PERSONA ENFERMA
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proved mechanically reliable.
One rotation of 2 and 3 was a full oscillation period. The usual speed was given by a 15 : 3 8
reduction between gears 1 and 2 , a gearbox ratio 2 :125 , and 250 rpm motor speed. This gave a period of 3 8 seconds.
The whole system is seen in Figures 2. 8 and 2.9.
2.6 Power Control
The original series connection of Globar elements was a cause of recurring trouble. Element resistance increases with age , and in the series connection the highest resistance element takes most power. Aging is therefore accelerated till failure takes place. In parallel connection , on the other hand , elements with less resistance take more power, and the process of
aging remains stable within the whole element set. This greatly increases element life.
A complication is introduced by the temperature coefficient of resistance for silicon carbide , the basic element material. From room temperature to 800°c the
coefficient is negative , so that in parallel connection there is a tendency for thermal runaway. It has been observed that during heating up some of the elements do run at a higher temperature than others , but this condition
is only held for about one hour. Above
soo
0c ,
the coefficient turns positive , so in the working region the parallel configuration is stable both to aging and to temperature distribution .To meet the current requirements for parallel running , a 1 : 4 step-down trans forme r with 6 0 amp secondary was added . About 500 hours running was done with this with no loss of elements , compared to 3 replacements in 100 hours with series connection .
on-off control is rather drastic , so it was decided to use a Variac as a more stable element .
The oscillating control point equation can be obtained in practice by using the Variac voltage as control
setting F , and driving with a constant speed reversible motor. The small relay of the controller gives an
output suitable for operating the drive motor.
A 4 kVA Zenith Variac with 15 amp secondary was used. The torque required to change its setting was of the order of 10 kg-cm, compared to the 2 kg-cm
maximum of Philips gearboxes . To link these a 1 :16
gear reduction followed by a 1 � 4 belt drive was used. The spring-tensioned belt protected the Variac i f driven to its limits . The usual gearbox reduction was 1 : 125 , the motor being a Philips AU 5 100/22 of speed
250 revs/minute. At the final Variac driving speed of 1.9 revolutions per hour, several seconds were required for taking up backlash. This placed a lower limit on the period of the control oscillations .
Current limiting was required during the initial heating time. A relay operated by the furnace current to override controller logic for excessive currents proved to have too high a differential, so a transistor Schmitt trigger circuit (Figures 2. 10, 11) to do the same job was built by Mr. R. A. Vincent. A 0. 2n, 100 watt nichrome resistor was made and inserted in the Variac output circuit. The voltage across this was fed through a 1 : 1 transformer to the trigger, which operated a change over relay in the motor connections. The logic can best be seen from Figure 2. 12.
The low and high control wires from the controller were the outputs of a changeover relay, so that in normal operation one was at mains voltage and the other floating
with a motor winding . When the triggered relay operated,
the low control side of the motor was connected to the mains, so the Variac voltage decreased regardless of the controller setting.
To measure the current, an insulated wire in the
chassis so that a clip-on 50 cycles ammeter could
be used , as in Figure 2 . 1 3 . The A. C . voltage input to the trigger was also available on the front of the panel . Figure 2 . 13 shows the Variac , switching chassis with Schmidt trigger, and the furnace itself. Figure 2 . 14 shows the whole system.
The triggering voltage could be set by the pot . marked ADJUST in Figure 2 . 10 . The range of adjustment was 11 to 22 amp in the 0 . 2n resistor. Normally
15 amps was used to give the fastest heating possible without overloading. The same accumulators were used
4 4
for the
o.c.
supply as provided the sweep and oscillating voltage . The heaviest current drain was the 1/2 amptaken to operate the relay , but as this was not required after the initial heating , the control was not affected .
An 18 amp circuit breaker was added for emergency protection .
The behaviour of the final system can be seen
diagrammatically from Figure 2 . 15 . At point A, with the Variac set to a low voltage , the furnace and control are switched on . The Variac voltage increases ti ll the set maximum current is reached, and then cycling about this point continues , due to the current limit control , till the furnace has attained the set temperature . Less power
is required to maintain the temperature than to heat up , so the current drops towards point B . The current control is now not needed , but it is easiest to leave it ready to operate .
At point A the oscillator motor was turned on ,
but not the sweep . From the time the set temperature
is reached, then , the furnace is under temperature
control . After a suitable time , the sweep motor is
switched on at B, and the controlled temperature decreases . At point C , when the temperature is low enough , the
whole system can be switched off . Operator intervention is required at A , B , and C only .
A chromel-alumel thermocouple could be inserted
through the observation hole of the furnace for direct
measurement , as shown in Figure 2 . 14 . The voltage from this was recorded on a Heathkit chart recorder , Figure 2 . 16 .
A record of one run i s shown in Figure 2 . 17 . The two decreases after heating up are due to manual resetting
of the sweep circuit . Fluctuations are less than
s
0c.
Worse fluctuations than this ( 10°c or more ) were occasion
ally recorded . These were usually traced to a loose belt drive .
2 . 7 Crucibles and the Use of a Thermal Shield
For most of the crystal growing , 30 ml Johnson Matthey platinum crucibles with lids were used. They were cleaned by soaking in concentrated AnalaR HCl
for several days , and then washed several times in deionised water.
AnalaR zno and Extra Pure PbF2 were used with-
out further puri fication . For nickel doping the nitrate or oxide were used , and carbonate or nitrate for cobalt . At first the crucible lids were put on by hand only , and over 60% of the flux was lost each time . Subsequently the lids were crimped on with pliers , and losses reduced to 2 0% or les s .
A small crucible with a conical bottom was used for the third growing attempt , which was the first to produce a large crystal . A shaped piece of magnesia brick was used to hold the crucible upright , and it was suggested that thi s aided the growing process . To test this , the following runs were tried with Johnson-Matthey crucibles set into a brick as in Figures 2 . 18 and 19 . It was found that large crystals were more likely to be produced with the brick than without . It is presumed that two effects contributed.
First , a thermal slug action reduced temperature fluctuations. Consider as a guide the relaxation time for a temperature change across a slab, thickness L, of the brick itself.
T = p S LK 2
= 2½ minutes for p 20 lb/cu.ft.