GB2084395A - Electron emission composition and process of producing it - Google Patents

Abstract

An electron emission composition comprises a cubic form solid solution of the formula (Y, La)2O3 containing from 85 to 95 mole % of Y2O3 and from 5 to 15 mole % of La2O3. The composition may be prepared by reacting oxalic acid with a mixed aqueous solution containing Y and La ions to co-precipitate a mixed oxalate which is subsequently fired, suitably at from 800 DEG C to 1,300 DEG C, to give (Y, La)2O3 in the form of finely divided particles. A number of specific examples of emissive compositions are given that are suitable for utilising in fluorescent discharge lamps and cathode ray tubes, and they have the advantage that no gas-evolving activation stage is required during the manufacturing processes of the devices.

Description

SPECIFICATION Electron emission composition and process of producing it This invention relates to an electron emission composition and to a process for producing it.

Conventional electron emission compositions typically comprise oxides of two or three alkaline earth metals, thoriated tungsten, or aluminates or tungstates of alkaline earth metals. The oxides of alkaline earth metals and thoriated tungsten are used in emission compositions in, for example, fluorescent lamps, high pressure mercury vapour lamps and cathode ray tubes, and the alkaline earth metal aluminates and tungstates are used in accordance with their characteristics concerning current load, operating temperature and others.

Electron emission compositions are generally required to have both a low work function and low operating temperature and further to be adequately resistant to impacts of ionized gases in gas-filled discharge lamps. These requirements are due to the facts that emission compositions should be capable of stable operation for long periods and that they should give rise to little contamination due to substances vapourized therefrom and should also have a small consumption rate. The emission compositions are selected in accordance with their particular end applications and are coated, for example, on an associated electrode or cathode electrode. Then the coated compositions are formed and activated under appropriate conditions after which they are put to practical use.

For example, a conventional emission composition for use in fluorescent lamps comprises a ternary mixture of the carbonates of barium, strontium and calcium. The coiled or coiled coil tungsten filament in the fluorescent lamp is coated with ternary mixture. Then a current is applied to the filament to heat the coated carbonates whilst evacuating the lamp to decompose the carbonates into their elements. Subsequently the reduced elements, Ba, Sr and Ca are activated and aged under predetermined conditions. This gives a fluorescent lamp containing the electrode which is capable of stable operation.In this process, it is to be understood that the composition and amount of the ternary mixture coated onto the filament are strictly prescribed whilst the decomposition, activation and ageing are conducted under strictly prescribed conditions and time intervals for passing appropriate currents through the filament.

Conventional fluorescent lamps, such as described above, tend to become blackened at their ends and therefore have a relatively short lifetime. This is because the conventional emission compositions used therein are deteriorated and scattered or vaporized during operation. Further cathode ray tubes, the electron guns of which are coated with conventional emission compositions may have relatively short lifetimes for the same reasons.

It is an object of the present invention to provide an improved electron emission composition.

According to one embodiment thereof, the present invention provides an electron emission composition formed of a solid solution in cubic form represented by the formula (Y, La)2O3 and containing from, 85 to 91 mole % of yttrium oxide (Y203) and from 5 to 1 5 mole % of lanthanum oxide (La203).

The invention also provides a process of producing an electron emission composition comprising the steps of reacting oxalic acid with an aqueous mixed solution including ions of yttrium and lanthanum to produce a mixed oxalate of yttrium and lanthanum and firing the resulting mixed oxalate in air thereby to form an electron emission composition in the form of finely divided oxide particles of the formula (Y, La)203. Preferably, the firing is effected at a temperature of from 8000 to 1 ,3000C.

In the following description reference will be made to the accompanying drawing in which: Figure 1 is a fragmental side elevation of a fluorescent lamp using an emission composition, with the end portion of the lamp broken away to illustrate the inner construction thereof; and Figure 2 is a graph illustrating the X-ray diffraction pattern of an electron emission composition produced in accordance with the invention.

The present invention provides an electron emission composition composed of oxides of rare earth metals which have not previously been considered to be generally suitable for forming electron emission compositions. It is well known that oxides of rare earth metals have a work function as low as the order of from 2.1 to 2.8 eV, which figure is suitable for electron emission compositions. However as these oxides have a high melting point, not less than about 2,4000 C, emission compositions composed thereof have been required to be operated at high temperatures, generally exceeding about 1 ,7000C, and at such high temperatures, the rare earth metal oxides are dissociated and abruptly vapourized resulting in a pronounced consumption thereof. Thus they have not been used in emission compositions except in extremely special cases.

It has been found, however, that the emission compositions of the present invention can be used in fluorescent lamps in the same manner as the abovementioned carbonates of alkaline earth metals and moreover can lead to a marked decrease in end blackening whilst give a long operating life.

Yttrium oxide (Y203), alone, is of cubic form whilst lanthanum oxide (La2O3), alone, is of hexagonal form because the crystal form is dependent upon the radius of the ions involved. Yttrium ions have a radius of 0.893 A and lanthanum ions have a radius of 1.01 6 A. It has been found, however, that La203 forms a solid solution with Y203 whilst maintaining a cubic form provided that the Lea 203 forms not more than 1 5 mole % of composition. If the La203 is present in an amount of more than 1 5 mole /0, it combines with Y203 to form LaYO3 of rhombic form having the perovskite structure to form a mixture comprising the LaYO3 mixed with the remaining Y203 of different crystal form from the LaY03.

Whilst we do not wish to be limited by theoretical consideration it is believed that the emission composition of the invention has good characteristics for the following reasons: l (1 ) The melting point of yttrium oxide is decreased because it forms a solid solution with the lanthanum oxide. Therefore the emission composition (Y, La)203 has a relatively lower operating temperature.

(2) Lanthanum (La) ions intrinsically tend to form a rhombic crystal but are confined to the cubic form in the compositions of the invention. Therefore upon an increase in momentum thereof at and adjacent to the operating temperature the lanthanum ions are easily activated.

(3) Lanthanum in the form of a metal has a melting point of 9250C so that lanthanum atoms existing in the activated state have a comparatively low melting point and tend to be diffused at and adjacent to the operating temperature.

(4) Lanthanum has a vapour pressure of 3 x 10-" mmHg at 1 ,0000C which is markedly lower than that of barium (2.6 mmHg at 1 ,0000C). Yttrium has a vapour pressure as low as 4 x 10-10 mmHg at 1 0000 C.

(5) Lanthanum dissociated in (Y, La)203 exists in its diffused state and contributes to the electric conductivity due to electrons. Therefore lanthanum has an electric conductivity required for emission compositions of the oxide system.

Notwithstanding theoretical considerations, it has been found that, when the emission compositions of the present invention are used to coat the electrodes of fluorescent lamps in the same manner as conventional emission compositions the spot temperature is about 1,1 000C during lighting and black spots and end-blackening are much reduced even when such fluorescent lamps are operated for iong periods of time.

As noted above, the present invention also provides a process of producing an emission composition as mentioned above. This process comprises the step of reacting of oxalic acid with an aqueous mixed solution containing yttrium and lanthanum ions to co-precipitate the oxalates of yttrium 'and lanthanum, and firing the oxalates in air to form an emission composition in the form of the oxides of yttrium and lanthanum.

The resulting emission composition is in the form of finely divided particles including two dissimilar ions coupled to each other through oxygen ions interposed therebetween. In contrast, if an oxide of yttrium is merely mixed with one of lanthanum followed by the firing of the resulting mixture, a high temperature, approximately 2,0000C, is required. Further the emission composition thus produced is in the form of large particles due to sintering and unsuitable for use as an emission composition.

The temperature of the aqueous mixed solution of yttrium and lanthanum ions and a quantity of dissolving water required for the solution do not require to be strictly determined.

It has also been found that, when the oxalates are decomposed at a firing temperature of less than 8000 C, an undecomposed product tends to remain and at a firing temperature of above 1 3000 C, the particie size grows after the initiation of the firing. Therefore the firing is preferably carried out at a temperature of from 8000 to 1 ,3000C.

In order that the invention may be well understood the following examples are given by way of illustration only.

EXAMPLE 1 26.40 Grams (0.95 mole) of yttrium oxide (Y203) and 1.87 grams (0.05 mole) of lanthanum oxide (La203) were placed in a glass beaker and dissolved in 55 millitres of concentrated nitric acid poured into the beaker. Then pure water was added to the beaker to give a mixed solution having a total volume of 1.5 litres.

50 Grams of oxalic acid were dissolved in 1.5 litres of pure water and the resulting mixture was heated and stirred at 800C to give an aqueous solution of oxalic acid.

After having been heated to about 800 C, the mixed solution was slowly poured into the aqueous solution of oxalic acid. An immediate reaction ensued resulting in the precipitation of white mixed oxalates.

The mixed oxalates were filtered, rinsed and dried and then charged into an alumina crucible. The oxalates in the crucible were then fired in air at 1 ,1000C for one hour to give an emission composition in accordance with the invention in the form of finely divided particles having a white colour and of the formula (Yo9sLaoo5)2o36 EXAMPLE I The process as described above was repeated except that 25.0 grams (0.90 mole) of Y2C3 and 3.75 grams (0.10 mole) of La203 were employed and the firing was effected at a temperature of 1,3000C.

The resulting emission composition had the formula (Yo.go, LaO 10)2 3 and exhibited an X-ray diffraction pattern as shown in Figure 2 wherein the ordinate represents the relative intensity of a diffracted X-ray and the abscissa represents twice the angle of diffraction (2 0).

EXAMPLE Ill The process of Example I was repeated except that 23.6 grams (0.85 mole) of Y203 and 5.6 grams (0.15 mole) of La203 were employed and firing was effected at 8000C.

The resulting emission composition had the formula (to 85, La015)2O3.

Each of the emission compositions produced in Examples I-Ill was used to coat the electrode of a fluorescent lamp such as shown in Figure 1, wherein there is illustrated one end portion of a 40W standard fluorescent lamp. The arrangement illustrated comprises a glass tube 10 shown as including one end connected to a base or cap 12 and an electrode 14 in the form of a coiled filament disposed within the glass tube 10 perpendicular to the longitudinal axis thereof. The ends of electrode 14 are connected to a pair of iead wires 1 6 extending and sealed through a step 1 8 which is connected to hermetically seals the one end of the glass tube 10. The electrode 12 is coated with an emission composition.

Each of the emission compositions produced in Examples I-Ill was mixed with a nitro-cellulose lacquer and butyl acetate. The resulting mixture was pulverized into a paste-like liquid by means of a ball mill. The process of forming the paste-like liquid has been previously practised with powdered emission compositions of the carbonate system. Thus the conventional technique as it stands has been applied to the present invention.

Then the electrode 14, in the form of a coiled filament, is dipped into the paste-like liquid to coat it with a uniform layer of paste. Thereafter the glass tube 10 with the coated electrode 14 is subjected to the series of steps for manufacturing fluorescent lamps. More specifically, bulb sealing, evacuating, tipoof, ageing etc. are successively effected. In this way, a number of fluorescent lamps as shown in Figure 1 were produced for each of the emission compositions of Examples I-Ill. The evacuation was effected in accordance with a heating schedule for conventional carbonate emission compositions.

The fluorescent lamps thus produced were tested with regard to starting voltage and to endblackening after operation for 5,000 hours. A number of identical fluorescent lamps were produced in a conventional emission composition of the ternary carbonate system and similarly tested for comparison purposes. The results of the test are given in the following Table.

TABLE

En d'Bl acken ing Coating Av. Start after Weight Volt. in Electron Emission in mg per V for 10 No. of Composition Used filant Lamps Class lamps Example I 4.1 163 0.5 2/10 Example Il 3.9 161 0 0/10 Example Ill 4.3 159 0 0/10 Temary Carbonate 4.8 160 1 3/10 The class of end-blacking was estimated on a scale of O to 4, 0 indicating no blacking and 4 indicating maximum blacking.A class of 0.5 implies an extent end-blackening to an extent that only a slight black spot-like stain was observed. The numerator in the fraction under the heading "No. of lamps", indicates the number of lamps having the class of end-blackening in a total of ten lamps tested.

From the above Table it may be seen that the fluorescent lamps incorporating emission compositions of Examples I-Ill had starting voltage characteristics very similar to those of fluorescent lamps commercially available up to now and that there was very little end-biackening. It has also been found that the fluorescent lamps incorporating the emission composition of the present invention are substantially equal in variations in lamp voltage and current and total luminescent flux to those using the conventional carbonate system.

An electrode coated with an emission composition of the present invention does not require to be subjected to the decomposing step effected used with electrodes using conventional carbonate systems during the evacuation of the associated fluorescent lamps. Accordingly the load on the associated evacuating unit can be decreased while at the same time permitting the mass production of fluorescent lamps.

Since the emission compositions of the present invention are used with the electrodes of fluorescent lamps to exhibit electron emission characteristic similar to those of mixed alkaline earth oxides as described above, they may also be used with the cathodes of cathode ray tubes. In this case, as no evolved gas has to be removed during the evacuation of the cathode ray tubes, the evacuation may be effected with a lower load. Further since lanthanum has a low vapour pressure, the amount of metal vapourized therefrom is small and therefore the mating electron guns can be effectively prevented from being contaminated resulting in improvements in breakdown voltage of the cathode ray tubes.

Furthermore, the cathode ray tubes can have an increased life because the emission composition on the cathode has only a small consumption.

While the present invention has been described in conjunction with some preferred embodiments thereof it is to be understood that numerous changes and modifications may be resorted to. For example, an oxide or oxides of a rare earth metal or metals other than yttrium or lanthanum may be added in a small amount to the oxides of the latter to form (Y, La)2O3. However if the addition of such an oxide or oxides results in a crystal form other than the cubic form or in a mixture with such a crystal form sufficient to be indicated by an X-ray diffraction pattern then it is not suitable. Also, since samarium, europium and yttribium have a relatively high vapour pressure and praseodium and terbium are oxidised into higher vaient oxides, the use of such elements is not desirable.

Claims (5)

1. An electron emission composition of a solid solution in cubic form of the formula (Y, La)203 and containing from 85 to 95 mole % of yttrium oxide and from 5 to 15 mole % of lanthanum oxide.

2. An electron emission composition as claimed in claim 1 substantially as hereinbefore described with reference to the examples.

3. A process of producing an electron emission composition comprising the steps of reacting oxalic acid with an aqueous mixed solution containing ions of yttrium and lanthanum to produce a mixed oxalate of yttrium and lanthanum and firing the resulting mixed oxalate in air to form an emission composition in the form of finally divided particles of oxides of the formula (Y, La)203.

4. A process as claimed in claim 3, in which firing is effected at a temperature of from 8000 to 1 ,3000C.

5. A process as claimed in claim 3 substantially as hereinbefore described with reference to the examples.