BLP Gas-Phase Replication Effort - Run 7….19JAN98 ………………..(best viewed with 1024 pixel display width)
Legend
Total Input Power (electrical)
Output Power (heat)
Chamber Wall Temperature (C)
Gas Temperature (C)
Gas Pressure (torr)
Filament Resistance (ohms)
Room Temperature (C)
Inlet water Temperature (C)
Run 7 was a diagnostic run using Ar gas instead of H2. No catalyst was present and the chamber was cleaned before the run to remove gross contamination from previous runs. For the first 2.1 hours of the run the cartridge heater was set at 30 watts and the chamber was being pumped down. Then we changed to 15 watts in the tungsten filament and 15 watts on the cartridge heater…still pumping with the vacuum pumps. At hour 3.3, we filled the chamber with 2 torr of welding grade Ar gas and put 30 watts on the filament and turned off the cartridge heater. Interestingly this action killed the Tgas signal*. As you can see from the Tfil trace (the measured parameter is actually the filament resistance), some erosion started almost immediately…but it was a lot slower than with the H2 gas in the chamber. The erosion continued and necessitated periodic adjustments to the filament voltage to keep Pin approximately constant at 30 watts. At hour 5.0 we decided to evacuate the chamber again to see if the erosion would stop. We opened the valve to the vacuum system and pumped on the chamber for the next hour. Curiously this did not slow the erosion rate significantly! Earlier, (between hour 2.1 and 3.3) the filament had operated in vacuum with no signs of erosion.
At precisely hour 6.0 we closed the valve to the vacuum pump isolating the chamber but we did not fill the chamber with anything. The idea was to see if something was outgassing from the chamber walls that would increase the erosion rate noticeably. Unfortunately at hour 6.15 the filament burned out…rather unexpectedly. This early burn-out may indicate that there was a very thin spot on the filament that was very very hot in the last hour of the run. Perhaps the apparent erosion then (when the chamber was actively evacuated) was just simple evaporation of the tungsten.
One thing appears certain from this run: something in our system that is not present in significant quantity when we are pumping on the chamber initially begins attacking the filament when we close off the chamber.
* As mentioned before, the Tgas thermocouple is an ungrounded thermocouple suspended in the middle of the experiment chamber near the filament. Apparently the hot filament with ~13 VAC on one end emits enough electrons, even in a 2 torr atmosphere, to deliver a significant current to this thermocouple. We unplugged the thermocouple from its amplifier and measured a DC voltage of about -1.8 volts and an AC voltage of about 1.6 volts from one side of the thermocouple to ground…apparently some rectification was occurring, too.