WO1999014781A1

WO1999014781A1 - Alumina arc tube seal having increased resistance to thermal shock

Info

Publication number: WO1999014781A1
Application number: PCT/US1998/017619
Authority: WO
Inventors: William H. Rhodes; Jeffrey T. Neil; Paul H. Ingalls; Frederick C. Ii Dimock
Original assignee: Osram Sylvania Inc.
Priority date: 1997-09-15
Filing date: 1998-08-26
Publication date: 1999-03-25
Also published as: KR20010023251A

Abstract

A ceramic arc tube (10) for arc lamps has a body (12) formed of substantially alumina material having a bimodial grain structure with an average grain size of about 41 microns and sealing buttons made of substantially alumina having an equiaxed grain structure and a smaller grains size; i.e.: about 14 microns. The smaller grain size provides buttons having greater strength and reduced tendency to form cracks.

Description

ARC TUBE SEAL HAVING INCREASED RESISTANCE TO THERMAL

SHOCK

This application claims the benefit of Provisional Application S. N. 60.058,991 , filed September 15, 1997.

TECHNICAL FIELD

This invention relates to ceramic arc tubes and more particularly to such arc tubes having increased resistance to thermal shock.

BACKGROUND ART

Some lamps utilize a translucent alumina (aluminum oxide) ceramic tube to contain the arc plasma between electrodes. High pressure sodium lamps are the most common of this type. A newer type of such lamp is one employing metal halide technology. A typical arc tube comprises a tubular ceramic body having end caps or sealing buttons closing off the body. Electrodes are sealed into the sealing buttons. The portion of the electrode that is sealed to the buttons is typically made of niobium metal since niobium has a good thermal expansion match to alumina, which minimizes the stress levels developed during heating and cooling. Usually, the niobium is sealed to the alumina by a glass-ceramic frit such as a calcium aluminate glass ceramic.

Currently manufactured arc tubes have exhibited cracks in the sealing area that have been attributed to thermal shock. These cracks usually originate from the inside diameter of the button and propagate outward into and down the arc tube. To correct this problem it has been necessary to increase the sealing cycle time to keep the level of thermal shock below the fracture strength of the ceramic generally and more particularly below the fracture strength of the sealing button.

This decrease in cycle time greatly adds to the cost of the arc tubes.

DISCLOSURE OF INVENTION

It is, therefore, an object of the invention to obviate the disadvantages of the prior art. It is another object of the invention to enhance the manufacture of ceramic arc tubes.

Still another object of the invention is the provision of an arc tube that is less subject to cracking in the seal area.

Yet another object of the invention is the provision of a method of making an improved arc tube that is less subject to cracking in the seal area.

These objects are accomplished, in one aspect of the invention, by providing a ceramic arc tube that comprises a hollow body having from 0.02 to 0.08 vvgt. percent MgO as a first sintering aid; from 0 to 0.035 wgt. percent of a second sintering aid; and the balance A1₂0_3. If desired, a third sintering aid, such as ZrO₂ can also be used. Cylindrical buttons seal the ends of the body, the buttons having from 0.02 to 0.5 wgt. percent MgO and the balance Al₂O₃ and a smaller grain size than the hollow body.

In a method of making improved arc tubes, the steps comprise forming a ceramic arc tube having a hollow body having from 0.02 to 0.08 wgt. percent MgO as a first sintering aid; from 0 to 0.035 wgt. percent of a second sintering aid; and the balance A1₂0₃; forming a pair of buttons having from 0.02 to 0.5 wgt. percent MgO and the balance Al₂O₃, said buttons having a smaller grain size that said body; inserting said buttons into the ends of said hollow body to form an assembly; and firing said assembly to seal said buttons to said hollow body.

The use of the smaller grained sealing buttons increase the strength of the sealed area greatly reducing cracking and allowing increased speed in the sealing cycle, thus reducing manufacturing costs. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a diagrammatic, elevational sectional view of an arc tube in accordance with an aspect of the invention; and

Fig. 2 is an enlarged photomicrograph of the seal area of an arc tube manufactured in accordance with an aspect of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims taken in conjunction with the above-described drawings.

Referring now to the drawings with greater particularity, there is shown in Fig. 1 a ceramic arc tube 10 having a tubular body 12 with ends 14 and 16. The ends are sealed by sealing buttons 18 and 20, respectively.

The body 12 is composed of alumina and contains at least one sintering aid such as MgO in an amount of from 0.02 weight percent (wt. %) to 0.08 wgt. % . Additionally, a second sintering aid may be added. The second sintering aid can be Y₂O₃ in an amount from 0.035 to 0.07 wgt. %. In these amounts, the second sintering aid is present in an amount sufficient to cause liquid phase sintering. The body 12, after sintering, has a bimodal grain structure with an average grain size of 41 μ.

The sealing buttons 18 and 20 are formed from CR-6 alumina (obtainable for Baikowski International Corp. , Charlotte, NC) and contain from 0.02 to 0.5 wgt. % MgO as a sintering aid. A second sintering aid, such as Y₂O₃ can be employed so long as the amount is insufficient to promote liquid phase sintering, i.e. , 0.005 to O.Olwgt. % . While the buttons can contain up to 0.5 wgt. % MgO, it is important the body 12 contain MgO in the same concentration range as the buttons, preferably, as noted above, in the range of 0.02 to 0.08 The cylindrical buttons make an interference fit with the body 12 and, preferably, have an outside diameter that is within 5 to 12% of the inside diameter of the body 12. Sealing is accomplished because the shrinking of the body 12 under free sintering is a greater quantity than the buttons.

Employment of the CR-6 alumina containing 0.02 to 0.5 wt. % MgO provides the buttons with an equiaxed grain structure having an average grain size of 14 μ after sintering. The interface between the buttons and tube body shows no voids and excellent bonding between the relatively large grains of the body and the smaller grains of the buttons, see, for example, Fig. 2.

After assembly of the green buttons to the green body, sintering takes place by firing the assembly at 1800 to 1830°C in hydrogen for three hours. The buttons after firing are opaque (not critical since they are not part of the light producing region) and have a measured 98.8% density which assures hermeticity in lamp operation. The arc tube bodies averaged a total transmittance of 95.6% and an in-line average of 7.5 % . These values are average for high pressure sodium lamps.

Thus, the provision of sealing buttons having a different grain structure and increased strength allows faster sealing cycles and greatly reduces the costs of manufacture of ceramic arc tube.

While there have been shown and described what are at present considered the preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications can be made herein without departing from the scope of the invention as defined by the appended claims.

Claims

CLAIMSWhat is claimed is:

1. A ceramic arc tube comprising: a hollow body comprising from 0.02 to 0.08 wgt. % MgO as a first sintering aid; from 0 to 0.07 wgt. % of a second sintering aid; from 0 to up to an effective sintering amount of ZrO₂ and the balance Al₂0₃; and cylindrical buttons sealing the ends of said body, said buttons comprising from 0.02 to 0.5 wgt. % MgO and the balance Al₂O₃ and said buttons having a smaller grain size than said body.

2. The arc tube of Claim 1 wherein said hollow body has a bimodal grain structure having an average grain size of about 41 ╬╝ and said buttons have an equiaxed grain structure and an average grain size of 14 ╬╝.

3. The arc tube of Claim 1 wherein said second sintering aid is Y₂O₃.

4. The arc tube of Claim 2 wherein said second sintering aid is Y₂O₃.

5. The arc tube of Claim 3 wherein said Y₂O₃ is present in an amount of about 0.035 wgt. % .

6. The method of making a ceramic arc tube comprising the steps of : forming a ceramic arc tube comprising: a hollow body having from 0.02 to 0.08 wgt. % MgO as a first sintering aid; from 0 to 0.07 wgt. % of a second sintering aid; and the balance A1₂0₃; forming a pair of buttons having from 0.02 to 0.5 wgt. % MgO and the balance Al₂O₃, said buttons having a smaller grain size that said body; inserting said buttons into the ends of said hollow body to form an assembly; and firing said assembly to seal said buttons to said hollow body.

7. The method of Claim 6 wherein said firing takes place in hydrogen at 1800 to 1830┬░C for about 3 .hours.