Electric discharge lamp

Electric discharge lamps of extended life can be produced or cheaper materials used in their manufacture by providing on the internal surface of the envelope and on the exposed surfaces of internal components a protective coating of a metal phosphate or arsenate glass. External surfaces can also be protected against corrosion in air or in the atmosphere within an outer jacket, where this is employed. The coating may be applied in a liquid medium which, preferably after draining, is warmed to evaporate the medium and thereafter baked to form the vitreous coating.

The present invention relates to electric discharge devices and more 
especially to discharge lamps the envelopes of which contain a fill of a 
reactive gas or vapour. The invention is concerned with the provision of 
protective coatings on internal surfaces of such lamps. 
Various types of discharge lamp have gas fills containing gases or vapours 
which are reactive or potentially reactive with materials commonly used 
for lamp envelopes and the internal components of the lamps. Among these 
types may be particularly mentioned metal halide discharge lamps, and 
metal vapour discharge lamps containing the vapours of reactive metals, 
such as sodium. 
In metal halide discharge lamps the radiation is produced by an electric 
arc between two primary electrodes extending into an envelope, or arc 
tube, containing mercury and one or more metallic halides which are at 
least partially vaporised and dissociated by the heat of the arc during 
operation of the lamp. The envelope is commonly constructed of vitreous 
fused silica, although sometimes another transparent or translucent glass, 
ceramic or crystalline material is used. 
One problem which limits the performance of such lamps is reaction between 
the material of the envelope, or of the electrodes and supports, and the 
metallic halide contents. Such reactions can cause any of the following: 
darkening or obscuration of the envelope wall, with consequent loss of 
light; erosion of the electrodes and supports; loss of the constituent 
vapours in the discharge causing changes of colour or electrical 
characteristics; or mechanical failure of the envelope and destruction of 
the lamp. 
Other discharge lamps have arc tubes formed of a translucent ceramic 
material or translucent crystalline material and frequently contain highly 
reactive metallic elements, such as sodium or other alkali metal. These 
elements cannot be used in arc tubes constructed of vitreous silica or 
most glasses, because of the tendency of the metal vapour to react with 
these materials. 
In accordance with this invention, attack by reactive fill substance on 
portions of the structure of a discharge lamp or device is reduced or 
prevented by providing, on at least those portions of the internal surface 
of the discharge envelope or the exposed surfaces of internal components 
which tend to react with the fill, a coating of a metal phosphate or 
arsenate glass. The surfaces to be covered will usually include the 
internal surfaces of the envelope and the electrode supports. 
In accordance with a further aspect of this invention, in discharge lamps 
where the arc tube is enclosed in an outer envelope which itself contains 
a gas or gases, external surfaces of the arc tube or components thereof 
may also be protected by a coating of metal phosphate or arsenate glass 
and thereby preserved from attack by gases or vapours in the outer 
envelope. 
The protective coatings provided in accordance with this invention may be 
applied to conventional materials used for the fabrication of lamp 
components, for example to protect them from highly reactive fill 
substances, or they may enable cheaper and more readily available 
materials to be substituted for conventionally used materials without 
unacceptable loss in performance or life. 
The coating is preferably derived from an aluminium phosphate complex as 
described in German Offenlegungschrift (DOS) No. 2,028,839 (British 
Patents Nos. 1,322,722 and 1,322,724), one or more of the metal phosphate 
or arsenate compositions prepared in accordance with DOS No. 2,235,651 or 
from a composition comprising an aluminium phosphate and containing a 
titanium compound prepared in accordance with DOS No. 2,331,954. 
Combinations of these compositions can also be used. 
For the purposes of this invention, preferred metal phosphates and 
arsenates are those of atomic number 12 to 14, 20 to 32, 39 to 50, 56 to 
80, 90 or 92. The term `phosphate` is here meant to include ortho-, meta- 
and pyro- phosphates together with phosphinates and phosphonates. 
Especially preferred sources of metal phosphate coatings are 
solvent-soluble complex phosphates containing coordinated solvent groups, 
such as water or polar organic solvents, as described in DOS Nos. 
2,028,839 and 2,235,651. Not only are the isolated complex phosphates 
themselves suitable, but the compositions which are therein described 
containing phosphate precursors may also be used. 
Liquid coating compositions may be used which comprise a solution, of (a) a 
metal compound and (b) an oxyacid of phosphorus or arsenic, or a compound 
capable of forming such an oxyacid in the solution. At least part of the 
solvent may be organic. These compositions are capable of decomposing to a 
metal phosphate or arsenate on being heated. 
The solvent is selected from water or the wide range of organic solvents 
which dissolve the components of the composition. The organic solvent, 
when used, is preferably selected from alcohols, esters, ketones, 
aldehydes, nitrocompounds and ethers, especially monohydric alcohols of 
the structure ROH, esters of the structure R.sup.1 COOR.sup.2, ethers of 
the structure R.sup.1 OR.sup.2, ketones of the structure R.sup.1 
COR.sup.2, nitrocompounds of the structure R.sup.1 NO.sub.2 and ethers of 
the structure OR.sup.3, where R, R.sup.1 and R.sup.2 are alkyl groups or 
substituted alkyl groups containing from 1 to 10 carbon atoms each, and 
R.sup.3 is a divalent alkyl group having from 4 to 7 carbon atoms one of 
which may be replaced by an oxygen atom. Mixtures of one or more solvents 
may be used. Diluents may also be present, provided they do not bring 
about precipitation of the components of the composition. 
Aliphatic alcohols containing 1 to 10 carbon atoms are particularly 
convenient, especially lower molecular weight alcohols containing 1 to 4 
carbon atoms, for example methanol, ethanol, n- or iso-propanol and 
substituted alcohols especially methoxy- or ethoxy-ethanol. Suitable 
esters are ethyl acetate or carbonate. Acetyl acetone may be used. 
Tetrahydrofuran is the most preferred ether to use, though dioxan may also 
be used. Aromatic hydroxy compounds can be used, but solubility is low in 
such materials. 
The composition may be formed by dissolving an isolated complex of the type 
described in the specifications referred to above in a solvent. The metal 
compound may itself be a phosphate and so provide the oxyacid of 
phosphorus or arsenic, in which case an additional acid may be required to 
form a homogeneous solution, e.g. hydrochloric or nitric acid. 
A wide range of metal compounds may be used. Simple inorganic compounds 
including oxides and hydroxides are suitable, as are salts such as 
halides, carbonates, nitrates, phosphates, perchlorates and cyanates. 
Sulphates may be used in some cases but they can be disadvantageous owing 
to the difficulty with which they are thermally decomposed. 
Also suitable are salts of organic acids such as acetates, benzoates, 
oxalates, propionates or formates. Alkoxides are also useful. 
Alternatively co-ordination complexes of the metal may be used, for example 
complexes having ligands derived from acetylacetone, ethylenedithiol, 
ethanolamine, carbon monoxide or phosphines. 
Preferred compositions are those in which the metal and oxyacid are present 
with atomic ratios of metal to phosphorus or arsenic from 1:0.1 to 1:2.9. 
Preferred metals are aluminium, iron, chromium, titanium, vanadium and 
tin. 
A solvent-soluble aluminium phosphate may be used, for example the acid 
orthophosphates Al.sub.2 (HPO.sub.4).sub.3 and Al(H.sub.2 PO.sub.4).sub.3, 
and mixtures containing them. 
Normal aluminium orthophosphate is insoluble in water but soluble in dilute 
mineral acids (for example hydrochloric and nitric acids) and in some 
carboxylic acids (for example citric acid) and such solutions may be used 
for the purpose of this invention. Moreover, solid complex aluminium 
phosphates containing the anion of the acid and chemically-bound water or 
alcohol (or a mixture thereof) may also be used. 
Where the complex contains an alcohol group, it is preferred that it be an 
aliphatic alcohol containing from one to four carbon atoms, for example 
methyl alcohol, ethyl alcohol, n-propyl alcohol or isopropyl alcohol, 
although complexes with higher alcohols are known and may be used if 
desired. 
The complex phosphates most commonly contain from three to five molecules 
of the hydroxy compound per phosphorus atom, for example water-containing 
complexes may have an empirical formula corresponding to AlPO.sub.4 
.HCl.(H.sub.2 O).sub.x where x is in the range 3 to 5. 
The complex aluminium phosphates containing alcohol and their solutions may 
be prepared by reacting aluminium compound, preferably halide, with an 
alcohol and phosphoric acid. One such compound has the empirical formula 
Al P Cl H.sub.25 C.sub.8 O.sub.8. 
The complex phosphate containing water can be made as above or by 
hydrolysing the alcohol-containing complex phosphates or, for example, by 
contacting aluminium phosphate hydrate with gaseous hydrogen chloride. 
Iron, chromium, vanadium, titanium and tin phosphate-containing coatings 
may be prepared by dissolving a salt, preferably a halide, in an alcohol 
and adding phosphoric acid or a source thereof. 
The glass layer should be free from pin-holes or other defect or 
imperfection which might cause it to break down during operation of the 
lamp. In one preferred method of making lamps according to this invention, 
the desired portions of the internal surface of the envelope and the 
surfaces of internal components which are exposed in the finished lamp are 
coated either separately or after assembly with a liquid composition 
capable of generating the desired metal phosphate or arsenate, and 
subsequently heated to evaporate the solvent and cure the composition to 
form a defect-free metal phosphate or arsenate coating. It has been found 
valuable in the production of defect-free coatings to allow the applied 
liquid coating composition to drain thoroughly and thereafter to bake 
initially at a relatively low temperature to remove the solvent and 
subsequently at a controlled higher temperature to complete the formation 
of the protective coating. The preferred baking temperatures vary with the 
particular composition of coating material employed, but can be determined 
by experiment.

In accordance with one aspect of this invention, surfaces of electric 
discharge lamps and devices or components thereof tending to react with 
the lamp contents are provided with coatings of the phosphate or arsenate 
glasses described above. 
In FIGS. 1 to 3 are shown three examples of discharge devices or lamps. In 
each case the arc tube or envelope 10 is constructed of vitreous fused 
silica, into which are sealed electrodes 11 on tungsten shanks 12, 
electrically connected to external leads or connectors 13 through 
molybdenum foil pinch seals 14. An exhaust tube 15 is provided in the wall 
of the tube 10 and is sealed off in the finished lamp, as shown in the 
drawing. 
The form of lamp shown in FIG. 1 commonly contains a mixture of the iodides 
of scandium, sodium, thorium and mercury, in addition to metallic mercury 
and a quantity of argon gas. An additional auxiliary electrode 16 may be 
sealed through one end of the tube 10 for starting purposes. An arc tube 
of this kind is normally sealed in an outer glass jacket (not shown) which 
is either evacuated or filled with an inactive gas, and which may be 
coated with a phosphor. Lamps of this type are commonly designated MBI or 
MBIF lamps. Other lamps of this construction contain the halides of 
aluminium or tin, and are commonly referred to as molecular arc lamps. 
The form of lamp shown in FIG. 2 may contain the halides of sodium, 
gallium, thallium and mercury, together with metallic mercury and a rare 
gas such as xenon. These lamps are very compact and operate at a higher 
pressure than those of FIG. 1. They are not normally operated in an outer 
envelope (although they may sometimes be) and are commonly designated 
Compact Source lamps, or CSI lamps. 
The form of lamp shown in FIG. 3 has a longer and narrower arc tube than 
those of FIG. 1, and is designed to operate without an hermetically sealed 
outer jacket, but usually in a special closed fitting. These lamps may 
contain similar halide mixtures to those used in the type of lamp shown in 
FIG. 1, but other versions intended for photochemical and other special 
purposes contain other halides, such as the iodides of gallium, indium or 
bismuth. Lamps of this type are commonly designated MBIL lamps. 
In FIG. 4 is shown a further type of arc tube 10 which is made of a 
translucent ceramic material, such as alumina, or a transparent 
crystalline material, such as crystalline alumina or sapphire, and in 
which electrodes 11 are sealed, either through metallic caps 17 or ceramic 
plugs sealed to the ends. The metallic caps are often of niobium or a 
niobium alloy, which is particularly reactive with metallic halide 
compounds. Lamps of this type frequently contain highly reactive metallic 
elements, such as sodium or the other alkali metals, which cannot be used 
in arc tubes constructed of vitreous silica or most glasses. The arc tubes 
of these lamps are commonly fitted within outer jackets. 
Many possible chemical reactions may limit the performance of such lamps. 
Most of these are not fully understood, but certain likely reactions are 
believed to be responsible for the observed effects. 
As an example, when a lamp of the type shown in FIG. 1 containing aluminium 
trichloride was operated the tungsten electrode supports were rapidly 
eroded, the surrounding areas of the silica envelope were darkened and a 
more general attack and devitrification of the envelope was apparent. 
Although these effects have not yet been positively identified as caused 
by specific chemical reactions, the following types of reaction are known 
to be possible: 
##STR1## 
where x and y correspond to several chlorides of tungsten. (The first part 
of reaction I probably occurs in the higher temperature parts of the 
discharge, while the second part occurs at the cooler metal surface as a 
result of incomplete recombination of the chlorine with the monochloride). 
These could readily account for the main features of the observed 
behaviour. 
Such interactions with the lamp components are not limited to normal 
chemical reactions with the halides in their solid, liquid or gaseous 
forms. The presence of the discharge and the associated electric fields 
permits a much wider range of interaction, erosion or attack, involving 
the products of dissociation of the compounds, and excited or ionized 
species derived from them. Electrolytic processes may produce further 
reactions, and may increase the rate at which some of the interactions 
occur. 
As an example, the presence of an electric field with a definite mean 
polarity across the tube wall, associated with the current leads to the 
discharge, is known to greatly enhance the migration of sodium, derived 
from the dissociation of sodium iodide in such lamps, through the silica 
walls. This gives rise to a loss of sodium from the arc, with consequent 
deleterious changes in colour and electrical characteristics. Near the 
electrodes, electrolytic action of this kind is a frequent cause of 
mechanical failure of the lamp. 
Also, impurity gases or vapours commonly present in lamp envelopes such as 
water vapour or oxygen may give rise to still further reactions, and 
enhance the rate of others. Traces of such impurities are commonly left in 
the gaseous filling during processing, or are introduced with the halides, 
or may be released from the various components by the action of the 
discharge. 
As an example, oxygen-containing impurities in lamps containing aluminium 
trichloride can in some circumstances produce chlorine by the reaction: 
EQU 2AlCl.sub.3 +30 .fwdarw. Al.sub.2 O.sub.3 + 3Cl.sub.2 IV 
leading to a greatly increased rate of erosion of the electrodes by 
reactions of the type I. 
The present invention can be utilized to prevent, control or reduce the 
incidence of, deleterious interactions of these kinds between the metallic 
or halide contents and the components of the lamp in contact with them, by 
coating the surfaces in contact with the metals or halides with a 
protective layer of metal phosphate or arsenate glass composition which is 
resistant to interactions of these kinds. 
The surfaces to be protected usually include the internal surface of the 
envelope and the exposed surfaces of the electrode leads or supports, and 
metal end caps when present, together with, when possible, the regions 
where different components join. The active surface of the electrodes will 
not normally be covered. 
In some cases there are advantages also in coating the external surfaces of 
the arc tube, leads or adjoining components, as a protection against the 
surrounding atmosphere. For example, the metallic caps 17 of lamps of the 
type shown in FIG. 4 are susceptible to reaction with impurity gases or 
vapours such as water vapour or oxygen in the surrounding outer jacket. 
Similarly the leads adjacent to the arc tube in lamps of the type shown in 
FIG. 3 are liable to reaction with the surrounding air in the fitting. 
Both these may be protected by external coatings of this type. 
The protective layer should be effectively free from pinholes or other 
defects or imperfections which will cause it to break down during 
operation of the lamp, although a substantial degree of protection, and 
consequent improvement in lamp quality may be obtained in some cases when 
such perfection is not fully achieved. 
In a preferred method of providing the coating, it is applied to the whole 
of the interior of the arc tube after the components have been 
substantially assembled. This has the advantage that the regions where the 
components join, which are often particularly susceptible to attack, are 
fully coated. The active areas of the electrodes may also be coated in 
this process, but the coating on these will normally be removed when the 
arc is first struck, or the lamps first operated. 
The protective coating may alternatively be applied by coating the 
components individually before assembly. This method might be used, for 
example, where it is essential that the active surface of the electrode is 
not brought into contact with the coating material or any products of 
reaction associated with it, or it might be used when required by a 
particular manufacturing technique. 
The coating can be applied by filling, injecting or spraying the inside of 
the arc tube with the complex or a solution of the complex and 
subsequently removing or draining any surplus, or it can be applied to the 
individual components by any of the methods described in DOS No. 
2,235,651. The complex is then decomposed by heating to yield a phosphate 
(or arsenate) layer by heating at a temperature below 1000.degree. C. as 
described in the same specification. The construction and processing of 
the lamp is then completed in the normal way. The following is one example 
of the application of the invention. 
EXAMPLE 
A 400 W aluminium chloride discharge lamp with an arc tube of the form 
shown in FIG. 1 is protected by coating the inside of the arc tube 10 in 
the following manner. A solution is prepared by adding slowly and with 
stirring, 4.646 g anhydrous aluminium chloride to 91.458 g methanol. 3.866 
G orthophosphoric acid (88%) is then added. The resultant solution is 
dispensed from a hypodermic syringe, through the lamp exhaust tube 15, 
before this is sealed off. It is distributed around the inside of the arc 
tube, which is then inverted and left to drain, leaving only a thin layer 
adhering to the inside surfaces. The resultant coating is baked at 
100.degree. C. in a vacuum for one hour and finally formed by baking at 
400.degree. C. for three minutes. 
The lamp is then processed in the normal manner for discharge lamps of this 
type to give an arc tube of volume 2.2 cm.sup.3 which contains 7.4 mg 
AlCl.sub.3 and 44 mg Hg together with a pressure of 20 Torr of argon at 
room temperature. During processing the arc is struck and the glass layer 
on the active surface of the electrode is removed, leaving the protective 
layer over the internal surfaces of the arc tube and the cooler parts of 
the electrode structures. 
The layer substantially increases the resistance of the cooler parts of the 
electrode leads from erosion by the chloride vapour, preventing blackening 
of the envelope surface by deposited tungsten and other reaction products. 
The silica is also protected from reaction with the aluminium chloride. 
Instead of the particular aluminium phosphate composition described in the 
above preferred example, aluminium phosphate or arsenate coatings may be 
used, prepared from solutions of halogen-containing complex phosphates or 
arsenates of aluminium as disclosed in DOS No. 2,331,954, coating the 
internal lamp surfaces, and heating to cure the coating under the 
conditions substantially as disclosed in the same Application. 
Instead of one of the above compositions, coatings may be used prepared 
from liquid compositions of other metal compounds and oxyacids of 
phosphorus or arsenic as disclosed in DOS No. 2,235,651, coating the 
internal lamp surfaces and heating under the conditions substantially as 
disclosed in the same Application, the remainder of the processing 
following the same general lines as in the above preferred example. 
It should be noted that it is not an essential part of the process of this 
invention to coat the envelope and internal components after assembly, as 
described in the above preferred example, and individual components may be 
coated before lamp assembly. The essential feature of the invention is the 
provision of a continuous layer consisting essentially of a metal 
phosphate or arsenate glass covering the interior surface of the envelope 
or any internal components that could react with the fill or contents of 
the lamp at the operating temperatures. 
Similarly, any external surfaces to be protected may be coated either 
before or after assembly.