Method of making a segmented heater system

A zig-zag picket graphite heater for use in high temperature furnaces of the type used for crystal growing, for example, utilizes a plurality of discrete graphite arcuate shell segments having vertically extending slots alternating from the upper and lower ends thereof. The edge pickets are adjacent slots which extend upwardly from the lower end of the segment and have extensions projecting below the other pickets so as to provide pads below the level of the slots. Graphite connectors span the adjacent pads of adjacent arcuate segments and certain of the connectors are also connected to electrical power supply terminals.

BACKGROUND OF THE INVENTION The present invention is directed to zig-zag 
picket type graphite heater elements for use in crystal growing furnaces 
and the like, and particularly to heaters wherein a plurality of like 
graphite cylindrical shell segments are connected in circumferentially 
separated position, but cylindrical formation, by graphite connector 
elements, certain of which also function to couple the electrical current 
source to the heater. The present application is directed to improvements 
in heater systems of the type disclosed in the present applicant's 
application Ser. No. 522,727, filed Aug. 12, 1983 now U.S. Pat. No. 
4,549,345 which is a division of Ser. No. 323,122, filed Nov. 19, 1981, 
now U.S. Pat. No. 4,410,796. 
DESCRIPTION OF THE PRIOR ART 
Present commercial zig-zag picket heating elements are provided in the form 
of a one-piece cylindrical shell, which surrounds the crucible holding the 
material to be critically heated. Such heaters are conventionally machined 
from graphite to function as an electrical resistance heater which 
produces the relatively high temperatures required, for example, in 
crystal growing operations. Because such heaters are very expensive, and 
quite brittle and subject to cracking or chipping while in use, which 
affects the uniformity of their application of heat to the crucible, the 
need for the frequent replacement of such heaters has added considerably 
to the expense of the crystal growing operation. Some of the following 
patents cited in the afore-mentioned applications are indicative of the 
present state of the art. 
SUMMARY OF THE INVENTION 
The present invention is concerned with a segmented graphite heater and a 
method of assembling a segmented heater system in cylindrical formation. 
The heater comprises a plurality of circumferentially separated graphite 
shell segments, each having vertical slots extending alternately from the 
upper and lower ends thereof, substantially the full length of the 
segments, to provide pickets in zig-zag array. The edge pickets are 
adjacent slots which extend upwardly from the lower end of the segment, 
and these pickets have extensions projecting below the other pickets, 
which are formed with pads below the level of the slots. Graphite 
connectors at the level of, and spanning the adjacent pads, are employed 
below the level of the inboard pickets and slots to connect the segments 
in cylindrical formation, and certain of the connectors are also utilized 
to supply electrical power to the pads. The connectors are installable 
from inside the cylindrically arranged segments and hold the pickets of 
adjacent segments in absolute vertical alignment so as to maintain 
substantially the same width between segments as the width of a slot, and 
thereby provide the uniform heating characteristics which are necessary 
for crystal growing. 
One of the prime objects of the present invention is to provide a segmented 
heater assembly which can be readily assembled in precise alignment in a 
very simple and easy manner from the interior of the cylindrically 
arranged segment. It should be appreciated that heaters of this character 
may weigh in the neighborhood of 25 to 80 pounds, and need to be installed 
by people who are not used to lifting heavy weights, and who must reach 
inside the puller or furnace chamber to accomplish the assembly operation, 
inasmuch as the exterior of the heater is, for the most part, inaccessible 
around most of its periphery in modern-day puller chamber installations. 
A further object of the invention is to design an improved segmented heater 
which connects the discrete segments in a manner which not only aids the 
assembly operation, but also insures the uniform heating and other 
attributes of monolithic zig-zag heaters, while avoiding their undesirable 
characteristics. 
Other objects and features of the invention will become apparent by 
reference to the following specification and to the drawings.

In the present applicant's aforementioned U.S. Pat. No. 4,410,796, which is 
incorporated herein by reference, a high temperature furnace is 
schematically illustrated, and shows a crucible for containing the 
crystalline material to be pulled, such as molten silicon, surrounded by a 
zig-zag picket type heater. It is to be understood that the presently 
improved heater system is to be used in the same manner, and for a more 
detailed description of the general environment in which such heaters are 
used, attention is directed to Arst U.S. Pat. No. 3,359,077 and Bochman et 
al U.S. Pat. No. 3,798,007. 
As best seen in FIGS. 1-3, my improved heater system or assembly includes a 
series of discrete arcuate segments, generally designated 10 (four being 
shown in the present drawings), disposed in cylindrical formation in 
circumferentially spaced relationship. Each of the segments is shell-like 
and has a first series of slots 11, extending axially downwardly from its 
top edge 12, and a second series of axially extending slots 13, extending 
axially upwardly from lower edge 14. The equal width slots 11 and 13, are 
uniformly circumferentially spaced from each other, and the axial length 
of slots 11 is equal to the axial length of slots 13. As best seen in FIG. 
3, the slots 11 and 13 which alternate circumferentially to provide a 
series of pickets in zig-zag formation, do not extend for the entire axial 
length of the heater. As FIG. 1 clearly discloses, the inboard pickets 15 
which are all formed are of the same axial length, whereas the outboard, 
or edge, pickets 16 of each segment 10 extend downwardly below the level 
of edge 14, and are formed with inwardly projecting, circumferentially 
enlarged pads 16a thereon, which project laterally at a spaced vertical 
distance below the edge 14. 
The heater element 10 may be constructed separately or in the manner 
previously described, by first machining a cylindrical blank of graphite 
to the desired final dimensions of the cylindrical shell and then cutting 
the slots 11 and 13. Four of the slots 11 may then be extended for the 
full length of the cylindrical heating element at uniformly spaced 
positions to separate the cylindrical element into four like cylindrical 
shell segments 10. The separated segments 10 are then reassembled by 
graphite connectors 17, and graphite power connectors 18, in a condition 
in which the spaces 19 between adjacent segments 10 is equal to the width 
of slots 11 and 13. Each of the segments 10 has n end slots 13 and n+1 
slots 11. In the present embodiment, each segment 10 has three slots 11 
and four slots 13, for example. 
Referring now more particularly to FIGS. 4-8, each of the pads 16a of the 
outboard pickets 16 projects radially inwardly of the wall of the picket 
to form a mount 20 of generally triangular configuration in plan view 
having a vertically flat mount surface 20a, which, for a purpose to be 
later described, is provided with an axially parallel vertical slot 21. 
Each pad 16a, further is provided with horizontally disposed threaded 
bores 22 on opposite sides of the slots 21 for receiving graphite screws 
23, which have integrated washers 23a and hexagonal heads 23b. 
Screws 23 are provided to secure the upstanding vertical wall 24 of each 
connector element 17, in bridging relationship to the pads 16a of the 
adjoining outboard pickets 16 of adjoining segments 10. Connectors 17 are 
also provided with horizontal feet portions 25 with horizontal flat 
surfaces 25a on which the flat lower surfaces 16b of the pad 16a are 
received. It is to be observed that mating vertical slots 26 are also 
milled in the connectors 17 and that the respective slots 21 and 26 
receive vertical alignment rib pins 27 to ensure the vertical, axially 
parallel relationship which guarantees that the space 19 will be uniform 
from top to bottom, and so of a width equal to the width of slots 11 and 
13. Bores 28 are provided in the wall 24, in alignment with the threaded 
bores 22 provided in the pad 16a, to receive the screws 23. 
The connectors 18 connect the adjacent pads 16a in exactly the same manner. 
Accordingly, the description will not be repeated, and the same numerals 
have been utilized to identify like parts. The connectors 18 differ in 
that the horizontally extending feet 25 thereof are extended and have 
vertical openings 29 for receiving power supplying electrodes 30 (see FIG. 
3). The outer ends of the feet 25 are preferably split as at 31 (see FIG. 
1), and inboard portions thereof are also slotted as at 32 to provide 
gripping capability. Also, a threaded bore 33 is provided in each power 
connector foot 25 for a screw 23 which is used to clamp the power 
electrodes 30 in position. Provided to support the connectors 17 from the 
bottom 34 of the puller chamber, are screw jacks 35 which extend into 
threaded openings 35a in the feet 25 and which are received on suitable 
ceramic or quartz insulator tubes 36. 
The arrangement disclosed finds the quadricylindrical segments 10 
detachably assembled to each other, with the individual segments 
concentric about a common central axis the zig-zag current flow path 
provides a more uniform heat circumferentially above level 14, in the 
space in which the crucible is disposed. Within this space, each picket is 
of the same width and the same thickness, and the spacing of the pickets 
is uniform. Cracking or chipping of an individual picket 10 requires only 
that that an individual segment be replaced, and does not require the 
scrapping of the entire heater. The electrical circuit may be that 
disclosed in the present assignee's aforementioned U.S. Pat. No. 
4,410,796, and any suitable source of power may be utilized. 
When installing the heater system, or replacing one of its segments, it 
should be apparent that assembly can be readily achieved by placing the 
segments 10 in formation, and simply inserting the pins 27 in the openings 
provided, which ensures correct alignment so that the screws 23 may be 
inserted and utilized to clamp the connectors and segments in assembled 
relation. 
While one embodiment of the invention only has been described, it will be 
apparent to those skilled in the art that the disclosed embodiment may be 
modified. Therefore, the foregoing description is to be considered 
exemplary, rather than limiting, and the true scope of the invention is 
that defined in the following claims.