Gebruikerscommissie-bijeenkomst STW-project...

Gebruikerscommissie-bijeenkomst STW-project VAF5804

‘Metal-hydrides for smart coatings’

Vaste Stof fysica, FEW, VU, De Boelelaan 1081

22 January 2004, 1100-1500 hr, S207

Program

1100 hr Coffee1115 hr Project summary Ronald Griessen1130 hr New understanding of the Wiebke Lohstroh/

switching of Mg2NiH4 Ronald Griessen1150 hr Optimization of optical

properties by using sputteredcompositional gradient films Bernard Dam

1210 hr Labtour1245 hr Lunch1400 hr H-diffusion in ZrO2 Beatriz Noheda1420 hr Discussion1500 hr Tea

Gebruikerscommissie

Paul van der Sluis (PRL)John Kelly (UU)WWim van Helden (ECN)Gert Jan Kramer (Shell)Beatriz Noheda (VU/STW)Dana Borsa (VU/STW)Bernard Dam (VU)Ronald Griessen (VU)Leo Korstanje

Metal-hydrides for smart coatings (combi-solar collectors)

V

Solar collector

PhotovoltaicVAREM

H2O H2O

Purpose of the project “Metal-hydrides for smart coatings”

optimise optical properties optimise switching kinetics optimise the structural stability of the Mg-Ni VAREM incorporate the optimised VAREM in a switchable device

History of the project

Idea: Summer 2001Start: 25 September 2002 Sputter apparatus: Sept. 2003One Postdoc: Beatriz NohedaNew postdoc: Dana BorsaTo hire: 1 OIO +1 TWAIO

Wiebke Lohstroh

New Understanding of the Switching of Mg2NiHx

Thin FilmsWiebke Lohstroh

vrije Universiteit amsterdam

1st generation switchable mirrors

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.51E-5

1E-4

1E-3

x=[H]/[Y]

Res

istiv

ity [m

Ωcm

]

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Transmission [V

]

H2Y Y YH3

Huiberts et al. Nature 380 (1996) 231

Switchable Mirrors

1st generation:

Huiberts, Amsterdam (1996)

RE-Hx (YHX)

2nd generation:

Van der Sluis, PHILIPS (1997)

RE-Mg-Hx (GdMgHX)

3rd generation:

Richardson, Berkeley (2001)

RE free-Hx (Mg2NiHX)Black state

The Mg2Ni - H system

J.J. Reilly and R.H. WiswallInorganic Chemistry 7 (1968), 2254

Mg2Ni ↔ Mg2NiH4

3.6 wt% H

Advantages of Thin Films• Transport properties

• Reflection & Transmission

~200nm

Pd

Mg2NiHx

Glass

Outline of this talk

The black state in Mg2NiHx

What is so special about itOptical measurementsSelf-organized layeringImplications for this STW project

This talk

The black state in Mg2NiHx

The Mg2Ni - H system

Mg2Ni Mg2NiH4

H2metallicsolid solutionMg2NiH0.3

insulatorEg~1.7eVMg2+[NiH4]4-

LT monoclinic

hexagonal~32%vol

HT>510Kcubic

10002000

30004000

50006000

3.0 2.5 2.0 1.5 1.0

-0.5-0.4-0.3-0.2-0.10.00.10.20.30.40.50.60.7

Ref

lect

ance

Loading time (s)

Photon energy (eV) Isidorsson et al. APL80 (2002) 2350

220nm Mg2.17Ni 3nm PdRT / up to 1bar H2

1.01.5

2.02.5

2000

4000

60008000

0.00

0.01

0.02

0.03

0.04

0.05Tr

ansm

issi

on

Time [

s]Energy [eV]H2

220nm Mg2.17Ni 3nm PdRT / up to 1bar H2

10002000

30004000

50006000

3.0 2.5 2.0 1.5 1.0

-0.5-0.4-0.3-0.2-0.10.00.10.20.30.40.50.60.7

Ref

lect

ance

Loading time (s)

Photon energy (eV)

0 1000 2000 3000 4000 5000 60000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Res

istiv

ity (m

Ωcm

)

Ref

lect

ance

/ Tra

nsm

ittan

ceat

1.2

4 eV

Time (s)

0.1

1

10

Mg2Ni Mg2NiH≈ 4

Black state

Mg2NiH~0.7

Hall Effect & H - concentration

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00

1

2

3

4

5

6

7

8

x = [H]/[Mg2Ni]

Nef

f [10

22/c

m3 ] @

100k

The black state is not due to an abrupt change in charge carrier concentration

Garcia et al. PRB 59(1999) 11 746

1 H removes1 electron

This talk

The black state in Mg2NiHx

What is so special about it

10002000

30004000

50006000

3.0 2.5 2.0 1.5 1.0

-0.5-0.4-0.3-0.2-0.10.00.10.20.30.40.50.60.7

Ref

lect

ance

Loading time (s)

Photon energy (eV)

Fig. 4: Optical reflectance spectra of the same film as in Fig.3 during slow hydrogen uptake atroom temperature. The hydrogen gas pressure is successively increased from a few millibar to 1bar. The contour plot shows that the minimum reflectance occurs for all photon energies atessentially the same hydrogen composition, i.e. Mg2NiH0.3. This implies that the film is blackover the entire visible spectrum.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.00.0

0.1

0.2

0.3

0.4

0.5

0.64.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

Photon energy (eV)

Ref

lect

ance

For a film with d=220 nm we need κ > 2 to explain T<0.0001

dceTκω2−

≅

Lambert-Beer

( )( ) 22

22

11

κκ

+++−

≅nnR

n: refractive indexκ: extinction coefficient

0.0

0.1

0.2

0.3

0.4

0.5

0.6

4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.00.0

0.1

0.2

0.3

0.4

0.5

0.64.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

Photon energy (eV)

Ref

lect

ance

Rthen R > 0.4 !

T

The Absorbing State of Mg2.1NiHx

Isidorsson et al. APL80 (2002) 2350

1000 2000 3000 4000 5000 6000

3.0

2.5

2.0

1.5

1.0

Time (s)

Phot

on E

nerg

y (e

V)

0 1000 2000 3000 4000 5000 60000.0

0.1

0.2

0.3

0.4

0.5

Ref

lect

ance

Time (s)

3.0

2.5

2.0

1.5

1.0

0.0 0.1 0.2 0.3 0.4 0.5

Reflectance

Ener

gy (e

V)

The impossible black state in Mg2NiHx

T

(n, κ)3 nm Pd

220 nmMg2NiHx

quartz

calculate R & T

compare it with experiment

R

0 1 2 3 4 50

1

2

3

4

5

6

κ

n

(n,κ)

No solution for a

homogeneous sample

T

R

The impossible black state in Mg2NiHx

no metallic reflection from the substrate side

H-gradient ?

H-rich

H-poor

Inhomogeneous ?

grain size ~30nmno diffuse scattering

This talk

The black state in Mg2NiHx

What is so special about itOptical measurements

The black state

The black state viewed from two sidesH2 200nm Mg2Ni / 5nm Pd on sapphire

substrate Pd

H loading starts close to substrate

This talk

The black state in Mg2NiHx

What is so special about itOptical measurementsSelf-organized layering

0.1

1

Res

istiv

ity

Energy [eV]

250nm Mg1.69Ni / 7.3nm Pd

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

Photon energy (eV)

Log

Res

istiv

ityReflection from the substrate-side

H

Reflection from the Pd-side

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

Photon energy (eV)

Log

Res

istiv

ity

250nm Mg1.69Ni / 7.3nm Pd

Res

istiv

ity

Energy [eV]

Two Layer ModelPd

Mg2NiH4

Pd

Mg2NiH0.3

Mg2NiH4

Pd

Mg2NiH0.3

Mg2NiH4

PdMg2NiH0.3

Mg2NiH4

Details of the black state

Mg2NiH0.3

Pd

<ε>

Pd

Mg2Ni

Pd

Mg2NiH4Mg2NiH0.3

Pd

Mg2NiH4

Reversibility

0.1

1

Res

istiv

ity [m

Ωcm

]

0.1

energy [eV]

unloading

reversiblesymmetric

H2

This talk

The black state in Mg2NiHx

What is so special about itOptical measurementsSelf-organized layeringImplications for this STW project

Implications

Mg2NiHx is a complicated systemInfra-red properties are favorableComposition is crucialOptimization is possibleSputtered gradient samples are the key to success

That’s all,

for this time