Patent Grant
9653280
The invention describes an electrode for use in a lamp and a method of manufacturing such an electrode. The invention further describes a lamp and a method of manufacturing a lamp.
In lamps such as gas-discharge lamps or halogen lamps, the body of the lamp is often made of quartz glass and encloses a burner or chamber with a filling. In the case of lamps such as a high-intensity discharge (HID) lamp, the fill gas or filling can comprise an inert gas as well as various metal salts. The electrodes, usually embedded in a sealed portion of the lamp, can become very hot on account of the high current that flows through the electrode during switch-on and during operation of the lamp. The hot electrodes cause the quartz glass to also heat up. The different coefficients of thermal expansion of the quartz glass and the electrode metal lead mean that these expand and contract at different rates during heating and cooling respectively. A known problem caused by these different expansion and contraction rates is that cracks appear in the quartz glass, since quartz glass expands and contracts to a lesser extent than metal. During the lifetime of the lamp, the cracks can become larger. For example, a number of small cracks can spread and join to form an enclosed region in the sealed portion in the form of a ‘bead’. Also, one or more small cracks can develop into a crack extending radially outward, known as a ‘radially extending crack’ or REC. A bead-like crack can also develop into a REC. Such cracks can ultimately lead to failure of the lamp or, in a worst case, to explosion of the lamp.
Much effort has been invested in finding a solution to the problem of lamp failure due to cracks in the sealed portion, since long lifetime and reliability of performance are extremely important factors, particularly in the case of gas-discharge lamps used for automotive purposes. Some efforts describe wrapping a coil around the electrode in the region that will be enclosed in the sealed portion. Such techniques are time-consuming and expensive, and therefore not practicable. In one approach, US 2007/0103081 describes an electrode treated so that cracks largely develop in a controlled manner. This document teaches the treatment of the electrode, for example by creating a deep pit in the electrode or a deep groove around the body of the electrode, in order to deliberately allow a bead-like crack to develop during operation of the lamp. However, observations have shown that such grooves in the electrodes are associated with a high proportion of electrode breakages, even during the manufacturing process, so that this type of electrode treatment is not particularly advantageous from the point of view of a prolonged lamp lifetime as well as a desirable production yield.
Other approaches are based on minimizing the contact between the electrode body and the quartz glass in the sealed portion. For example, WO 2008/032247 describes electrodes treated so that bristle-like protrusions, arranged in a spiral manner on the sides of grooves running around the electrode, result in a separation between quartz glass and electrode in a critical part of the sealed portion that is most subject to extreme temperatures during operation of the lamp. However, this type of electrode is also associated with an undesirably high failure rate, resulting in shorter life-time and an undesirably low production yield. The reason for this is that, to reach the necessary high pressure of inert gas in the lamp, a cooling step is required during manufacture. Cooling is carried out rapidly, for example by immersing the seal (or the entire lamp) in a liquid nitrogen bath. This means that, in the sealed portion, the quartz glass and the electrode both contract, but the electrode contracts to a greater extent. While the electrode contracts, axial forces are exerted on the electrode, which arise as a result of the adherence between the quartz glass and the electrode in the sealed portions on either side of the critical region. In effect, the grooved region is being held firmly at both ends, while at the same time being forced to contract. The relatively deep groove in the body of the electrode causes the electrode to behave as a notched tensile specimen. Often, this results in the groove developing into a crack or break in the body of the electrode during cooling, and the lamp is rendered useless. The same applies to US 2007/0103081, since any deep pit or groove in the body of the electrode increases the likelihood of failure during cooling.
It is therefore an object of the invention to provide an improved electrode which reduces the lifetime-related and production-related problems outlined above.
This object is achieved by the electrode of claim 1, the method of claim 7 of manufacturing such an electrode, the lamp according to claim 10, and the method according to claim 12 of manufacturing such a lamp.
According to the invention, the electrode for use in a lamp comprises a quartz glass envelope enclosing a chamber, which electrode comprises a tip for extending into the chamber and a base for embedding in a sealed portion of the quartz glass envelope, and the electrode is characterized in that the base comprises a plurality of essentially smooth concave channels arranged around the body of the electrode, and wherein the depth of a channel is preferably at most 8 percent (8%), more preferably at most 5 percent (5%), and most preferably at most 3 percent (3%) of a diameter of the electrode.
Here, the expression “arranged around the body of the electrode” is to be understood to mean that the channels are arranged circumferentially or helically about the electrode, as opposed to a longitudinal arrangement. Furthermore, a channel arranged around the body of the electrode such that it “wraps around” the electrode several times is regarded in this context as a “plurality of channels”, since, when viewed from any point along a side of the lamp, the electrode appears to comprise a plurality of channels. The term “smooth” in this context means that the channel floor is devoid of any ‘pits’ or ‘holes’, so that the channel floor is essentially uninterrupted by any such depressions or deeper areas.
An obvious advantage of the method according to the invention is that the channels formed in the body of the electrode ensure that the electrode, during a cooling step in manufacture, will not act as a notched tensile specimen and will therefore not be subject to critical stresses in the form of fracture stresses. This is because the shallow concave form of the channel increases the resistance of the electrode material to the triaxial stresses exerted on the electrode during cooling. This is in contrast to the prior art grooved electrodes, which, with their narrow, steeply pitched groves are more likely to break during cooling on account of the triaxial stresses being concentrated at the deepest part of the groove. Another advantage of the electrode according to the invention is that, since the depth of the channels is relatively shallow, the diameter of the electrode is only minimally reduced, so that a ‘core’ of the electrode is still large enough to withstand the forces exerted upon it during cooling. This is in contrast to prior art electrodes with deep grooves, or grooves with deep irregularities such as additional pits or holes, and a correspondingly narrow electrode core. These types of electrode may fail during a manufacturing cooling step since the narrow core of the electrode is often not large enough to withstand these forces.
The method according to the invention of manufacturing such an electrode for use in a lamp (comprising a chamber in a quartz glass envelope) comprises the step of removing material from the body of the electrode to form a plurality of essentially smooth concave channels around the body of the electrode such that the depth of a channel is at most 8 percent (8%), more preferably at most 5 percent (5%), and most preferably at most 3 percent (3%) of a diameter of the electrode. Here, the expression “removing material” can mean that the material is physically transferred from the channel to regions along the sides of the channel, or it can mean that electrode material is actually taken out of the electrode body, for example by being vaporized.
The lamp according to the invention comprises a quartz glass envelope enclosing a chamber and a pair of such electrodes disposed to extend into the chamber, and wherein each electrode is partially embedded in a sealed portion of the quartz glass envelope.
The method according to the invention of manufacturing a lamp comprises the steps of forming an envelope of molten quartz glass; forming a first sealed portion to partially embed such a first electrode in the sealed portion; introducing a filling into a chamber in the molten quartz glass; cooling the first sealed portion, and forming a second sealed portion to partially embed a second electrode and to seal the filling in the chamber.
In the manufacture of a lamp such as a HID lamp, first one electrode is pinched in a sealing portion, and then the filling is introduced into the chamber, which is then sealed. By cooling the first sealed pinch, the volume of the filling in the chamber is reduced. A second pinch can then be formed to embed the second electrode and to simultaneously reduce the volume of the chamber to the desired dimensions. When the lamp is complete, the filling thaws and the pressure increases accordingly.
The steps listed here need not be carried out in the order given, but can be carried out in any appropriate order. The advantage of the method according to the invention is that, during the cooling step, in which the sealed portion can be cooled slowly or rapidly to seal in the filling, the geometry of the channels arranged around the body of the electrode according to the invention is such that the triaxial stresses in the channels, arising as a result of the different cooling rates, can be successfully withstood by the electrode. This method is then associated with an advantageously higher production yield and prolonged lifetime, since the number of electrodes that crack during cooling can be greatly reduced.
The dependent claims and the subsequent description disclose particularly advantageous embodiments and features of the invention.
While the tip of the electrode can have any suitable shape, it is assumed in the following, without restricting the invention in any way, that the electrode body is essentially rod-shaped, and that the electrode is made of a suitable material such as tungsten.
The axial forces exerted on the electrode during cooling are deflected by the sides of the channel, as described above. A flatter channel cross-section may therefore be associated with an improved deflection of these forces. Therefore, in a preferred embodiment of the electrode according to the invention, the ratio of channel width to channel depth comprises at most 8:1, more preferably at most 5:1, and most preferably at most 2:1.
Since the channel floor is essentially concave, the geometry of the channel floor may be defined in terms of a radius or diameter. For example, that part of the channel comprising the channel floor can follow the shape of a segment of a circle. The triaxial stresses exerted on the electrode may best be withstood by a channel floor with a relatively flat curvature. Therefore, in a further preferred embodiment of the electrode according to the invention, the ratio of channel width to a diameter of the channel floor comprises at most 10:1, more preferably at most 2.0:1, and most preferably at most 1:1.
In the case of quartz glass lamps with embedded electrodes which become very hot during operation, it is known that, during operation of the lamp, the high temperatures of the electrode subject the quartz glass to stress, resulting in cracks in the sealed portion. Initially, attempts were made to avoid the development of cracks in the sealed portion, for example by aiming to reduce the contact between electrode and quartz glass by wrapping the electrode in a foil or coil. However, in experiments leading to the electrode and lamp according to the invention, it has been established that very small cracks in the quartz glass are in fact favorable, since these later act to absorb some of the forces acting on the glass as the electrode rapidly expands, therefore relieving the glass of some stress and actually preventing the spontaneous development of large cracks. For this reason, this type of micro-crack can be termed a “relief crack”. To successfully ‘grow’ relief cracks during the cooling step in manufacture, it was found that the manner in which the channels are formed in the electrode plays an important role. In one further preferred embodiment of the electrode according to the invention, therefore, at a transition between the surface of the electrode and a channel, material of the electrode is deposited to form a plurality of brush-like protrusions. Such spikes or tufts can favor the development of the desired micro-cracks.
In another particularly preferred embodiment of the electrode according to the invention, in an alternative to the brush-like protrusions formed at the channel sides, material of the electrode is deposited as the sides of the channels to form a low ridge. This low ridge has been shown experimentally to give very favorable results, allowing the controlled growth of relief cracks during the cooling stage in manufacture. Preferably, such a ridge has a height of at most 20 μm, more preferably at most 10 μm, and most preferably at most 6 μm at a transition between the surface of the electrode and a channel. This material can be deposited by being displaced during the formation of the channel. For example, if the step of forming the channel results in the electrode material being heated and transformed into its molten state, the molten material can be deposited along the outer edges of the channel where it can later cool and harden. Preferably, the material is deposited smoothly, so that the transition between the surface of the electrode and the channel is in the form of a slightly rounded ridge.
Channels can be arranged around the electrode body in a number of ways. For example, for a rod-shaped electrode, a series of neighboring channels can be arranged in a parallel fashion, so that each channel lies on a circular circumference of the electrode body. Evidently, the thickness of the electrode, measured in a cross-section taken along the middle of a channel, would then reduced by twice the channel depth. Therefore, in a preferred embodiment of the invention, the channels are formed such that at least one channel is arranged in a helical fashion around the body of the electrode. Such a helical channel can wrap around the electrode any suitable number of times. Viewed from any vantage point, even a single such helical channel appears to be a plurality of channels. Evidently, two or more helical channels can be ‘nested’. Another type of arrangement could even comprise channels arranged to run in opposite directions so that they intersect or cross over, given a cross-hatched pattern on the electrode surface.
While it is desirable, for the reasons already put forward, to reduce the contact areas between quartz glass and electrode in a critical region along the body of the electrode in the sealed portion, it is also desirable to ensure that the electrode is held firmly by the quartz glass. Therefore, in a further preferred embodiment of the invention, the base of the electrode comprises a region treated to comprise the plurality of channels, which channel region is flanked on at least one side by an untreated region in which the surface of the electrode is essentially smooth. In this smooth region, the quartz glass can satisfactorily adhere to the electrode surface. The lengths of the smooth regions and the channel region can be governed by the type of glass being used, the electrode material, and the type of lamp in question. All these factors contribute to the temperature that can be reached during operation of the lamp and therefore the stress to which the glass will be subject.
The channels can be formed in a number of ways. For example, the channels could be milled using an appropriate milling tool. However, since the electrode rod is rather fragile, being very thin, it is preferably not to unduly subject the electrode to mechanical forces. Therefore, in a particularly preferred embodiment of the invention, the channel is formed by directing a laser beam at the surface of the electrode to remove material of the electrode or to shift or move the material from the channel to give ridges or brush-like protrusions along the interface between the channel and the electrode outside surface. The laser beam is preferably generated such that material is only removed up to the desired depth of the channel. Preferably, the electrode is rotated and moved laterally while the laser beam is being directed at the electrode, so that the desired spiral channel is formed in the surface of the electrode.
Operating parameters of the laser will govern the rate at which the material of the electrode is transformed to its molten state. For example, a high power and high pulse frequency can result in the material being spattered out from the channel. This might be desirable if brush-like protrusions are to be formed.
Alternatively, the operating parameters of the laser for generating the laser beam are chosen such that a floor of the channel, made by removing material of the electrode, is essentially smooth. The skilled person will know how a laser is to be set up in order to achieve the desired effect. By appropriate choice of the laser operating parameters, the laser beam can be generated to transfer the electrode material into the molten state while at the same time ensuring that the molten material is ‘gently’ pushed to the sides where it is deposited in low ridges along the transition region between electrode surface and channel. Favorable results can be obtained by operating a q-switched solid state laser to give a pulsed laser beam with pulses of a duration in the range 10 ns to 80 ns, at a frequency between 10 kHz and 70 kHz.
The channels made in this fashion are particularly suited for use in lamps in which a very high electrode temperature is reached during operation, since the relief cracks formed during the cooling in the manufacturing stage will protect the lamp from failure due to bead-cracks or REC cracks during the lamp lifetime. Therefore, a preferred embodiment of the lamp according to the invention comprises a gas-discharge lamp in which the electrodes comprise tungsten rods with a diameter in the range of 200 μm to 500 μm and which are disposed in the lamp such that tips of the electrodes extend into the chamber from opposite sides, and the other end of each electrode is embedded in a sealed portion of the lamp such that channels arranged around the body of the electrode are enclosed in the sealed portion.
To manufacture a lamp according to the invention, the filling must be sealed in the sealed portion, and the electrodes must be embedded in the quartz glass. In a HID lamp, the electrodes intrude into the chamber from opposite sides, and two seals are formed. For a halogen lamp, on the other hand, both electrodes enter the chamber from the same side and are embedded in a single seal. For a HID lamp, in which the fill gas in the burner or chamber is under high pressure, the filling in the chamber is preferably frozen in a manufacturing step by exposing the partially completed lamp to liquid nitrogen, which may be directed over the parts to be cooled, or may be in the form of a ‘bath’ into which the parts to be cooled are briefly dipped or immersed. Cooling a sealed pinch region in liquid nitrogen results in the inert gas of the filling to be frozen, resulting in a smaller volume. After forming the second pinch, the inert gas of the filling returns to its gaseous state. In this way, the required fill gas pressure in the burner—usually in the region 10 to 20 bar—can be obtained.
Other objects and features of the present invention will become apparent from the following detailed descriptions considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention.
In the diagrams, like numbers refer to like objects throughout. Elements of the diagrams are not necessarily drawn to scale.
In contrast,
While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art from a study of the drawings, the disclosure, and the appended claims. For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.
| Number | Date | Country | Kind |
|---|---|---|---|
| 09179788 | Dec 2009 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2010/055707 | 12/10/2010 | WO | 00 | 6/15/2012 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2011/073862 | 6/23/2011 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 5404078 | Bunk | Apr 1995 | A |
| 6452334 | Ohshima | Sep 2002 | B1 |
| 9093256 | Muelders | Jul 2015 | B2 |
| 20010005117 | Nishida | Jun 2001 | A1 |
| 20020190654 | Kanzaki | Dec 2002 | A1 |
| 20030042856 | Kaneko | Mar 2003 | A1 |
| 20060208635 | Lenef | Sep 2006 | A1 |
| 20070103081 | Boroczki | May 2007 | A1 |
| 20070296339 | Grundmann et al. | Dec 2007 | A1 |
| 20080185950 | Claus et al. | Aug 2008 | A1 |
| 20080315771 | Tsukamoto | Dec 2008 | A1 |
| 20100045183 | Deprez | Feb 2010 | A1 |
| 20100201265 | Ishikawa et al. | Aug 2010 | A1 |
| 20100270921 | Shirakawa et al. | Oct 2010 | A1 |
| 20100295448 | Takagaki et al. | Nov 2010 | A1 |
| 20100295449 | Shido | Nov 2010 | A1 |
| 20110260613 | Deguchi et al. | Oct 2011 | A1 |
| 20140232260 | Muelders | Aug 2014 | A1 |
| Number | Date | Country |
|---|---|---|
| 1391254 | Jan 2003 | CN |
| 101523550 | Sep 2009 | CN |
| 102005013759 | Sep 2006 | DE |
| 102007010607 | Sep 2008 | DE |
| 1271595 | Jan 2003 | EP |
| 2254144 | Nov 2010 | EP |
| 2001-015067 | Jan 2001 | JP |
| 2004311034 | Apr 2004 | JP |
| 2008235128 | Oct 2008 | JP |
| 2009122318 | Jun 2009 | JP |
| WO-2006099850 | Sep 2006 | WO |
| 2007122535 | Nov 2007 | WO |
| 2008032247 | Mar 2008 | WO |
| 2009025073 | Feb 2009 | WO |
| 2008119375 | Oct 2009 | WO |
| Entry |
|---|
| First Office Action, China Application No. 201080057774.8, dated Sep. 3, 2014, 28 pages. |
| Office Action, European Application No. 10809057.2, dated Mar. 3, 2015, 5 pages. |
| Office Action, Japan Application No. 2012-543951, dated May 12, 2015, 6 pages. |
| Second Office Action, China Application No. 201080057774,8, dated May 6, 2015, 25 pages. |
| EPO as ISA, PCTIB2010/055707, filed Dec. 10, 2010, “International Search Report and Written Opinion”, mailed May 6, 2011, 12 pages. |
| Fourth Office Action, China Application No. 201080057774.8, dated Mar. 3, 2016, 8 pages. |
| Office Action, European Application No. 10809057.2, dated Jul. 13, 2015, 5 pages. |
| Third Office Action, China Application No. 201080057774.8, dated Nov. 4, 2015, 26 pages. |
| Number | Date | Country | |
|---|---|---|---|
| 20120256539 A1 | Oct 2012 | US |
