from the group of osmium, iridium, ruthenium, and rhenium, although osmium is preferred. The resultant cathode exhibits increased electron emission at the same temperature, or the same electron emission at a lower temperature, as compared with emission from a cathode without the layer.
[0003] In a development of the "M" type cathode disclosed in US Patent 3,497,757 the coating comprises a thin porous layer of an alloy of osmium and iridium or osmium and ruthenium which serves to prolong the life of the cathode and reduces manufacturing difficulties due to the tendency for osmium to form an extremely toxic oxide.
[0004] Another development of the "M"-type cathode is described in an article entitled "Surface and Emission Characteristics of the Impregnated Dispenser Cathode" (Jones, MacNealy, and Swanson) in "Applications of Surface Science 2 (1979)" pages 232-257, North-Holland Publishing Company. This development is an IDC (impregnated dispenser cathode) made by Spectra-Mat Inc. of Watsonville California USA. This cathode has a sputter coating of osmium-rhuthenium alloy, the coating having a random columnar structure. This cathode has improved emission which is attributed, at least in part, to the geometric form of the surface structure.
[0005] U.S. Patent 4 165 473 discloses a type of thermionic cathode different to "M"-type, the "mixed matrix" type. A preferred example of this cathode comprises particles of pure iridium mixed in fixed proportions with particles of pure tungsten. The particles are sintered to form a compacted porous matrix. The matrix is filled with an active material in the form of an alkaline earth aluminate. Some alloying occurs at the particle boundaries, but for optimum results such alloying must be incomplete. The emission of such a cathode is greater than that of an "M"-type cathode, the optimum proportions of iridium and tungsten being 20% iridium and 80% tungsten. The iridium and tungsten mixture may--be replaced by pure iridium, osmium, ruthenium, or rhenium or mixtures thereof or by a mixture of tungsten and one of those metals.
[0006] European Patent Application 80300910 (Publication No. 19992) describes an improved form of dispenser cathode having the advantage of enhanced electron emission and prolonged life. This form of cathode comprises an electron emissive layer formed of an alloy of tungsten and osmium, and an associated alkaline earth activator.
[0008] In accordance with one aspect of the invention there is provided a thermionic electron emitter including an electron emissive layer formed solely of an alloy containing osmium in the proportion, expressed in atomic percent, in the range from 32 percent to 34 percent inclusive, the balance being of tungsten; and an alkaline earth activator.
[0009] In an embodiment said electron emissive layer comprises a coating of said alloy provided on a porous refractory medium impregnated with said alkaline earth activator. Alternatively, the alkaline earth activator may be confined on the side of said porous refractory medium remote from said layer.
[0010] In another embodiment said electron emissive layer is porous and is impregnated with said alkaline earth activator. Alternatively said alkaline earth activator may be confined on the side of said porous, electron emissive layer remote from the electron emissive surface of said layer.
[0011] The alkaline earth activator may be a mixture of barium oxide, or a compound of barium, reducible on heating to said oxide; an oxide, or a compound which decomposes on heating to the oxide, of an alkaline earth metal other than barium and at least one of aluminium or boron oxide. The alkaline earth metal other than barium may be a metal selected from the group consisting of calcium, strontium and magnesium.
[0012] According'to a further aspect of the invention there is provided a method of making a thermionic electron emitter comprising the steps of forming an alloy of the kind defined in said one aspect of the invention and incorporating said alloy and an alkaline earth activator in the emitter.
[0013] According to a yet further aspect of the invention there is provided a thermionic cathode including a thermionic electron emitter of the kind defined in accordance with the said one aspect of the present invention.
[0014] In order that the invention may be carried into effect embodiments thereof are now described, by way of example only, by reference to the accompanying drawings of which:
Figure 1 shows how electron emission density of-a thermionic electron emitter varies as a function of alloy composition and
Figures 2 to 5 show thermionic cathodes having different forms of electron emitter constructed in accordance with the present invention.
[0015] Over the past few years an extensive programme of axperimental and theoretical research has been carried out by the inventors with the aim of producing a thermionic electron emitter which, as compared with hitherto known electron emitters of the kind described, for example, in the above-mentioned United States Patents, has enhanced electron emissive properties.
[0016] As described hereinbefore, European Patent Application No. 80300910 (Publication No. 19992) discloses a particularly useful thermionic electron emitter including an electron emissive layer, formed of an alloy of tungsten or molybdenum and osmium, and an associated alkaline earth activator. It was considered that optimum results would be achieved if the proportions of alloy constituents used fall in a preferred range - namely 20 to 30% osmium and 80 to 70% tungsten. Another composition considered to be useful was 40% osmium and 60% tungsten. While materials having these preferred compositions are found to have beneficial electron emissive properties the inventors have now discovered that, contrary to expectation; a superior material, with greatly enhanced electron emissive properties, is produced if the composition lies outside the above-mentioned preferred range.
[0017] The inventors have found, in accordance with the present invention, that greatly enhanced electron emission is achieved if the alloy comprises osmium in the range (expressed in atomic percent) from 32 percent to 34 percent inclusive, the balance being of tungsten.
[0018] The relative proportions of the alloy constituents used is found to be remarkably critical. This fact is demonstrated in Figure 1 of the drawings which shows how the zero field electron emission density e is found to vary as a function of alloy composition - assuming a cathode operating temperature of 1340K. It will be apparent that the electron emission density exhibits a striking peak, centred on an osmium composition of 33 atomic percent; in comparison, the electron densities achieved at compositions of 25 atomic percent osmium or 40 atomic percent osmium, for example, are relatively small.
[0019] The inventors find that this striking improvement in electron emission is achieved for alloy compositions lying in the above-defined, relatively narrow range (i.e. from 32 percent osmium to 34 percent osmium inclusive, the proportions being expressed as atomic percentages).
[0020] Figure 2 of the drawings shows a cross-sectional view through a thermionic cathode 10 including a cylindrical body 1 of molybdenum, for example, containing a heating element 2 in a cavity 3 of the body, and a thermionic electron emitter constructed in accordance with one example of the present invention.
[0021] The emitter, in this example, comprises a porous plug 4 of tungsten or another suitable refractory medium, such as molybdenum, which is impregnated with an alkaline earth activator, and an electron emissive layer 5 in the form of a coating provided at the exposed surface of the plug. Layer 5 comprises an alloy of 67 atomic percent tungsten and 33 atomic percent osmium formed at the exposed surface of the plug by cosputtering osmium and tungsten in the desired proportions. Alternatively, the alloy could be formed -by co-evaporating the constituent metals or by co-precipating the metals from reducible compounds thereof.
[0022] Typically, the thickness of layer 5 is around 4000 Angstrom Units, although alternatively a thickness in the range 2000 Angstrom Units to 25,000 Angstrom Units, for example, could be used.
[0023] The alkaline earth activator in this example comprises a mixture of barium oxide, calcium oxide and aluminium oxide in the molecular proportions 3:;:1 respectively, although, as will be described in greater detail hereinafter other constituents and proportions may usefully be employed.
[0024] In another example of the invention, shown in Figure 3 of the drawings, the alkaline earth activator is contained within a cavity 6 behind the porous plug 4.
[0025] In a yet further example of the invention, shown in Figure 4 of the drawings, the thermionic electron emitter comprises a porous layer 7 of the tungsten/osmium alloy impregnated with an alkaline earth activator - again a mixture of barium oxide, aluminium oxide and calcium oxide, in the molecular proportions 3:1:1 respectively is used. As before, in this example, the alloy contains 33 atomic percent osmium and 67 atomic percent tungsten.
[0026] The emitter of Figure 4 may be constructed by initially pressing a mixture of the powdered alloy constituents (W and Os) in the desired proportions at, for example, 10 tons per square inch. The mixture, so pressed, is then sintered (e.g. at 2500°C for 30 minutes) to achieve a porosity of approximately 20% and the sinter then furnaced at a temperature, and for a time, sufficient to allow full osmium/tungsten interdiffusion to occur (e.g. at a temperature in the range 1800° to 2000°C for 5 to 10 hours). The porous layer of tungsten/osmium alloy, thus formed, is then impregnated with the alkaline earth activator. Impregnation techniques are well known in the art and are described, for example, in US 3,201,639.
[0027] The emitter of Figure 4 may be constructed by an alternative method involving pressing a powder of an alloy of tungsten and osmium (in the proportions 33 atomic percent osmium and 67 atomic percent tungsten), sintering the powder, so pressed (e.g. at 25000C for 30 mins.) to achieve a porosity of about 20% and then impregnating the porous layer, so formed, with the alkaline earth activator.
[0028] In yet a further example of the invention, illustrated in Figure 5, the alkaline earth activator is contained within a cavity 8 behind the porous alloy layer, formed as described hereinbefore.
[0029] Although the examples, described hereinbefore, concern use of an alloy of tungsten and osmium in the proportions 67 atomic percent and 33 atomic percent respectively, the present invention, as described hereinbefore, encompasses a thermionic electron emitter including an alloy of osmium and tungsten in the proportions of 33 + 1 atomic percent and 67 + 1 atomic percent respectively (i.e. tungsten in the proportion (expressed in atomic percent) in the range from 66 percent to 68 percent inclusive and osmium in the proportion (expressed in atomic percent) in the range from 34 percent to 32 percent inclusive).
[0030] Furthermore, although an alkaline earth activator in the form of barium oxide, calcium oxide and aluminium oxide in the molecular proportions 3:½:1 respectively has been described, other proportions and materials may be used. For example, the barium oxide, calcium oxide and aluminum oxide may be in the proportions 4:1:1 or 5:3:2 respectively. Furthermore, instead of using calcium oxide, another oxide of an alkaline earth metal (other than barium) may be used; for example, the oxide of strontium or magnesium or a mixture of the oxides of any two or more of calcium, strontium and magnesium. Alternatively the carbonates of the alkaline earth metals, mentioned hereinbefore, which decompose on heating to the oxide, could be used. Instead of using aluminium oxide, boron oxide may be used.
1. A thermionic electron emitter including an electron emissive layer (5) and an alkaline
earth activator (4) characterised in that said layer (5) is formed solely of an alloy
containing osmium in the proportion, expressed in atomic percent, in the range from
32 percent to 34 percent inclusive, the balance being of tungsten.
2.' A thermionic electron emitter according to Claim 1 wherein said layer (5) is provided
as a coating on a porous refractory medium impregnated with said alkaline earth activator.
3. A thermionic electron emitter according to Claim 1 wherein said layer (5) is provided
as a coating on a porous refractory medium (4) and said alkaline earth activator (6)
is confined on the side of said medium (4) remote from the layer (5).
4. A thermionic electron emitter according to Claim 1 wherein said layer (7) is porous
and is impregnated with said alkaline earth activator.-
5. A thermionic electron emitter according to Claim 1 wherein said layer (7) is porous
and said alkaline earth activator is confined on the side of said layer remote from
the electron emissive surface of said layer.
6. A thermionic electron emitter according to any one of Claims 2 to 5 wherein said
porous refractory medium is of tungsten or molybdenum.
7. A thermionic electron emitter according to any one of Claims 1 to 6 wherein said
alkaline earth activator is a mixture of barium oxide, or a compound of barium reducible
on heating to said oxide; an oxide, or a compound which decomposes on heating to the
oxide, of an alkaline earth metal other than barium and at least one of aluminium
and boron oxide.
8. A thermionic electron emitter according to Claim 7 wherein said alkaline earth
metal other than barium is selected from the group of metals consisting of calcium,
strontium and magnesium.
9. A method of making a thermionic electron emitter including the steps of forming
an alloy of a composition defined in Claim 1 and incorporating said alloy with an
alkaline earth activator.
10. A thermionic cathode including a thermionic electron emitter according to any
one of Claims 1 to 8.