1. No category

sample mounting
Marek Bartkowiak
What this talk will NOT be about
• will not provide THE solution
• will not provide drawings for a holder
• will not give a specific design
Sample mounting
from the scattering point of view
only sample is exposed to
the beam
=> held by “nothing”
•
any other amount of
material in the beam needs
to be minimized
=> surrounded by “nothing”
•
additional material should
be far away from sample
=> for better differentiation
•
from the cryogenic point of view
sample mounted on a well
conducting holder
CONFLICT => can be significant amount in
beam
•
assure good thermal contact
between sample and holder
=> use of some sort of glue or
fixture
=> even more material in beam
•
reduce other sources of heat
=> radiation shields around the
sample
•
For neutron scattering SE the situation is much better than for X-ray
there exists a selection of metals and insulators with low neutron absorbtion or
scattering cross-section
Thermal Conductivity
Pobell 1992
http://www.lakeshore.com/Documents/LSTC_appendixI_l.pdf
sample holder
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sample holder
contact resistance
sample
thermal
radiation
sample heated mainly by thermal
radiation
radiation heat flows through the
contact between sample and
holder
thermometer usually not mounted
on sample
thermometer measures
temperature of holder
radiation heat load
black body radiation spectrum
300K
77K
4K
radiation power of a black body
radiator (Stefan-Boltzmann law)
Stefan-Boltzmann constant
for a grey (non-ideal) body
radiation heat transfer
heat transfer between surface A1 and
A2
P… radiation power
F… view factor
using
for 2 long concentric cylinders:
material
typ. ε
paint
0.9
glass
0.9
Copper polished
0.05
Copper oxidized
0.85
Aluminium foil
0.04
Aluminium as bought
0.1
T of hot surface
max. heat load
(K) neglecting Tcold (mW/cm2)
300
46
77
0.2
4
1.4x10-6
(1nW/cm2)
radiation shield
radiation heat load is taken
by the shield
sample will equilibrate to a
temperature close to
thermometer
BUT
How long does this take?
How can this time be
shortened?
exchange gas
solid contact
exchange gas
hydrostatic limit
free molecular
regime
l << d; l..mean free path
l >> d; l..mean free path
thermal conductivity
independent of
pressure
mean free path for Helium
Ekin 2006
thermal conductivity is a
function of pressure and
energy transfer to the
surfaces
(accommodation
coefficients)
For 2 surfaces with 1
Surface at room
temperature and Helium
as gas:
White&Meeson, 2002
Catarino, Cryogenics 48 (2008),17-25
for Tlow : 4K
A : 1m2
p : 10-7mbar
heat load: 2mW
exchange gas
without radiation shield
•
•
è
è
thermal gradients in the gas
column
can drive convection
sample temperature can be
significantly higher than
thermometer reading
sample temperature can fluctuate
with radiation shield
BETTER!!
radiation shield keeps
thermal gradients small
superfluid film
Thermal conductivity is
huge
Thermal conductivity is
difficult to predict
depends on the actual
setup
thermal heat load drives
superfluid film flow
Nacher, J Low Temp Phys 97 (1994), 417
this is a good possibility
to couple a sample if the
heat load is small
solid contact
•
•
•
Solid/solid contacts at low temperatures
become limiting factor for heat flow
this is true even for metal/metal joints
interface heat transport proportional to
contact Area
Solder joints have highest conductivity
BUT may alter the sample
•
pressed m/m contacts show conductivity
100x higher than predicted by WiedemannFranz-law
BUT compression force is high 500N for Cu/Cu
•
varnish, grease, epoxy can be used to
increase contact area
BUT may introduce hydrogen compounds in
the beam (increased background)
•
usually not
applicable to sample
mounting
contact resistance
metal-metal
Bermann,Nature 182 (1958) 1661
Ekins
metal (soft)-dielectric(hard)
thermal transport across interface by
phonons only
acoustic mismatch theory applicable
==> Kapitza resistance
Kapitza resistance
acoustic mismatch theory
Thermal Boundary or Kapitza-Resistance
Snell‘s law of refraction
critical angle of total reflection
not all acoustic waves hitting the
for lq. helium and copper
4°
Kaptiza-Resistance occurs
boundary
are
allowed
to
penetrate
at any solid-solid, liquid-solid
interface
some
will be reflected (totalfraction
reflection)
of phonons incident within critical angle
Partciular problematic for
liquid helium because of
the low sound velocity
Pobell
Lammi 2009 - 28
powder samples
Powder is like a bunch of single crystal samples
most of the grains are not in direct contact with holder
cooling relies only on
the thermal contact
between powder
grains
è
for metals
è
sintered materials
coolant fills the voids
between grains
è
increases contact area
è
adds to background
additional radiation
shield reduces heat
load on sample
container
lower thermal
gradients
filler materials
metal powder (Cu) can be mixed with sample and pressed to a pellet
è
these pellets can be conductive even if the sample is insulating
con: background of Cu + crystal peaks
mixtures of deuterated methanol+ethanol
è
easy to apply can be removed by evaporation, not crystalline
background
con: may be similar amount of material as sample, expensive
Helium wet cell
è
optimal filling of voids, fully removable
con: needs He leak tight container + gas handling and filling capillaries,
inelastic signal (rotons)
Helium film (overpressure filled)
è
only thin film on sample (low background) fully removable, no GH
and capillaries
con: leak tight container at overpressure (10bar+); wetting of sample
depends on grain size and sample amount
takeaway lesson
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choose the right sample holder material (background vs. thermal
coupling)
Copper clamp contact surfaces should be gold plated
use of varnish or glue greatly enhances thermal contact to
sample
Thermometer should be in close proximity to the sample
sample in exchange gas preferred over sample in vacuum
a radiation shield should always be used (an Aluminium foil
around the sample might sometimes do..)
Aluminium becomes superconducting below 1K ==> should
only be used below 0.1K in magnetic fields > 150mT
Summary
Sample in vacuum
use radiation shield
use varnish
Sample in exchange
gas
better with radiation
shield
100mbar at 100K is
more than sufficient
Powder sample
use filler material
better with radiation
shield