A Simple Method of Repair of Graphite HeatersVed Prakash Citation: Review of Scientific Instruments 41, 1256 (1970); doi: 10.1063/1.1684791 View online: http://dx.doi.org/10.1063/1.1684791 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/41/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A simple heater for use in a thermoreflectance spectrometer Rev. Sci. Instrum. 62, 2034 (1991); 10.1063/1.1142360 A Simple Vacuum System Substrate Heater Rev. Sci. Instrum. 43, 148 (1972); 10.1063/1.1685417 A Simple, Inexpensive Heater for Solid Samples in a Mass Spectrometer Rev. Sci. Instrum. 40, 1515 (1969); 10.1063/1.1683853 Graphite Heater for MHD Studies Rev. Sci. Instrum. 35, 846 (1964); 10.1063/1.1746814 Simple Method of Repairing a Cracked Glass Accelerator Tube Rev. Sci. Instrum. 32, 91 (1961); 10.1063/1.1717161

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

132.206.7.165 On: Thu, 18 Dec 2014 18:01:55

1256 NOTES

extends into a clearance hole bored into a bronze ball bearing. The ball is free to rotate in a spherical bearing cavity (pivot bearing) machined in an aluminum block. The spherical bearing cavity is attached to a microposi­tioner which uses two standard micrometer heads to move the micropositioner bed ± 1.27 cm along two orthogonal directions in a plane parallel to the vacuum flange.

For the goniometer shown here the angle change is 7.28° /cm travel. This corresponds to an angle change of 0.0185° for each division on the micrometer head (0.001 in.) and allows a total travel of ±9° from the central axis of the unit. With the bearing tolerances allowed on the unit shown (25 /.I), the reproducibility of an angle setting is 0.02° or less. The differential error in an angle change is considerably less than 0.02° except near 0° where the bellows are in their relaxed position, and there is no force exerted on the shaft by the pivot bearing. Because this goniometer has two orthogonal directions of motion it allows rapid and precise alignment of the crystal.

1 E. Bogh and E. Uggerh~j, Nuc!. lnstrum. Methods 38,216 (1965).

A Simple Method of Repair of Graphite Heaters

VED PRAKASH

National Physical Laboratory, New Dethi-12, India

(Received 1 April 1970)

M OST high temperature laboratory furnaces have graphite heating elements when the ambient is

either high vacuum or an inert gas. A cylindrical grid type heater (Fig. 1) was first developed by Stockbarger1

for growing calcium fluoride single crystals. The labora­tory fabrication of such a heater has been described by several authors (Mackenzie and Bockris,2 Marshall and Wickham,3 and Prakash).4 However, this graphite grid heater is rather fragile; a minor jerk or strain on some part of it while it is being fabricated in the workshop or being fixed in the furnace can result in a broken limb of the heater and consequently in the loss of a considerable amount of workshop time in making another one. Usually damaged grid heaters have to be discarded since there is no way to weld or solder the material. The use of a con­ducting adhesive cement is also ruled out because these heaters are generally used for temperatures above 1500°C. The grid type graphite heaters (Fig. 1) have been in use in this laboratory for a number of years.4 To save time and effort a method of repairing damaged heaters has been tried with considerable success.

FIG. 1. The graphite heater.

Figure 2 shows the cross section of the repaired part when the stress is at the periphery of the cylindrical heater. The damaged grid is slipped on a wooden mandrel and fixed with paraffin wax to avoid further stress. Two drill holes 3.57 mm diam are made at a suitable distance, one on each side of the line of the crack. A couple of molybdenum strips of thickness 0.13 mm is prepared. The length and breadth are made to suit the width of the grid element. Two 3.57 mm holes in each of these strips are made at the same distance as in the graphite part. The heater is carefully removed from the mandrel by heating it slightly and the strips are fixed to it by the help of two 3.17 mm graphite screws and nuts which can be easily made in a laboratory workshop. Before placing the mo­lybdenum strips, a thick coating of Aquadag is applied on the faces in contact with the graphite so that the crack and other voids are filled after tightening the screws and nuts. The distance between the two holes should be as small as possible to avoid any appreciable change in the resistance of that part of the grid. The size of the screws and nuts and the thickness of the molybdenum sheet are not critical and can be varied as desired.

If the breaking point is in the middle of the length of the grid, the repaired part introduces some distortion in the thermal field. This renders the method unsuitable whenever stringent thermal field conditions are required. If the repair is done near the ends of the heater, no serious distortion is expected.

One heater with two such molybdenum joints has given quite satisfactory service for more than 200 h of running

LINE OF CRACK

4QUAOAG COATING

MOLYBDENUM STRIP

/'

/ /

(,~,",-- ~G.'PHITE SCREWS

---.,:,,,,,,'-.; "'" ", AOUADAG COATING

MOLYBDENUM STRIP

FIG. 2. Exploded diagram of the repaired assembly.

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

132.206.7.165 On: Thu, 18 Dec 2014 18:01:55

NOTES 1257

and is still in good condition. The joint with molybdenum is suitable up to about 1800°C above which some other refractory metal such as tantalum can be used.

The author wishes to thank Professor A. R. Verma, Director National Physical Laboratory, for permission to publish this note.

1 D. C. Stockbarger, 1944, cited in "Artificial Optical F!uorite," Office of Scientific Research and Development, Report No. 4690 (Publication Board, Department of Com~erce, 'Yashington, D. C.).

2 J. D. Mackenzie and J. O'M. Bockns, J. SCl. lnstrum. 35, 109 (1958).

3 K. H. J. C. Marshall and R. Wickham, J. Sci. lnstrum. 35, 121 (1958).

4 V. Prakash, Indian J. Tech. 2, 46 (1964).

A Holder Device for Polarized Fluorescence Studies of Rotating (Dye-)Polymer Films

A. RUPPRECHT

Bacteriological Bioengineering, Department of Bacteriology, Karolinska Institutet, S-10401 Stockltolm 60, Sweden

(Received' 27 February 1970)

A POLARIZED fluorescence method for the study of various types of orientation in (dye-) polymer films

has been developed by Nishijima et al.l,2 It involves excitation with polarized light of the chromophoric groups in a rotating polymer film and recording the intensity after passage of the fluorescent light through an analyzer. These measurements were performed with specially con­structed instruments. The theory of polarized fluorescence of oriented polymers has recently been further developed by various authors.3- 7

. In this note a simple device with rotating film holders is described which makes possible polarized fluorescence measurements with an ordinary Zeiss ZMF 4C spectro­fluorometer. This device has been used in type C measure· ments2 on oriented dye-LiDNA films to obtain informa­tion about the molecular arrangement of acridine dyes in the uniaxially oriented DNA matrix.8 Figure 1 shows the construction of the holder device.

A round quartz plate (17 mm diam) has been used as carrier for the dye-LiDNA film which was about 10 mm wide. A Zeiss scale holder (i.d., 18 nun; hole diam, 15 mm) kept the plate with sample film in place on the brim of the holder cylinder. To eliminate reflexes from the trans­mitted light the surface of the cylindrical cavity of each holder was covered with a black cloth. A Fleischmann power pack, type 712, was used as dc source for the driving motor; the speed of rotation of the holders was adjusted to about 0.5 rpm.

Before starting an experiment the driving motor was operated via the microswitch which stopped the rotation

FIG. 1. Device with rotatable holders for polymer films (as seen from above) which fits into the ordinary cuvette holder space of a Zeiss spectrofluorometer. A-Precision electric motor. (Graupner, type Micro T 05) with reduction gear box (485: 1 ratIO), B-cam on a holder pipe cogwheel, C-microswitch which stops the motor when activated by the cam. The various components are mounted on a brass plate which is inclined 45°. This is fast~ned adjustably with two sidearms (at the arrows) to each back slde of the brass cuvette holder stand; a separated sample holder is shown to the right.

in a defined position of the holder pipes as determined by the cam. Thereafter the sample holders were put in place and turned manually in the stationary holder pipes until the direction of molecular orientation was horizontal. A bypass switch which overrides the microswitch was closed to start the driving motor. After the fluorescence intensity curve was recorded, the bypass switch was opened to make the microswitch automatically stop the rotation after completion of the revolution of the holders. In this way the recorded curve could be correlated with the angle of rotation of the sample.

This work was supported by the Swedish Natural Science Research Council (Dnr. 2509).

1 Y. Nishijima, Y. Onogi, and T. Asai, J. Polymer Sci. Pt. C 15, 237 (1966).

2 Y. Nishijima, Y. Onogi, and T. Asai, lnt. Syrnp. Macromol. Chem. 7, 161 (1966).

3 Y. Nishijirna, Y. Onogi, T. Asai, and R. Yamazaki, Rep. Progr. Polymer Phys. Japan 10,465 (1967).

4 C. R. Desper and 1. Kimura, J. Appl. Phys. 38, 4225 (1967). 6 R. S. Stein, J. Polymer Sci. Pt. A-2 6, 1975 (1968). 6 A. Rupprecht, European J. Biochem. 10, 297 (1969); 71. Kimura, M. Kagiyarna, S. Nomura, and H. KaWai, J. Polymer

Sci. Pt. A-2 7, 709 (1969). 8 A. Rupprecht, R. Rigler, B. Forslind, and G. Swanbeck, European

J. Biochem. 10, 291 (1969).

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

132.206.7.165 On: Thu, 18 Dec 2014 18:01:55