M.J.Zehetbauer, in cooperation with
Faculties of Physics & Chemistry, Vienna University, Austria
FFG Project “OptiBioMat” AIT Wr. Neustadt, Austria; ETH Zurich, Switzerland
Institute of Solid State Physics, Univ.Technology, Vienna, Austria
Network S 104 „High Performance Bulk Nanostructured Materials“
Austrian Science Fund, 2008-2012
EU MC IT Network „Ti-Alloys for Biomedical Applications in Orthopaedics“ 2011-2014
Designing Bulk Functional Nanomaterials
By Severe Plastic Deformation
Final BioTiNet Meeting
“Low-Rigidity Ti-based
Biomedical Materials“
4.–8.Nov.2014, Dresden,
Germany
Two main approaches to produce bulk nanostructured materials
1. Bottom-up
Inert gas condensation (Gleiter, 1984)
Electrodeposition (Erb et al, 1989)
Consolidation of nano-powders (Koch, 1990)
Crystallisation from amorphous state
2. Top-down
Shock wave loading
Severe Plastic Deformation, “SPD” (Valiev et al, 1991, 1993)
Equal
Channel
Angular
Pressing
(V. Segal, R. Valiev)
Hydrostatic
Extrusion
(M. Pachla,
K. Kurzydlowski) V
High
Pressure
Torsion
(N.Bridgman,
R.Valiev) Accum. Roll
Bonding (N. Tsuji)
Bulk Nanomaterials (BNMs) SPD - Methods
Advantages of SPD–Nano–Processing
(1) Bulk Materials for applications
(2) No noxious handling with powders, 100% dense & pore-free
(3) SPD not only generates grain boundaries, but also other defects
like single dislocations and vacancies/vacancy agglomerates
...thus creates new phases and new phase transitions, achieves
enhanced diffusion….
….benefits for functional nanomaterials !?
Enhanced Properties of SPD-BNM
• Mechanical properties (strength, ductility, fatigue)
• Biocompatibility, Biomedical Apps
• Diffusion & changes in phase stability
• Hydrogen storage, and stabilisation of BNM by
hydrogenisation
• Irradiation resistance
• Magnetic properties
• Thermoelectricity
Magnetic domains
grains
in SPD soft
magnetic steel P800
300 nm
R. Pippan et al.,
Mater.Sci.Forum
2006
1) SPD apps for magnetic materials
• Exchange coupling possible for D < 100 nm
• SPD reduces/increases anisotropy: magnetic softening/hardening
2) coercivity increases with stress caused by SPD
0.01 0.1 1 10 10010
100
1000
10000 Ni Fe
Fe-3Si
Fe-6.5Si
Fe-17Co
Hc
(A
/m)
Grain size (m)
~ D-1
~ D6
300 350 400 450 5001000
1500
2000
2500
3000
3500
4000
0.5Hz
Hc(
A/m
)
T(K)
Ni(Unv)
Ni(RT)
Ni(N2)
Ni(450°C)
1) coercivity decreases with grain size; Hc ~ D
6
Subsummary 1:
Effect of SPD-Nanocrystallization to Magnetism
Soft Magnetism needs:
Low Coercivity, grain sizes D
Nano-Powders:
Enhanced diffusion and kinetics through grain boundaries
Low ab-/desorption temperature
Surface contamination
Commercially expensive, environmental risks
Bulk NMs T.Klassen, et al., Z. Metallkd., 94, 610 (2003)
Coarse grained
structure Nanocrystals
2) SPD apps for H2 storage materials
PCT diagram of ECAP processed ZK60 Four new plateaus 200°C, 220°C, 240°C and 260°C
(Black Lines) V.M.Skripnyuk et al., Acta Mater. 52, 405 (2004)
(Coloured Lines) M. Krystian, M. Zehetbauer, G. Krexner, H. Kropik,
B. Mingler, J.Alloys Comp. (2011)
Enhanced (ab- &) desorption of Hydrogen
From: M.Skripnyuk, E.Rabkin, Y.Estrin, R.Lapovok, Acta mater. 52, 405 (2004)
M.Krystian, M.Zehetbauer, G.Krexner, H. Kropik, B. Mingler, J.Alloys Comp. (2011)
ECAP:
Long-Time
Cyclic Stability of
Ab/De-sorption !
Sub-Summary 2: SPD apps for H2 storage materials
• apply SPD: - similar or even better kinetics than ball milled materials
(avoid impurification especially with O2 !)
- better long-term stability than ball milled one:
no deteriation of storage capacity and/or kinetics,
even after 1000 cycles !
• add catalysators: - better ad/de-sorption kinetics also in SPD materials
• add H2 before SPD: - much smaller grain sizes (use of stabilisation,
effect of foreign atom sort)
!! BUT: ultrafine grain size may not be stable for long-time cycling !!
• increase the surface: - allowing initiation of the H2-ad/desorption;
dissociation of H2-molecule
3) SPD Apps for Thermoelectrics
Applications -
requirements for TE
materials
• Respectable TE properties:
ZT > 1, high DT, high efficiency
• Stable at high temperatures
• Cheap material + availability,
easy to produce (synth. proc.)
• Thermal expansion coefficients of
n, p legs in same range
• Sufficient mech. strength for device
integration (stiffness, stress,
Young‘s mod., bulk mod.)
• High relative density (>95%)
Ulysses
spacecraft
thermosflask battery charger Echostar X wristwatch
TEG: up to 1 KW saves about 5% fuel
VW
TE coolbox
TE efficiency: Figure-of-Merit ZT how to increase ?
Skutterudite: cubic structure with formula TPn3
T = Co, Fe, Ni … Pn = P, As or Sb (Space group : Im –3,
sites: T: 8c (¼, ¼, ¼) , Pn: 24g (0, y, z), void: 2a (0,0,0) or (½, ½, ½)
T
(Co,Fe,Ni...)
Pn (P,As,Sb) Filler atom (in voids)
p- and n-type filled skutterudites
Comparison of ZT of HH and nanostructural HH alloys
Nanocrystallization by HPT ? (Zhang et al., 2010)
Problem:
Extremely high
resistivity due to
cracks
ZT = S2T/(κ)
ZT values after different HPT treatments of ball milled skutterudites
(A, B, C represent different strains achieved by HPT)
Solution:
High-Temp
HPT !
Which defects are lost during annealing ?
check by dilatometry & SEM/TEM (Rogl et al., Oct. 2014)
vacancies
cracks & pores
World‘s Record in ZT of Skutterudites !
Thermal expansion, a tool to analyze defects via
their specific free volume (Sprengel et al, 2013)
Thermal Stability (DD0.60Fe3CoSb12, p-type)
I II III
a.w. crystallite size [nm] : 152 53 122
dislocation density [m-2] : 31013 21014 1.61014
II
after HPT
III
after heat
treatment
I
before HPT
II
I
III
X-ray Profile
Analysis
Sub-Summary 3/4: HPT Thermoelectrics
• p- and n-type skutterudites HPT-processed at T 500 C exhibit a
strongly strengthened nanocrystalline structure, with oriented, lamellar-
shaped crystallites of 50 nm in size and an enhanced dislocation
density.
• In comparison with ball-milled plus hot-pressed skutterudites, the HPT-
processed samples show a reduction of the thermal conductivity up to
40%.
• This and the slightly higher Seebeck coefficient allow to enhance the
figure of merit (ZT) values by up to a factor of 2, in spite of a markedly
enhanced electrical resistivity.
• Thermal stability can be kept within (ZT) = -10%, thanks to the
stability of dislocations being higher than that of vacancies and even
that of crystallite boundaries !
Stents
Bone Tools
Problem: Time of Biodegradation
= days & weeks, weak Mg alloys
= months up to 1 year, pure & ultrapure Mg
Problem: Low strength
- Strengthening by SPD limited to 10 %
- SPD induced precipitates by thermal treatment ?
Summary
1) SPD is a very suitable tool to achieve bulk and 100% dense
functional nanomaterials
2) With some functional properties, not only the SPD
achieved grain boundaries but also the SPD induced
defects (dislocations, vacancy agglomerates) essentially
contribute to their improvement !
Functionalising BNM : What is needed ?
• Biomedical Apps: good biocompatibility, low E, high
strength, texture control
• Hydrogen Storage: - Small crystallite size / Large surface
for high / low (de-)sorption kinetics /
temperature
- SPD for low-cost processing
• Magnetism - soft: Small grain size, stress-free
nanocrystallization
- hard: Small grain size, high anisotropy by
strain and texture control
• Thermoelectrics: - Decrease of resistivity – flaw free SPD
- Low-dimensional crystal defects (?)
- Low-T TEs: control of texture