Charge behavior on insulating monocrystallic surfaces by Kelvin probe force microscopy
M. Mirkowska 1,2 , M. Kratzer 2 , C. Teichert 2 , H. Flachberger 1
1 Chair of Mineral Processing, Department of Mineral Resources and Petroleum Engineering, Montanuniversität Leoben, Leoben, Austria
2 Institute of Physics, Montanuniversität Leoben, Leoben, Austria
Experyment 2 z 18.07
Monika Mirkowska, e-mail: monika.mirkowska@unileoben.ac.at Christian Teichert, e-mail: christian.teichert@unileoben.ac.at www.unileoben.ac.at/~spmgroup>
Contact
Literature
Measurement procedure:
• Successful charging by static contact as well as by rubbing confirmed by CPD change (static contact: CPD ~-1.5 V , rubbing : CPD ~-6.5 V).
• Charging by rubbing appears to be more effective than charging with static contact
(correction procedure for different geometry of charged regions has to be developed).
• Charging can be reversed by application of opposite tip bias.
• For rubbing, a CPD „saturation“ level is observed which is not the case for static contact charging.
• Charge decays roughly exponentially with time.
AFM cantilever
a mineral particle
X X X X
KPFM signals: scan size 10 x 10 µm2
Before charging Charging: -10 V; 4 times by 15 min
Charge decay after charging (each scan took 28 min)
After time stop (12 h)
scan size: 50 x 50 µm2
shows scanning influence
-9 -4 1 6
-200 300 800 1300
average min
max
CPD [V]
Time [min]
X X X X
KPFM signals: scan size 10 x 10 µm2
Before charging Charging: -10 V; 4 times by 15 min
Charge decay after charging (each scan took 28 min; every second scan is shown)
2nd charging and charge decay 3rd charging and charge decay After time stop (18 h)
+10V after 140 min -10V after 30 min
Introduction Experimental
Detailed knowledge about the contact charging behavior of dielectric materials is of great interest for technological applications like tribocharging separation[1,2] of mineral particles. The underlying mechanisms are still not well understood.
Here, an attempt is made to study the electric charging of well-defined surfaces (calcite monocrystals) upon contact with a conductive AFM tip.
Kelvin probe force microscopy (KPFM)[3] was applied to verify the electrostatic characteristic of the surfaces before and after contact charging. Both, tribocharging due to rubbing and static contact charging with applied tip bias have been investigated.
Equipment:
Asylum Research MFP-3D AFM
Probes:
TiN coated tips for noncontact AFM,
spring constant 70 N/m, tip curvature radius 35 nm Samples:
monocrystalline calcite, CaCO3 (100), MTI Corporation, USA Conditions:
air, room temperature, 50 % r.H.
applied forces: 2-3 µF, applied voltage: ±10 V
*) CPD – contact potential difference
sample
First pass Second pass
sample
h
VDC VAC
KPFM measurements
Surface height Surface potential Topography signal CPD* signal
KPFM charging KPFM
measurements (rubbing or static contact) measurements
repeating
Results
Charging by rubbing
The AFM tip with or without applied bias (0 V or ±10 V) is dragged on chosen surface area (4 x 4 µm2) with defined force (~3 µN) and speed (0.30 Hz).
Charging by static contact
The AFM tip with applied bias (±10 V) brought into contact with defined force (~2 µN) and for defined time (15 min).
A prototype of the coaxial triboelectrostatic
separator [4]
[1] M.J. Pearse, M.I. Pope, Powder Technol. 14, 7-15 (1976) [2] B.A. Kwetkus, Part. Sci. Technol. 16, 55-68 (1998)
[3] M. Nonnenmacher, M.P. O’Boyle, H.K. Wickramasinghe, Appl. Phys. Lett. 58, 2921 (1991) [4] A. Oberrauner, Dissertation, Chair of Mineral Processing, Montanuniversität Leoben (2011)) [5] Y.S. Zhou, et al., Nano Lett. 13 (6), 2771–2776 (2013)
before charging charge decay time stop charge decay
charging -10V (12 h)
-1,7 -1,2 -0,7 -0,2 0,3
-50 150 350
average min
CPD [V] max
Time [min] 1000 1200
0.3
-0.2
-0.7
-1.2
-1.7
0
Evolution of the CPD signal with time.
0
before charging charge decay time stop charging time stop charge charging -10V (14 h) +10V -10V (18 h) decay
Evolution of the CPD signal with time.
Summary Outlook
• Investigation of the influence of parameters like contact force, humidity, rubbing speed, and temperature.
• Performing contact charging with crystal particle attached to the AFM cantilever.
