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THERMAL CONDUCTIVITY OF SILICATE BONDED SAMPLES

Status of measurements with the thermal conductivity facility in Firenze

June, 7th 2007

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MEASUREMENT & SETUP

Recall the measurement main features:• steady state measurement• PT1000 sensors connected in a Wheatstone bridge• sample homogeneity and heat flux homogeneity

INSULATOR

BOXHEATER

COPPERBOX

CLAMP

SAMPLE

THERMOMETERS

SAMPLEHEATER

cryogenic liquid

cold plate

Elba GWADW 2006 test measurement on pure Si reed

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SILICATE BONDING MEASUREMENT

By moving to the new peculiar geometry of silicate bonded disks, several difficulties arise:

Sample with 1” diameter, 12 mm thick

• PT1000 position against the lateral surface should be known as precisely as possible, since:

• Very high conductance requires the power flowing to be pretty high for temperature gradient to become measurable; •Thermal contacts are a determinant issue, and in general heat flux must be kept homogeneous;• Sample (bonding) inhomogeneities spoil the measurement accuracy by bending the heat path.

S

lK (K is the conductance)

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CURRENT SETUP

PT1000

INDIUM FOILS

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CURRENT SETUP

1 mK

With the current setup (with old copper heating coil), we obtained the first good measurement at room temperature:

Pure Si across Bonding

160 W/mK 150 W/mK

(Si @300K has ~150 W/mK)

%2%4%1%1

calibration

power

geometry

PT1000 calibration

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HEATING TOOL

Long time has been spent in understanding the behaviour of the setup, testing and improving almost all of its parts.

Homogeneity of heat flux has proved to be determinant.

The first heater we made

contact surface

Rear with Joule coils

Carved groove for hosting a PT1000

Aluminum heater now employed

thickness helps the flux to become homogeneous

flat constantan coil

flat contact face

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TEMPERATURE SENSORS

PT1000

PT1000 flat surfaces are too wide for having a precise determination of sensing point on the sample…

• sharp contact region• two PT1000s in series (doubled sensitivity)• PT1000 wires soldered in a robust way

With aluminum rings:

Three point measurement allows to be evaluated on pure Si and across bonding

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A GOOD THERMAL CONTACT

In order to have a homogenous heat flux, thermal contacts must be realized carefully.

We found that the contact between sample and heater (and sink) was not good, even with grease.

By inserting indium disks, contact is good all across the surface

SINKHEATER

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HISTORY OF SAMPLES MEASUREMENTS

• N° 1 & 2, 1” samples - test of the apparatus, no good data - both broke after cooling cycle - both showed inhomogeneity in the bonding layer

• N° 3, 1” sample- First good measurement at room temperature- first cooling cycle seemed to result in worsening of the bonding:

date pure Si bonding actions

8/3 160 150 Final setup but with copper heating coil

11/4 186 102 After the first cooling cycle

26/4 184 115 After a new cooling cycle, sample has been removed and placed again. New indium disks, new constantan heating coil

17/5 180 134 New calibration, acquisition problems resolved, indium disks have been changed with better ones

4/6 179 109 After two cooling cycles

ROOM T MEASUREMENTS

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INHOMOGENEITIES IN THE BONDING LAYER

If the bonding realizes a bad thermal contact in some point between the two disks, measurements can be severely altered.

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Temperature on an aluminum sensor with a 2mm cut in the bonding layer (1K is set across the whole sample)

INHOMOGENEITIES IN THE BONDING LAYER

On the ring sensor, a 2mm lateral cut in the bonding results in 30 mK spread of T over a 1K temperature gradient on the whole sample. Depending on the position of PT1000s, that leads to an error as high as 3%.

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FIRST CONDUCTIVITY CURVE

A first complete curve of conductivity for the third sample has been obtained:

Temperature [K]

Therm

al C

onduct

ivit

y [

W/(

m K

)]

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CONCLUSIONS

• In its final version, the facility seems to behave

well, even for

1” disk samples;

• We can go ahead by measuring ½” samples, but

that needs

new sensors, new heater…;

• Going down to helium will request new sensors

(like

CERNOX, already available) and represents a

major

improvement;

• A possible way ahead: samples with new geometry.