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