OME

The Organic Material Effusion Cell OME is specially designed for the evaporation or sublimation of high vapor pressure materials, notably sensitive organic substances, at operating temperatures up to 300°C.
It can likewise be used for other elements and compounds typically evaporated at very low temperatures, e.g. alkali metals.

Due to Thermal Conduction Cooling (TCC) with encapsulated heater unit, a patented design by MBE-Komponenten, the OME offers drastically improved operation properties below the lowest temperature limit of common Knudsen-type effusion cells.
Due to the T⁴-dependency of emitted radiation power according to the Stefan-Boltzmann law, conventional effusion cells with Thermal Radiation Cooling (TRC) show a poor cooling efficiency at low operation temperatures. This results in a low cell cooling rate that renders precise temperature control below 150°C very difficult.

By contrast, the OME uses the linearity of the heat transfer between a heated reservoir and a cooled heat sink to obtain a high cooling rate even at very low operating temperatures. Rapid cooling down, low thermal time constants and a stable temperature control are thus achieved.
The novel TCC concept involves a liquid metal used as a thermal conductor that provides a direct thermal connection between crucible and heat reservoir, leaving no isolating voids. This excellent thermal contact also guarantees a very uniform temperature distribution within the crucible.

Precise temperature measurement is provided by a direct touching TC in the center of the heater area, near the crucible bottom. Accurate temperature indication is crucial for materials with steep vapor pressure curves.

The data underlying the TCC curves in the following two diagrams were measured on a 2 cm³ OME 40-2-25 cell.

The first diagram on the right compares the cooling rates of the TCC cell OME and a typical TRC effusion cell.
While the cooling rate of the TRC cell only slowly increases with cell temperature in agreement with Stefan-Boltzmann law, the cooling rate of the TCC cell increases linearly with temperature.

Despite its larger thermal mass the cooling rate of the OME is much higher at low temperatures and reaches 10°C/min even at 70°C cell temperature. 

TCC cell OME and a TRC cell during a rapid cooling down procedure are compared in the second diagram on the right. It shows the resulting temperature decrease of both cells starting at the same cell temperature of 150°C.

After switching off the cell heaters the OME is cooling down about four times faster than a TRC cell comparable in size.

The diagramm on the left-hand side shows the excellent long term temperature stability of the OME. It displays a test run at setpoint 100°C with a constant cooling water flow of 0.5 l/min.

The measured stability over a period of 10 hours of operation with PID controller is better than  ±0.02K.

The fast heating up and cooling down times of OME cells are shown on the second testing chart.

Remarkably, cells with TCC technique allow heating up and cooling down steps with absolutely no temperature overshoot. This is the qualification to prevent waste and potential destruction of the evaporants.