Bez tytulu slajdu

1
O n t h e T r a c k o f M o d e r n
P h y s i c s
In.var and re.var
Do you remember old spring-moved clocks? Their
pendulum was attached to thin, high-precision
spings, made of INVAR. Old material, but new
discoveries!
Nature 400 (1999) 46
Invar is Fe-Ni, 65-35 alloy. Ab-initio
calculations 1 showed that magnetic moments of
nickel (blue here) are aligned, while those of
iron (red) seem to be chaotic. And this
disorder-in- order assures the minimum volume
(and energy) Read below.
USAF Aircraft Pictures - http//sun.vmi.edu/hall/a
fpics.htm
INVAR Effect- after 100 years finally understood
            In 1897 the Swiss physicist Charles
Edouard Guillaume discovered that fcc Fe-Ni
alloys with a Ni concentration around 35 atomic
, now called INVAR, exhibit anomalously low,
almost zero, thermal expansion over a wide
temperature range. This discovery immediately
found widespread application in the construction
of calibrated, high-precision mechanical
instruments, such as seismographs and hair
springs in watches. Today, Invar alloys are used
in temperature-sensitive devises, such as shadow
masks for television and computer screens. In
1920 Guillaume was awarded the Nobel Prize in
Physics for the discovery of these Fe-Ni alloys.
It was realized early on that the explanation
of the Invar effect is related to magnetism. Yet,
though it has been 100 years since this effect
was discovered, it was not understood. In a
recent article published in Nature ''Origin of
the Invar effect in iron-nickel alloys (Nature
400, 46 (1999)), I. Abrikosov and B. Johansson
from Uppsala node of the Network, in
collaboration with Mark van Schilfgaarde from
Sandia National Laboratories, Livermore, USA,
presented results of ab initio calculations of
volume dependences of magnetic and thermodynamic
properties for the most typical Invar system, a
random fcc Fe-Ni Invar alloy, where they allowed
for noncollinear spin alignments, i.e. where the
spins may be canted with respect to the average
magnetization direction. They have found that the
evolution of the magnetic structure already at
zero temperature is characterized principally by
a continuous transition from the ferromagnetic
state at high volumes to a disordered
noncollinear configuration at low volumes, and
that there is an additional, comparable
contribution to the net magnetization from the
changes in the amplitudes of the local magnetic
moments. The noncollinearity gave rise to an
anomalous volume dependence of the binding energy
curve, and this allowed Mark van Schilfgaarde, I.
Abrikosov and B. Johansson to explain the
well-known peculiarities of Invar systems.
http//psi-k.dl.ac.uk/TMR1/summary_report.htm Th
e result on INVAR has been obtained within EU TMR
Network Ab-Initio Calculations of Magnetic
Properties of Surfaces, Interfaces and
Multilayers guided by prof. Walter Temmerman
from Daresbury Laboratory, Warrington, UK We
thank for the permission.
Some material shrink with temperature, another
come back to their old shape. We call them
shape-memory alloys.
The most common shape-memory metals are
Nickel-titanium 50-50 alloys or copper alloys,
like CuZnAl, and CuAlNi, but even Pt is used.
The wire, twisted at low temperature, when
heated will come back to the original shape. And
the original shape? It is fixed bending the
wire at 500ºC. It can be bent and unbent a
million times.
Memory-shape flaps do not require huge hydraulic
actuators but only heating wires. They are used
in USA military aviation from sixties Other
applications of memory-shape alloys span from
robot actuators, hydraulic fittings to medical
protesis.
deformation
deformation
Shape memory
Normal material are subject to a permanent
plastic deformation, once stress exceeds the
limit of elasticity. In shape-memory alloys, a
thermal treatment, after the stress has been
removed, brings the object to the original
dimensions.
Some material do not expand with temperature,
some of them even shrink, like Germanium at low
temperatures. Sometimes, they expand in one
dimension but shrink in another
direction. Negative expansion is often connected
to the presence of some sub-crystal structures,
moving and rotating independently. An axample is
silver-copper oxide, of the cuprite symmetry
structure 2.
After K. Ireland, University of Wollongong,
Material Engineering http//www.uow.edu.au/eng/mm/
matl/ShapeMemoryAlloys.pdf
In shape-memory alloys two sub-phases coexist
hard, high-T austenite and low-T, plastic
martensite. Cooling brings all austenite to
martensite. Subsequent deformation keeps the
martensite structure intact. It return to the
original austenite after heating.
Nitinol Devices and Components http//www.nitinol.
com/images/3slide3.gif
http//www.ifm.eng.cam.ac.uk/people/sc444/
Innovative Manufacturing Research Centre
Cambridge University Engineering Department
www.fz-juelich.de/iwv/iwv1/datapool/page/9/fgl2.jp
g Forschungszentrum Jülich
Ag2O Cu2O
We can see these tiny changes of interatomic
distances with an even smaller sonde (X-rays).
One of the techniques is called Extended Fine
Structure X-Ray Absorption It works in the
following manner 1. X-ray (synchrotron
radiation) is tuned to the energy of an
internal-electron level of the probe atom. 2. An
absorbed X-ray quantum releases an electron from
this atom. 3. This electron (i.e. the quantum
wave objects) interfers with itself, getting
scattered on neighborourhood atoms. 4. A fine
structure, depending on the interatomic
diustances is observed in the X-ray absorption.
In this mode the neighbourhood of the X-ray
absorping atom is exploited.
This (left) picture from Transmission Electron
Microscopy shows coexistence of martensite (long
needles) and austenite (patches) phases. In pure
titanium only one phase exists (Scanning EM).
This Fe-Cr-Ni-Mo dual-phase stainless steel
elongates better than the chewing gum
Two inter-penetrating networks of corner sharing
M4O tetrahedra with O-M-O linear coordination
And this cosmic rubber is soft, if torn slowly,
but springs, if hit. The Material Science came to
the playground!
Figures are from prof. P. Fornasini, University
of Trento, Physics Department, Thanks!
1 M. van Schilfgaarde, I. A. Abrikosov,
B. Johansson, Origin of the Invar effect in
ironnickel alloys, Nature 400 (1999) 46 2 S.
a Beccara, G. Dalba, P. Fornasini, R. Grisenti,
A. Sanson, and F. Rocca, Local thermal expansion
in a cuprite structure the case of Ag2O, Phys.
Rev. Lett. 89, 25503 (2002)