Stave Mechanical Issues
Meeting UCSC
November 10, 2005
ATLAS Upgrade Workshop
Silicon Tracker
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VG1
Basic Questions
•
Preliminary Indications
– Radiation level increasing by factor of 10
• Potential silicon damage increases markedly
– Substantial increase in leakage current
» Suggesting potentially colder operation
– Radiation length issue
• Material in detector volume increased beyond original expectations
– Impetus for considering fundamental change to detector mounting,
detector support, and electronic circuit design
» Long, individual stave structure with embedded cooling tubes
•
Key questions, not all inclusive, could well be:
–
–
–
–
Is evaporative cooling still possible?
If single-phase cooling were desirable, which coolant is rad-hard?
Will detector thermal runaway require special cooling considerations?
In satisfying the stringent requirements for detector cooling, thermal runaway,
detector stability will a long stave structure be low mass?
– What are the best construction materials to withstand the radiation environment?
Meeting UCSC
November 10, 2005
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VG2
First Stave Geometry Studied
Electronic chip load 108W (Pixel stave 110W 80cm length)
Module heating 1mW to 1W over life time
96cm
Recommend option being a 2m
version of this arrangement, with
central support
Coolant in and out
Meeting UCSC
November 10, 2005
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VG3
Analysis
•
Key initial considerations
– Stave stiffness (sag and thermal
stability)
• Negative CTE materials
– Composite box (facings)
• Positive CTE materials
– Cooling tubes (Aluminum
or PEEK)
– Silicon wafers
– Cable bus
– BeO hybrid
– Ceramic dielectric
– Stave Cooling
• Length and size of cooling
tubes
• Thermal resistance of cable
bus
Meeting UCSC
November 10, 2005
Step A: First order calculation of gravity sag
Step B: Thermal model of cross-section
resulting from Step A
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Gravity Sag
•
Objective is to check proportions
of composite structure
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–
–
–
•
Sag affected by orientation—we
address worst case
Sag affected by end support
conditions and the unsupported
length---we look at simple and
fixed end conditions for 1m length
(pins at each end provide
something in between)
Sag is affected by non-structural
uniform mass distributed along
length---an estimate is made
simple support
Composite material-use a very
modulus graphite fiber, quasiisotropic layer
–
For present ignore the negative
CTE effect
Meeting UCSC
November 10, 2005
Fixed support
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Stave Sag
Item
Description
Al Tube
Al Tube
Hollow Composite
Partial stave
Partial stave
Partial stave
Partial stave
Stave
4mm ID, 0.203mm wall
Tube+Liquid Coolant
65.5mm by 5.52mm by 0.203mm
Box + tubes (coolant)
Box +tubes(coolant)+ modules
Box+tubes+modules+bus
Box+tubes+modules+bus+BeO
Box+tubes+modules+bus+BeO+foam core
Simple
Support Sag
(mm)
2.5
8.5
0.24
0.301
0.361
0.378
0.456
0.477
Delta
(mm)
0
0.061
0.060
0.017
0.078
0.021
Composite: 24Msi modulus, ρ=1700kg/m3, 120Msi modulus fiber 60% fraction.
Modules: 64mm by 280microns, double sided mounting
BeO: 24mm by 64mm 380microns, double sided mounting
Foam: carbon, 3% solid fraction
Coolant: liquid ρ=1630kg/m3
Meeting UCSC
November 10, 2005
Item
Description
Al Tube
Al Tube
Hollow Composite
Partial stave
Partial stave
Partial stave
Partial stave
Stave
4mm ID, 0.203mm wall
Tube+Liquid Coolant
65.5mm by 5.52mm by 0.203mm
Box + tubes (coolant)
Box +tubes(coolant)+ modules
Box+tubes+modules+bus
Box+tubes+modules+bus+BeO
Box+tubes+modules+bus+BeO+foam core
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Fixed
Support Sag
(mm)
0.5
1.699
0.048
0.060
0.072
0.076
0.091
0.095
Delta
(mm)
0
0.012
0.012
0.004
0.015
0.004
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VG6
Sag Summary
If the separation between composite facings is increased, the sag can be
decreased within acceptable bounds. This option will force the consideration: how
best to package the cooling tubes
Stave
L=1m, hc=4.6mm, w=6.4cm
L=1m, hc=10mm, w=6.4cm
L=1m, hc=20mm, w=6.4cm
L=2m, hc=20mm, w=6.4cm
L=2m, hc=40mm, w=6.4cm
Normal Gravity Sag (mm)
Simple
Fixed
Cooling tubes OD 4.6mm
0.397
0.0795
0.107
0.0214
0.032
0.0064
Cooling tubes OD 4.6mm
0.4586
0.0917
0.1441
0.0288
wL4
sag  C
C=5 for simple support, C=1 for fixed support
384 EI
w=uniform loading, N/m
L=span length
E=Elastic Modulus for composite box
I=Moment of Inertia of composite box I  h 2
Meeting UCSC
November 10, 2005
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VG7
Material Properties in Thermal Model
Properties used in 3D model
Item
Silicon Chip
Silver-filled AdhesiveA
Dielectric (Ceramic)
BeO
Cable bus
Composite Wall
CGL 7018
Coolant Tube
Coolant
Silicon Detector
Cable Bus
Composite Wall
CGL 7018
Coolant Tube
Coolant
Meeting UCSC
November 10, 2005
K-W/mK
Station 0
185 (X/Y/Z)
1.55 (X/Y/Z)
5 (X/Y/Z)
240 (X/Y/Z)
0.12 (X/Y/Z)
250 (X)/250 (Y)/1 (Z)
1 (X/Y/Z)
204 (X/Y/Z)
Station 1
185 (X/Y/Z)
0.12 (X/Y/Z)
250 (X)/250 (Y)/1 (Z)
1 (X/Y/Z)
204 (X/Y/Z)
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Thickness-mm
0.25
0.0508
0.229
0.38
0.125
0.75
0.076
0.3048
0.28
0.125
0.75
0.076
0.3048
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VG8
Thermal Model
•
Model Makeup
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–
–
–
–
–
–
–
•
Array of 6 chips, two on back and one
array on front, offset by the stagger in
the silicon wafers
Silicon wafers 3cm axial with 6.4cm
width (280 microns)
Electronic chip, 7mm by 10mm
(250microns)
BeO, 2cm by 6.4cm (380 microns)
Dielectric beneath chips, 229microns,
k=5W/mK
Cable bus, 126microns, k=0.12W/mK
Aluminum cooling tubes, 12mil wall
PEEK-fiber filled k=0.9W/m*K
Objectives
–
First order determination of gradient
• Effects of various material layers
• Compare PEEK versus Al coolant
tubes
• Coolant film gradient
Meeting UCSC
November 10, 2005
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VG9
Thermal Solutions (No Wafer Heating)
Aluminum coolant tubes
Tube surface referenced to 0ºC
0.5W per chip
Side B
Side A
Meeting UCSC
November 10, 2005
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VG10
Thermal Solutions (No Wafer Heating)
•
Solution---Aluminum Cooling Tubes
–
–
Added convective cooling on interior of
tube surface (3000W/m2K)
• Increase in peak chip temperature
of 0.98ºC. An approximate calc of
this component is 0.85ºC
Notice small temperature variation for
center wafer, however essentially
uniform
• Side A electronic chip array is
used to provide proper heating of
the cooling tube between the two
chip arrays on Side B
Meeting UCSC
November 10, 2005
ATLAS Upgrade Workshop
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VG11
Thermal Solutions (No Wafer Heating)
•
Solution---PEEK Cooling Tubes
–
–
With convective cooling on interior of
tube surface (3000W/m2K)
• Peak temperature 8.4ºC, above
zero
• Increase in peak chip temperature
of 2.2ºC.
The small temperature variation in
center wafer, among areas, suggest a
more refined mesh should be
considered in the final analysis
Meeting UCSC
November 10, 2005
ATLAS Upgrade Workshop
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VG12
Comments
•
Thermal Model
– Model is a slice from the stave
• Unfortunate but the heat load is unbalanced between Side A and Side B
• Gives the appearance that the silicon wafer temperature on Side A is not
uniform
– The middle wafer on Side B has electronics on both sides of a wafer
and here the surface temperature is nearly uniform
– Deviation of the chip temperature from wafer temperature is about 4ºC for both Al
and PEEK cooling tubes.
– Heat flux is highest at the chip, but even here it is not very high (0.7W/cm2)
• BeO serves as heat spreader
– Flux between BeO and cable bus 0.23W/cm2 average
• Composite facing spreads heat along the cooling tubes
– Facing thickness is about the average thickness for the Pixel stave
thermal management surface, but there are now two facings for
distributing heat
» Thru thickness conductivity is lower, 1W/mK versus 10 to 20W/mK
– Heat flux at cooling tube wall reduced further over Pixel stave because of the two
pass system within one stave
Meeting UCSC
November 10, 2005
ATLAS Upgrade Workshop
Silicon Tracker
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VG13
Wafer Heating (Next on List)
•
Wafer Heating
– Two sources
• Leakage current: peak estimated to be 1W per module from leakage current
(temperature dependency)
• Free convection for surrounding gas
– Open for suggestions on gas temperature to be used
– Need to develop more confidence in results of thermal modeling before
recommending the coolant inlet temperature
• Primary issues are portions chosen for stave structure and cooling tubes
– Secondary issues are the material thermal properties being used
» Must maintain reality check throughout analysis
– What follows:
• Refinement in thermal model mesh
• Calculation of tendency for thermal runaway, although at the present no
strong evidence this will be an issue within present geometry
Meeting UCSC
November 10, 2005
ATLAS Upgrade Workshop
Silicon Tracker
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VG14
Things to Watch For
•
Stave cross section-2m length
– Need greater separation between facings for increased stiffness
– Poses interesting issue in integration of cooling tube- simple circular
tube geometry not likely
•
Wider Staves
– Again integration of coolant passages
– Possible wafer thermal runaway
– Temperature gradients
•
Thermal distortion as a general issue
– CTE mismatch of materials
– Spatial temperature variations both lengthwise and transverse
Meeting UCSC
November 10, 2005
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VG15
Things to Watch For
One must be mindful that stave temperature varies in Z-direction (internal pressure drop
and 2-phase convection coefficient varies along stave length)
Meeting UCSC
November 10, 2005
ATLAS Upgrade Workshop
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VG16
Current Emphasis
•
Structural model of 1m stave
–
•
Compare to analytical solution
Set up of thermal and structural
coupling solution
• Predict CTE effects
– Axially
– Transverse
• Thermal gradient established by
location of cooling tubes
– Tube geometry for
increased separation
between stave faces
Meeting UCSC
November 10, 2005
ATLAS Upgrade Workshop
Silicon Tracker
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VG17