Cathodes and emissive material therefor

United States atétitb F Ice
l’
1
,
2,871,196
' Patented Jan. 27, 1959
2
tween which the electric discharge or ?ash takes place.
2,871,196
To reduce the undesirable sputtering characteristics
of barium oxide, it has been proposed to add aluminum
CATHODES AND EMISSIVE MATERIAL
THEREFOR
oxide (A1203) to the barium oxide tov form a barium
aluminate. As aluminum oxide is added to increase its
-
Dimitrios M. Speros, Willoughby, Ohio, assignor to Gen
eral Electric Company, a corporation of New York
01f rapidly and there is also a sharp reduction in the
electron emission of the barium aluminate system. For
example, if one mole of aluminum oxide is added to
10 three moles of barium oxide to form the compound
No Drawing. Application April 29, 1957
Serial'No. 655,534
percentage in the compound, the melting point drops
_
4 Claims. (Cl. 252-512)
3BaO-Al2O3 the melting point of the compound drops
to approximately 1750° C. If, however, the parts of
This invention relates to cathodes for electric dis
charge devices and more particularly to cathodes pro
vided with an improved electron emissive material, to’
methods of activating a cathode for emission of elec
trons, and to the emissive material itself. The inven
barium oxide and aluminum oxide are made equal to
form the compound BaO-Al2O3 it is found that the melt
ing point is comparable to the melting point of the orig
tion is especially adapted for use in photographic ?ash
inal starting material barium oxide. Although the com
pound BaO-Al2O3 has the very desirable high melting
point, it is very low in electron emissivity and is un
tubes operating with cold cathodes.
suited for use as an activating material for cathodes of
'
Photographic ?ashtubes give a high intensity ?ash 20 electric discharge devices such as ?ashtubes because of
of very short duration and may be ?red intermittently
the high starting voltage required to ?ash the tube.
by means of a portable power pack as pictures ‘are
If barium oxide (BaO)‘ is added to increase the emis
taken. They comprise a small diameter glass tube,
sivity it will be found, as previously indicated, that the
melting point of the compound drops rapidly. If the
?lled with a rare gas such as xenon, the tube being pro
vided at each end respectively with a cathode and an 25 amount of barium oxide in the barium aluminate is
anode between which the electric arc discharge in the
further reduced to less than one mole ,a further reduc
gas takes places. When used with portable photographic
tion in emissivity takes place but the melting point re
equipment, it is desirable to provide a ?ashtube operat
mains ,at a satisfactory level about 1800" G. How
ing at low voltages which increases the reliability ‘of
ever, it has been found according to this invention that
the unit particularly when batteries in the power pack 30 the emissivity of the barium aluminate may be greatly
‘become weak. Moreover, it is desirable t'o'operate the
increased without an appreciable reduction in the melt
?ashtube with a cold cathode to simplify the ?ring cir
ing point by converting part of the barium oxide in the
cuit and to so design the cathode and emissive coating _ barium aluminate ‘to elementary barium. It is believed
as to eliminate sputtering and deterioration of the "arc
that the barium is dispersed throughout the barium alu-_
minate crystal, the result being that the crystal provides
life of the ?ashtube and reduces end blackening adja- ‘ ‘ a highly e?fective emissive material. The emissivity of
cent the cathode.
the aluminate system, including elementary barium, is
supporting electron emissive material; this increases the
Therefore, the instant invention has among its ob-'
in excess of the emissivity of barium oxide alone. Al
jects to provide an emissive material for cathodes of
though the theoretical reason for this is not precisely
electric discharge devices such as ?ashtubes which makes \ known, it is believed that the increasedemissivity is
possible operation of the tube at low voltage and which ' due to the increased amount of elementary barium
also provides for e?ective electron emission under cold
which may be bound in the crystal matrix of BaO-Al2O3
cathode operating conditions‘.
,
as distinguished from the amount of barium which can
- Another object is to provide a highly effective emis
be held by barium oxide (BaO) crystals. The emis
sive material for ‘discharge cathodes which utilizes bar
ium aluminate having an excess of aluminum oxide
together with some elementary ‘barium bound into a com—
posite crystal matrix.
45
sive material thus described for activating cathodes may
be prepared by _mixing commercially available com
pounds of barium oxide-aluminum oxide (BaO-Al2O3)
with powdered aluminum and then activating the material
Another object of the invention is to provide a new
by__heating to convert part of the barium oxide to ele
and improved method of coating or activating a dis 50' mentary barium. In manufacture of cathodes for elec
charge cathode with emissive material utilizing a'barium
tric discharge devices a mixture may be deposited on the
aluminate system in which the emissive material is acti—'
.metal cathode and activated by heating in position.
vated by heat when in position on the cathode.
'
-‘ Still another object is to provide a highly effective
activated emissive material for discharge cathodes which
allows latitude in choice of size and shape of the cath
ode.
It is well known that barium oxide itself (BaO) is
an eifective emissive material for cathodes of electric
i’iThe‘ activated emisisve material of the invention is
characterized by the formula (xBaO.Al2O3).yBa wherein
55 there is an excess of aluminum oxide in the barium oxide
aluminum oxide crystal to provide a compound having
a high melting point. -x therefore must be equal to or
less than 1. The lower limit of x is determined by the
minimum quantity of barium oxide in the emissive ma
discharge devices but this material has the disadvantage 60 terial which will give satisfactory emission characteristics.
that it is subject to sputtering from the cathode to the
walls of the discharge tube.v In the case‘ of photographic
?ashtubes, such emissive material is deposited on the
Walls of the glass tube adjacent the cathodes with the
resultant blackening of the tube thereby reducing the
light output, and therefore the e?iciency of the ?ash
tube. Barium oxide does have the advantage of a rela
tively high melting point which is in the neighborhood
x may range from 1 mole to as low as 0.1 mole; however,
optimum results are obtained when x is in the range of 1
mole to 0.34 mole. The limits y of elementary barium
are believed to be determined by the required number
of emitting centers bonded in the barium aluminate
crystal to give good emissivity.
_
v The upper limit of the elementary barium does not
appear to be critical as some excess of elementary barium
merely acts as a getter. An excess of elementary barium
of l'900° C. so that this emissive material can withstand
may be desirable for this reason; a large excess of barium,’
the high temperatures which occur at the spots on the 70 however, will inevitably evaporate from the cathode and
cathodes where electron emission takes place and be
cause blackening on the glass walls of the tube.
This
' -=2,871,196
4
3
occurs when this excess exceeds the moles of x'Ba'O'by 2
applied, such as radiation, passing current through a
to 2.6 times. Other getters may be used, for example, a
titanium anode will act as agetter making excess elemen
tary barium unnecessary. The lower limit of elementary
barium ideallydepends- on‘ the stoichiometric'yalue offone
mole of elementary vbarium to be Tboundiin each ‘mole of
BaO. Therefore y ideally approaches 1:: as the lowertlimit
cathode coil, conduction from a .hot surface, or by low
pressure are and glow discharges from the coil in argon.
Activation may take place in a hydrogen atmosphere,
argon or other inert atmosphere, or in a vacuum. Another
advantage of the low_temperature activation point is
that it is possible to activate the coated cathode .by means
of heat ‘conducted to the cathode from the sealing ?res
used to seal the cathodes in the glass tube.
it will be seen that y-may vary between 0.5x and ‘2.6x. 10 Cathodesprovided with an electron emissive-material
h ave'been constructed ‘as ‘follows:
In preparing ‘the emissive material, it is possible 'to
xample 1.-—A coiled tungsten wire cathode having an
use an available barium aluminate compound containing
for optimum results; however, ‘1y may be as lowlas 505x
and produce satisfactory results. :For'the reasons .given,
emissive material with approximately the composition
1.43 (0.34BaO.Al2O?')l.27Ba (has been constructed by?
has substantially the formula l.76BaO.Al2Q3. Because
this material is unstable in air it must ‘lbe "stored and 15 ?rst grinding 94.2.grams of commercially available barium
aluminate having the formula l.76BaO.Al2O3 into a
‘shipped in evacuated ampoul'es to protect it from de
powder and mixing it with 5.8‘grams of aluminum powder
terioration by water vapor. This barium aluminate com
,(average particle size 0003 mm.). This mixture was
pound is ?rst ‘ground'to a powder and then mixed with
thenpressed into pellets with'a pressure of 10 kilopounds
aluminum powder which is ‘preferably of a particle size
an excess of barium oxide as ‘a starting material which
about 0.003- mm. in properproportion v‘as described below. 20 per square inch ‘and the pellets'recrushed ‘in a mortar.
Thclg'rinding, weighing and pressing was done inan air
The amount of aluminum powder added to the above
conditioned, low-humidity room, andthe material stored
compound may vary between the limits of 2.7 to 7.3%
in a desiccator. The barium aluminate-aluminum mixture
maximum by weight of the mixture. ‘However, aluminum
powder in amounts between the limits of 2.7% and 5.8%
by weight of ltheimixture gives optimum results. The
was‘thenapplied to ‘the cathode tungsten coils by dipping
25 thecoils into‘ the dry powder. The emissive material was
thenactivated by heating the coated-coil to about 660“ C.
by means’ of the ?res whichvlseal the cathode 1to? "the glass
tube of. the ?ashtube.
respectivelyfor'each mole of 1.76BaO.Al2O3. I'l‘he chemi<
Whilean emissive material prepared with the above
cal reaction occurring upon heating to between 600° C.
to 660° -C.tactivates-the compound and is as follows in the 30 startingmaterials issatisfactormbarium aluminate con
taining an ‘excess of bariumv oxide. is unstable in air as
two cases given:
limits of 2.7% ‘to 5.8% of aluminumpowder above ‘
stated correspond: to 0,738 and 0.85 moles of aluminum
‘previously mentioned, and may present some production
difficulties. Barium aluminate represented essentially by
the-formula BaO.Al2O3|is available and has been found
35 to-be a-good starting material.
-"Example ~2-.—A cathode having
emissive material
withapproximately the composition
If vthefarno'u'nt vof aluminum’ powder-‘is ‘decreased sub
stan'tia'lly below the ‘limits given "an e'missionmaterial
rich in barium oxide rather than rich in aluminum oxide 40 has .beenconstructed in the same manner 'as described
above in connection with Example 1. Barium aluminate
results which, as previously mentioned, lowers themelting
ofthe formula BaO-Alz‘Og may be mixed with powdered
point. ‘On the other hand the maximum ‘weight of alumi~
aluminum wherein the aluminum ranges [in proportion
num which may be utilized should not exceed theabove
by weight ofthe total compound between 2.30% and
limit since this will deplete the barium oxide content of
4.85%, with 4.06% as the optimum. At optimum‘con
the cathode beyond the point of desirable emission and
ditions there is 0.4 mole of aluminum powder for’ each
stability characteristics.
mole of barium aluminate. The reaction occurring
The electric‘discha‘rge device or ?ashtube is provided
upon heating under optimum conditions is .as follows:
with conventional cold cathode constructions such as
nickel grids, iron ‘cups or tungsten coils. “The emissive
material is applied to the cathodes before activation. 'This 50
This corresponds to.0..33Ba0-Al2O3-0.5Ba. This mixture
may be accomplished'by dipping'the cathode into a dry
was prepared as above and the dry mixture applied to the
cathode. It was then activated by heating to atempera
ture of about 660° .C.
pressed material into or onto the cathode, orin'any other
conventional method. Because of the good adhesion 55 An emissive material of barium ,aluminate may be
powder mixture, by dipping it into a slurry of the mixture
with butyl acetate, by insertingparticles or pellets or the
of the dry powder to tungsten coils, the dry method is
synthesizedand partially reduced-by aluminum .during
probably ‘easiest and works well ‘fonportable I?ashtubes‘
having tungsten c'oil "cathodes. For vdischarge lamps
the activation of thecathode .itselfin .a single ‘step.
Compounds consisting ofmixtures of. barium.‘ carbonate
(BaCOg), .aluminum oxide (A1203), and aluminum (Al)
wet method is preferred but care should be taken to dry 60 powder have been successfully reacted to produce upon
being heated activated cathodes of the same compositions
the cathodes thoroughly before the-heating operation
where a large quantity‘of emissive material is needed, the
hereinafter described or the emissive material may dis
as the ones'described above.
color and ?ake off. An important advantage to the
This procedure, however, is not as susceptible to exact
control as those already described. The reasons for this
‘emission material according to the present invention is
that a coating thickness hardly visible to the naked eye
are that two solid state reactions must .now take place;
(1‘) the reduction of the carbonate‘ to the oxide at about
is suf?cient.
After coating, the cathode is heated for activation of ‘
660° C. ‘as represented by the equation 7
the emissive material. The cathodemay be heated to a
temperature between 600° C.?and ‘1200" C.;'however,
about 600° C. appears to be a su?iciently high temperature. 70
and (2) the partial reduction-of'the bariumox'ide to
Activation at this comparatively low temperature makes
give
it possible to construct the cathodes of low meltingpoint
metals such as iron or nickel. -An' important advantage
of this cathode is that no gases are evolved .duringka'cti
vation. Any'conventional method ‘of heating may ‘be, 75
2,871,196
5
Depending on diffusion factors, such as particle size, de
gree of pressing, rate of heating and the like, the less
thermodynamically favorable Reaction 2 above may take
6
the described standard lamps used as a basis of com
parison showed that their light output was reduced b
about 50% at 5000 ?ashes.
'
place concurrently with Reaction 1 above thereby de
Still another type of ?ashtube was designed for repeti
pleting the amount of aluminum present and necessitat Cl tive. application and made with an emission material in
ing the increase of activation temperature to approxima
accordance with this invention. The average life of
tely 1300" C. for the decomposition of unreacted barium
these tubes before they began missing was 765,000
carbonate.
?ashes; whereas, similar lamps with standard'cathodes
Photographic ?ashtubes provided with cathodes having
do not exceed 250,000 ?ashes and are accompanied by
an emissive material have been constructed, tested, and
early blackening. The barium aluminate cathodes ‘showed
compared with ?ashtubes heretofore manufactured but
very little blackening until near the end of their life.
in which the cathodes comprised a nickel screen on
The test results show that photographic ?ashtubes
which a barium-aluminum alloy had been vaporized.
having cathodes provided with an activated emissive ma
One standard type of photographic ?ashtube now com
terial composed of Al2O3 and Ba bound in a BaAl2O4
mercially available and provided with such cathodes has
crystal matrix in the appropriate proportion range show
a rated starting voltage of 305 volts with a standard de
very good maintenance in operation with little bulb black
viation in the starting voltage of 20.3 volts. An allot
ening making the ?ashtubes particularly adaptable for
ment of 100 photographic ?ashtubes in which the cathodes
use with portable photographic equipment.
were provided with emissive material comprising ‘the
While there have been described and illustrated speci
barium-aluminate-barium compound according to the in 20 ?c embodiments of the invention, it will be obvious that
vention showed an average starting voltage of 286 volts
various changes and modi?cations may be made therein
with a standard deviation of 18.2 volts. The results
without departing from the spirit of the invention which
show that ?ashtubes provided with cathodes constructed
should be limited only by the ‘scope of the appended
in accordance with the instant invention have a lower
claims.
starting voltage than the identi?ed standard ?ashtube' '
What I claim as new and desire to secure by Letters
and this provides a more reliable ?ashtube for use with
Patent of the United States is:
portable power packs used in photography. A sampling
1. A cathode comprising a conducting core and an
of twelve of the ?ashtubes were tested for 5000 ?ashes.
emissive material applied thereto corresponding to the
Those provided with the barium aluminate cathodes
formula (xBaO-Al2O3)-yBa wherein x is between 0.1
showed little blackening on the ends whereas the stand
and 1 and y is between 0.5x and 2.6x.
ard tubes with the above-described nickel cathodes were
2. A cathode comprising a conducting core and an
blackened to an extent such that their light output was
emissive material applied thereto having essentially the
reduced 30 to 50%. This means that the barium alumi
formula 0.33BaO-Al2O3-yBa wherein y is between 0.5
nate emissive material on the cold cathodes is very 'e?ec
and 0.89.
tive in eliminating sputtering and deterioration of the 35 3. An activated emissive material for a discharge
cathode under repeated ?ashing.
cathode corresponding to the formula (xBaO-Al2O3) ~yBa
Another type of standard photographic ?ashtube cur
wherein x is between 0.1 and 1 and y is between 0.5x
rently available has a starting voltage of 182 volts with
and 2.6x.
a standard deviation in starting voltage of 14.8 volts.
4. An activated emissive material for a discharge
Another allotment of 30 ?ashtubes was constructed in 40 cathode represented by the formula 0.33BaO-Al203-yBa
which the nickel screen cathode was replaced with
wherein y is between 0.5 and 0.89.
tungsten coil cathodes having a barium aluminate emis
References Cited in the ?le of this patent
sive material of the composition according to the in
vention. The average starting voltage of this allotment
UNITED STATES PATENTS
of ?ashtubes was 168 volts with a standard deviation of
10 volts. Again it is seen that by use of the instant in
vention, the starting voltage of the ?ashtube is reduced
thereby increasing its reliability and effectiveness. Oper
ation of the improved ?ashtubes up to 7000 ?ashes 50
showed very little blackening of the ?ashtube whereas
1,921,066
1,934,830
1,946,603
2,085,605
Bedford ______________ __ Aug. 8,
Spanner et al _________ __ Nov. 14,
Wedel ______________ __. Feb. 13,
Ramsay ____________ __ June 29,
1933
1933
1934
1937
2,142,331
2,300,959
Prescott ______________ __ Jan. 3, 1939
Pirani ________________ __ Nov. 3, 1942

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