CARBO-NIR simulation chamber
An environmental simulation chamber 'CARBON-NIR' for sublimation-condensation measurements down to -230°C was designed and built in 2010 as a complement to the reflectance spectrometer.
Pictures: CARBONIR enviromental chamber: drawing, general view and top and side pictures showing the different windowns (top and lateral) and the inside simulation cell
The CarbonIR cell is in copper for a high thermal conductivity (about 400 W.m-1.K-1 at 200 K). The aim being of studying centimeter-scale thick and large samples, the cell is a cylindrical box able to contain samples 8 cm in diameter and 6 cm high. It has an only aperture in its side for injection or outlet of gas via a tube (1 mm of diameter). Through the cell wall, the tube is hermetically fixed with silver. Silver is chosen for limiting thermal conduction between the tube and the copper.
A transparent window encircled by a metallic flange composes the cover. The interest of studying analogs of Mars seasonal condensates with different texture and compositions has been mentioned previously. From this perspective, the lid was conceived to be removable. It can be hermetically screwed up to the cell using high vacuum grease as a seal. Its mobility makes possible the insertion of samples created outside. Therefore it significantly increases the possibilities regarding the components and structures to be studied under controlled atmosphere.
The large transparent disk of the cover is chosen for the spectral measures from above. It also favors the evacuation of radiative flux emitted inside the cell, which is necessary for the purpose to reproduce radiative cooling that occurs on Mars. The controlled atmosphere inside the experiment cell leads to the impossibility of a direct spectral measure of the sample. Indeed, the incident signal sent by the spectro-gonio radiometer goes through two transparent windows to reach the surface, and again for the reflected light before reaching the detector. To minimize perturbations of the light signals, windows are made of sapphire. This material offers a good transmission (> 80 % at each interface) in the wavelengths range of interest.
Two diametrically opposed 3.4-cm diameter glass windows are located through the vertical cell wall. They make possible to follow the vertical evolution of the sample. A 18.0 megapixel digital single-lens reflex camera with 70-300 mm lens and intervalometer remote enables to monitoring it through one of those windows. Moreover, with those lateral apertures translucence can be defined by the use of transmission of a light beam (possibly a laser) through the ice. These windows complete the sapphire cover to offer a wide and easy view of the inside of the cell. The total effective volume of this cell is 312 cm3.
The copper environmental cell is placed inside a bigger metal chamber can be hermetically closed and bound to a vacuum pump to create high vacuum inside (about 1x10-6 mbar). Vacuum makes thermal isolation between the experiment cell and the surrounding environment. The vacuum chamber is a stainless steel cylinder with lateral openings for pumping, as well as for connections of the copper cell with other devices (for pressure and temperature controls). It is closed by a tip includes a sapphire window. The environmental cell is hold up at the center of the vacuum chamber by being fixed on a small bent copper piece with its other end in contact with the cooling head -- located inside the chamber -- of a cryostat.
Cryostat and temperature
We use a cryostat from Sumitomo that can cools down to -230°C by means of a helium compressor (#HC-4E1, and cooling head #CH-104). The cryostat power is about 45 W. The bent copper piece on which lays the simulation cell allows a good thermal connection between the bottom of the cell and the cryostat. The cryostat is used with dynamic high vacuum inside the external chamber in order to reduce thermal loss and limit condensation inside, especially on the cell windows and cryostat parts.
The temperature of the chamber is controled with a heating resistance (model Lakeshore HTR-50) inserted inside the copper under the environmental cell, at the bottom center. This 25 ohm resistance is connected to an external control box where the required temperature value can be configured to 0.1°C. This box also displays the temperature measured by a resistive thermometer Pt-100 also located under the cell but near the cylinder edge. This second probe helps to measure the thermal gradient along a radius of the cell. The temperatures are displayed with +/- 0.2°C uncertainty.
note: the temperature sensors will be changed (january 2015) for Si diodes that will allow to measure and contol down to -240°C with a high accuracy temperature controler.
The thermodynamic system of the SERAC cell is also used here. It is composed of a glass container that acts as a reservoir for 10 liters of gas, connected to a gas bottle, a primary pump and a turbo pump and an absolute pressure sensor MKS Baratron 390HA series, 100 Torr type. It measures pressures between 10-4 mbar and 133 mbar. Operating and display are done on an electronic box MKS #270C with 0.08% uncertainty. Valves between all the components enable to isolate them when required by the experimental protocols.
A mass flow controller (MKS MF1-type) can be inserted between the thermodynamic system and the CARBONIR cell. It enables to measure the gas injection rate towards the cell. In such a case a pressure sensor (MKS Baratron 100-Torr #626A) need to be added close to the cell to read the pressure downstream this flowmeter, and thus inside the cell. Adjacent to this sensor, a security valve avoids accidental over-pressure that could damage the cell.
The mass flow controller and the absolute pressure sensor are analogically connected to a data acquisition card (DaqBoard1005, 0-10 volts) linked to a computer. Real-time graphic display and record of the measured values are then possible, at a chosen frequency, using a LabView interface.