Densification of dense nano crystalline zinc oxide under electric field

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Engineering Conferences International ECI Digital Archives Electric Field Assisted Sintering and Related Phenomena Far From Equilibrium Proceedings Winter 3-11-2016 Densification of dense nano

1 Engineering Conferences International ECI Digital Archives Electric Field Assisted Sintering and Related Phenomena Far From Equilibrium Proceedings Winter Densification of dense nano crystalline zinc oxide under electric field Olivier Guillon Forschungszentrum Jülich GmbH Follow this and additional works at: Part of the Engineering Commons Recommended Citation Olivier Guillon, "Densification of dense nano crystalline zinc oxide under electric field" in "Electric Field Assisted Sintering and Related Phenomena Far From Equilibrium", Rishi Raj (University of Colorado at Boulder, USA) Thomas Tsakalakos (Rutgers University, USA) Eds, ECI Symposium Series, (2016). This Abstract and Presentation is brought to you for free and open access by the Proceedings at ECI Digital Archives. It has been accepted for inclusion in Electric Field Assisted Sintering and Related Phenomena Far From Equilibrium by an authorized administrator of ECI Digital Archives. For more information, please contact

2 Mitglied der Helmholtz-Gemeinschaft Priority Programme SPP 1959 Manipulation of matter controlled by electric and magnetic fields: Towards novel synthesis and processing routes of inorganic materials

3 Programme committee Prof. Dr. Olivier Guillon* Institute of Energy and Climate Research (IEK-1) Forschungszentrum Jülich Prof. Dr. Christian Elsässer Fraunhofer-Institut für Werkstoffmechanik IWM Freiburg Prof. Dr. Oliver Gutfleisch Fachbereich Material- und Geowissenschaften Technische Universität Darmstadt Prof. Dr. Jürgen Janek Physikalisch-Chemisches Institut Justus-Liebig Universität Gießen Prof. Dr. Dierk Raabe Microstructure Physics and Alloy Design Max-Planck-Institut für Eisenforschung, Düsseldorf Prof. Dr. Cynthia A. Volkert Institut für Materialphysik Georg-August-Universität Göttingen * coordinator Duration: 2 x 3 years Start: Summer 2016 Total funding: ca IEK-1: Materials Synthesis and Processing 1

DFG Priority Programme Reactive sintering Phase formation Densification Coarsening Defect formation, structure, mobility Phase formation with local deformation Deformation

4 DFG Priority Programme Reactive sintering Phase formation Densification Coarsening Defect formation, structure, mobility Phase formation with local deformation Deformation Pressure assisted sintering What are the interplays between electric / magnetic fields and defect formation, structure and mobility? IEK-1: Materials Synthesis and Processing 2

5 Mitglied der Helmholtz-Gemeinschaft Densification of nanocrystalline zinc oxide under electric field Olivier Guillon Benjamin Dargatz, Christoph Schmerbauch, Jesus Gonzalez-Julian, Mar?n Bram Institute of Energy and Climate Research Materials Synthesis and Processing (IEK-1)

Mo#va#on To obtain dense ceramics with nano-grain size à improved properties Understanding of mechanisms involved in coarsening is critical to the retention

properties Ø Promote the densification mechanisms instead of grain growth - High mechanical pressure (but reduced sample size) - Assistance by electric

6 Mo#va#on To obtain dense ceramics with nano-grain size à improved properties Understanding of mechanisms involved in coarsening is critical to the retention of nanocrystallinity Powder quality, storage conditions, processing and sintering process all play a role Ø Doping, modification of chemical composition and properties Ø Promote the densification mechanisms instead of grain growth - High mechanical pressure (but reduced sample size) - Assistance by electric field/current Pt wire Alumina discs Pt electrodes S Sample Hea?ng elements FAST/SPS Sinter-forging (flash sintering) IEK-1: Materials Synthesis and Processing 4

7 Mo#va#on ZnO Full densification with final nanosized grains is very challenging Mechanical pressure decreases grain size for a given sintering density Reduction of initial particle size does not necessarily result into smaller grain size Objective: To find a new strategy to fully densify bulk ceramics with nano-grain size Electric field and water will be used to promote sintering of ZnO ceramic materials. IEK-1: Materials Synthesis and Processing 5

ZnO powder O 2- Zn 2+ 10 nm NG20; purity >99.99 wt.

TEM and XRD in agreement Storage in environmental chamber

8 ZnO powder O 2- Zn nm NG20; purity >99.99 wt.% Polyhedral, nearly spherical shape Same aspect ratio TEM and XRD in agreement Storage in environmental chamber (humid /dry conditions) IEK-1: Materials Synthesis and Processing 6

conditions 50 MPa uniaxial pressure Maximal temperature of 400 C or 800 C Heating rates of

9 FAST/SPS of ZnO DC pulsed current process External mechanical pressure Joule heating effect High heating rates and short dwell time Temperature control by pyrometer sintering conditions 50 MPa uniaxial pressure Maximal temperature of 400 C or 800 C Heating rates of 10, 50, 100 or 200 K/min 10 min isothermal sintering time IEK-1: Materials Synthesis and Processing 7

10 FAST/SPS: Effect of hea#ng rate Al 2 O 3 Guillon and Langer, J. Mat. Sci., 45 (2010) Full densification takes place only for high heating rates in presence of bound water Therefore, kinetics of water desorption may play a significant role and limit the temperature-time window in which crystal interfaces are modified. IEK-1: Materials Synthesis and Processing 8

11 FAST/SPS: Effect of water content Increasing amount of water enhances densification Loss of water can be tracked by measuring partial pressure in FAST/SPS chamber IEK-1: Materials Synthesis and Processing 9

FAST/SPS: Densifica#on mechanism 50-200 K/min Master-Sinter-Curve E a = 130 kj/mol Literature:

boundaries Hydroxide-Ion OH - Oxygen-Ion O 2- Hypothesis: Decrease of E a because of easier OH

12 FAST/SPS: Densifica#on mechanism K/min Master-Sinter-Curve E a = 130 kj/mol Literature: kj/mol Assumption: Limitation of mass transport by diffusion of O 2- along grain boundaries Hydroxide-Ion OH - Oxygen-Ion O 2- Hypothesis: Decrease of E a because of easier OH - diffusion (lower valence and ionic radius) 99 pm 140 pm IEK-1: Materials Synthesis and Processing 10

13 Defect analysis Photoluminescence at low temperature (5 K) Hydrogen-related defects are indicated in the Near Band Edge domain IEK-1: Materials Synthesis and Processing 11

14 Chemical analysis GD-OES H [at%] Zn [at%] O [at%] humid (T max = 400 C/ w = 1,6 %) 0,5 ± 0,3 51,2 ± 0,1 48,3 ± 0,1 dry ---* 50,8 ± 0,1 49,2 ± 0,1 * Detection limit ~ 50 ppm XPS Incorporation of hydrogen in humid samples Zn [at%] O [at%] C [at%] humid (T max = 400 C/ w = 1,6 %) 59,9 40,1 --- dry 53,8 41,0 4,3 Measurement of a fresh fracture surface in high vacuum No presence of carbonates in humid samples which normally hinder densification IEK-1: Materials Synthesis and Processing 12

15 FAST/SPS sintering: Field effect? Does the electrical current play a role? Alumina disks between punch and powder prevent the flow of electrical current through ZnO sample Same sintering behavior for aqueous and dry conditions No effect of current IEK-1: Materials Synthesis and Processing 13

16 Microstructure analysis Sintered dry powder shows more than one order of magnitude larger grain size than humid powder, as high temperature is required for densification Anisotropic grain morphology is observed for sintered humid ZnO. IEK-1: Materials Synthesis and Processing 14

17 Microstructure analysis Full densification of ZnO with nanosized grains is possible through the addition of water and high heating rates The process is rapid and requires only low temperature IEK-1: Materials Synthesis and Processing 15

18 Electric field assisted sintering of ZnO Sintering behaviour is strongly affected by the electric field Maximal density obtained for 40 V/cm IEK-1: Materials Synthesis and Processing 16

19 Electric field assisted sintering of ZnO June 2014 Flash Sintering of Zinc Oxide 1731 Fig. 3. Specimen temperatures estimated by thermal expansion as a function of current and current density. Electric Field E [V/cm] Temperature T [ C] Furnace Temp. T f Specimen Temp. T s Electric Field E Current Density J Power Density φ 80 V/cm 100 0,1 0, Time t (min) 10 1 Current Dens. J [ma/mm²], Power Dens. φ [mw/mm³] Current flows through the specimen and causes drop of electric field and temperature increase Massive Joule heating due to electric current flow IEK-1: Materials Synthesis and Processing 17

2 % 80 V/cm 500 nm 40 V/cm 500 nm AQer ﬂash

3 % 700 ±140 nm 500 nm 80 V/cm 2500 nm 970

3 % 1720 ±400 nm 500 nm 160 V/cm 2500 nm 404

microstructure Fine grains (< 1 µm) for low

20 Electric ﬁeld assisted sintering of ZnO Before ﬂash 82.2 % 80 V/cm 500 nm 40 V/cm 500 nm AQer ﬂash 96.8 % 0 V/cm 97.2 % 80 V/cm 206 ±20 nm 98.3 % 700 ±140 nm 500 nm 80 V/cm 2500 nm 970 ±210 nm 97.3 % 1720 ±400 nm 500 nm 160 V/cm 2500 nm 404 ± 50 nm 98.1 % 4690 ±960 nm Electric field/current determines the evolution of the microstructure Fine grains (< 1 µm) for low fields and large grains (> 1 µm) for high fields Thermally activated diffusional process (massive matter transport) +? IEK-1: Materials Synthesis and Processing 18

21 Photoluminescence of ZnO: Defects PL Intensity [norm. to NB E] V/cm 40 V/cm 80 V/cm 160 V/cm 40 V/cm insulated Wavelength (nm) Int. PL Intensity (norm.) Field Strength ( V/cm) NBE / DLE [norm.] NBE Near Band Edge DLE Deep level emission B-DLE Donor-acceptor pair transition G-DLE Zinc on oxygen site R-DLE Oxygen interstitial Increase of NBE/DLE à higher crystal quality (less defects) 0 and 40 V/cm insulated have same grain size (400 nm) but different defects distributions Mobility of point defects under electric field? IEK-1: Materials Synthesis and Processing 19

Conclusions Full densification of ZnO with nano-grain sizes can be attained at only 400 C using FAST sintering under high heating rate and the presence of water As evidenced by chemical analysis and

22 Conclusions Full densification of ZnO with nano-grain sizes can be attained at only 400 C using FAST sintering under high heating rate and the presence of water As evidenced by chemical analysis and photoluminescence, hydrogen is incorporated and modifies densification behavior, color and electrical properties of ZnO Sintering path and densification are not affected by electrical current in standard FAST/SPS In contrast, higher electric fields can significantly modify the sintering behavior of ZnO, especially if current flows through the sample, leading to different defect configurations and exaggerated grain growth. IEK-1: Materials Synthesis and Processing 20

23 Publica#ons FAST/SPS sintering of nanocrystalline zinc oxide - Part 1: Enhanced densification and formation of hydrogen-related defects in presence of adsorbed water B. Dargatz, J. Gonzalez-Julian, M. Bram, P. Jakes, L. Schade, R. Röder, C. Ronning, O. Guillon Journal of the European Ceramic Society, 36, pp , 2016 FAST/SPS sintering of nanocrystalline zinc oxide - Part 2: Abnormal grain growth, texture and mechanical properties B. Dargatz, J. Gonzalez-Julian, M. Bram, Y. Shinoda, F. Wakai, O. Guillon Journal of the European Ceramic Society, 36, pp , 2016 Flash Sintering of Zinc Oxide and its Influence on Microstructure and Defect Formation C. Schmerbauch, J. Gonzalez-Julian, R. Röder, C. Ronning, O. Guillon Journal of the American Ceramic Society, vol. 97[6], pp , 2014 Anomalous coarsening of nanocrystalline zinc oxide particles in humid air B. Dargatz, J. Gonzalez-Julian, O. Guillon Journal of Crystal Growth, 419, pp , 2015 Improved compaction of ZnO nano-powder triggered by the presence of acetate and its effect on sintering B. Dargatz, J. Gonzalez-Julian, O. Guillon Science and Technology of Advanced Materials, 16, (10 p.), 2015 IEK-1: Materials Synthesis and Processing 21

24 OPEN ACCESS IEK-1: Materials Synthesis and Processing 22

25 Thank you for your kind attention IEK-1: Materials Synthesis and Processing 23

Field effects during consolidation of metallic powders

Engineering Conferences International ECI Digital Archives Electric Field Assisted Sintering and Related Phenomena Far From Equilibrium Proceedings Winter 3-10-2016 Field effects during consolidation of