Variable Temperature Micro Probe System

Variable Temperature Micro Probe Systems

MMR's Variable Temperature Micro Probe Systems, VTMP, (or Low Temperature Micro Probe Systems, LTMP) allow the user to probe devices mounted on the cold stage of a micro refrigerator. Each manipulator allows the corresponding probe to be positioned anywhere on a 10 mm x 12 mm sample to a precision of 50 microns. The chamber has four probes.

Features

Can be used as a standard probe station.

Operating Temperature: 70 K to 730 K

Temperature Stability: ± 0.05 K with K-20 Programmable Controller

Temperature Response: 1 K/sec

Cooling Capacity: 250 mW at 85 K with nitrogen; 500 mW with Ar at 90 K

Cold Pad Size:14 mm x 10 mm

Window Material: Sapphire, KRS-5, Ge, etc.

Maxiumum Sample Size: 10 mm x 12 mm

Probe Travel – X/Y/Z: 25 mm x 25 mm x 6 mm

Positioning Accuracy: better than 50 microns

Electrical: SMA connectors with outer shield insulated from VTMP case. Mini-coax connects SMA to probe.

 

Overview

Micro Miniature Refrigerators are small, cryogenic refrigerators that derive their cooling power from the Joule-Thomson expansion of a high-pressure gas. This effect is amplified by using the cooled gas to pre-cool incoming gas in a counter-current heat exchanger. Temperatures down to 70 K can be achieved in devices a little larger than a matchbox in size. Typically, the cold stage is a ceramic pad 14 mm x 10 mm in size supplied with a temperature sensor and resistive heater.

Micro miniature refrigerators provide many advantages in scientific applications:

 

Fast cool-down and warm-up. <15 min.

Precise temperature control: ± 0.1 K

Absence of mechanical, acoustic, or electrical noise

Small size

Wide range of operation: 70K to 730K

Low cost of operation:  $0.50/ hour

No maintenance required

 

WHAT IS JOULE THOMSON COOLING?



When a real gas, as differentiated from an ideal gas, expands at constant enthalpy (i.e., no heat is transfered to or from the gas, and no external work is extracted), the gas will be either cooled or heated by the expansion. That change in gas temperature with the change in pressure is called the Joule-Thomson coefficient and is denoted by µ, defined as:

µ = (dT/dP) at constant enthalpy

The value of u depends on the specific gas, as well as the temperature and pressure of the gas before expansion. For all real gases, µ will equal zero at some point called the "inversion point". If the gas temperature is below its inversion point temperature, µ is positive ... and if the gas temperature is above its inversion point temperature, µ is negative. Also, dP is always negative when a gas expands. Thus:



If the gas temperature is below its inversion temperature:

-- µ is positive and dP is always negative

-- hence, the gas cools since dT must be negative

If the gas temperature is above its inversion temperature:

-- µ is negative and dP is always negative

-- hence, the gas heats since dT must be positive