No Slide Title · • Types of current : (1) Direct current plating (2) Pulse plating In...
Transcript of No Slide Title · • Types of current : (1) Direct current plating (2) Pulse plating In...
Electroplating/
Electrodeposition
Wei Yan
ABC’s of Electrochemistry
03/22/2012
2 Ohio University - Avionics Engineering Center
• Introduction
• Electroplating Setup
• Importance of Electrodeposition
• Electrochemistry Fundamentals
• Factors affecting the Electrodeposits
• Conclusions
OUTLINE
3
Introduction
Electroplating? Electrodeposition? Electrolytic deposition?
• Electroplating is often also called "electrodeposition,” a short version of
“electrolytic deposition,” and the two terms are used interchangeably.
• It’s a process using electrical current to reduce cations of a desired
material from a solution and to coat the material as a thin film onto a
conductive substrate surface.
Reference:
http://electrochem.cwru.edu/encycl/art-e01-electroplat.htm
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Definition
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Electroplating Setup
1. Is the core part of the
electroplating.
2. Cathode: the object to
be plated
3. Anode: (1)dissolvable
anode (2) inert
anode
4. Electrolyte/plating
bath
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Electrolytic Cell
Reference: http://electrochem.cwru.edu/encycl/art-e01-electroplat.htm
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Table =The metals can be plated
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Part of Periodic Table. Metals inside the frame
can be electrodeposited from aqueous solutions
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Importance of Electrodeposition
• 1. Decoration: Coating a more expensive metal onto a base metal
surface in order to improve the appearance. Such as jewelry,
furniture fitting
• 2. Protection: Corrosion-resistant coatings such as chromium
plating of automobile parts
• 3. Electroforming: Manufacture of sieves, screens, dry shaver
heads and dies.
• 4. Enhancement: coatings with improved electrical and thermal
conductivity, reflectivity etc.
In CEER,
Electrodeposition of Metals in Catalyst Synthesis
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Applications
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Electrochemistry Fundamentals
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Reference: http://chem1.eng.wayne.edu/~yhuang/Papers/Book_Plating_ECHP
Anode Cathode
Anode Cathode
A typical electroplating setup for plating copper one mistake?
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Electrochemistry Fundamentals
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• Cathode:
Cathode
• Anode:
Reduction reaction
Oxidation reaction Anode
Anode and Cathode
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Electrochemistry Fundamentals
According to the Faraday’s first law and second law of
electrolysis, the amount of material deposited on an
electrode is proportional to the amount of electricity used.
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Faraday’s Laws of Electrolysis
m is the number of moles of the metals
reduced by charge Q;
F is Faraday constant
Mw is the atomic weight
W is the weight of deposit
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Electrochemistry Fundamentals
• Current efficiency = 100 * WAct/WTheo
Wact is the weight of metal deposited or dissolved
WTheo is the corresponding weight to be expected from Faraday’s
laws if there is no side reaction.
Cathode efficiency
Anode efficiency
Current distribution
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Current Efficiency, and Current Distribution
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Electrochemistry Fundamentals
• Overpotential is the difference in the electrode potential of
an electrode between its equilibrium potential and its
operating potential when a current is flowing.
• It represents the extra energy needed to force the
electrode reaction to proceed at a required rate.
• A slow reaction will require a larger overpotential for a given
current density than a fast reaction.
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Overpotential and overvoltage
Reference : http://chem1.eng.wayne.edu/~yhuang/Papers/Book_Plating_ECHP
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Electrochemistry Fundamentals
• Overvoltage is the difference between cell voltage and the
open circuit voltage (OCP) when a current is flowing.
• It represents the extra energy needed to force the cell
reaction to proceed at a required rate.
• The overvoltage is the sum of the overpotentials of the two
electrodes of the cell and the ohmic loss of the cell.
Unfortunately, the terms overvoltage and overpotential are
sometimes used interchangeably.
.
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Overpotential and overvoltage
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Electrochemistry Fundamentals
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How to determine the potential of deposition
Fig. Typical voltammogram for a single-metal deposition
Reference: http://electrochem.cwru.edu/encycl/art-e01-electroplat.htm
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Electrochemistry Fundamentals
• How to determine the potential of deposition for two metals
codeposition ?
Key point- the onset of the reduction peaks in CV
(1) If the two reduction peaks of metals are not separated, both
of metals can be reduced when operating potential is more
negative than the onset potential.
(2) If the two reduction peaks of metals are clearly separated,
both of metals can be reduced (codeposition) at a more
negative potential than onset potential of the less noble
metal
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Electrochemistry Fundamentals
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How to determine the potential of deposition
Fig. CV for Ni-Co codeposition in the plating bath containing 0.06M NiCl2 and
0.06M CoCl2 at 25C.
Reference: Chi-chang Hu and Allen Bai, J.Electrochem.Soc. 149(615) 2002
E < -0.75 V, cathodic deposition
-0.75V <E< -0.3V double layer
response
E > -0.3V anodic dissolution
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Electrochemistry Fundamentals
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How to determine the potential of deposition
Fig. CV for Ni-Fe codeposition in the plating bath containing 0.06M NiCl2 and
0.06M FeCl2 at 25C.
Reference: Chi-chang Hu and Allen Bai, J.Electrochem.Soc. 149(615) 2002
E < -1.00V Fe and Ni
codeposition
-1.00V <E< -0.6 V Ni only
deposition
17 Ohio University - Avionics Engineering Center
Factors affecting the Electrodeposits
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Factors affecting the Electrodeposits
• Surface Preparation
• Physical & chemical nature of substrate surface
• Deposition temperature
• Deposition current / deposition potential
• Bath composition
• Power supply current waveform
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Factors affecting the Electrodeposits
“ one can make a poor coating perform with excellent
pretreatment, but one cannot make an excellent coating
perform with poor pretreatment”
Three basic steps:
1. Surface cleaning – chemical way and mechanical way
2. Surface Modification
3. Rinsing
Reference: http://electrochem.cwru.edu/encycl/art-e01-electroplat.htm
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Surface Preparation
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Factors affecting the Electrodeposits
Current density of deposition can make an influence on
(1) Current efficiency
(2) Thickness of deposit
(3) Structure of deposit
(4) Composition of deposit
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Deposition current
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Factors affecting the Electrodeposits
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Deposition current- Example
Reference: M.J.Rahman, S.R.Sen. J. Mechanical. Engineering. 40(1) 2009
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Factors affecting the Electrodeposits
• Ingredients of a Plating Bath
(1) Metal salt solution-To provide a source of the metal or metals being
deposited.
(2) Various Additives
• The purposes of Additives:
1.To form complexes with ions of the depositing metal
2. To provide conductivity.
3. To stabilize the solution e.g. against hydrolysis.
4. To act as a buffer to stabilize the pH.
5. To modify or regulate the physical form of the deposit.
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Bath composition and additives
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Factors affecting the Electrodeposits
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Bath composition and additives- Example
Apart from a buffer, boric acid can inhibit zinc deposition
and promote nickel discharge
without boric acid with boric acid
Reference: M.J.Rahman, S.R.Sen. J. Mechanical. Engineering. 40(1) 2009
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Factors affecting the Electrodeposits
• Types of current : (1) Direct current plating (2) Pulse plating
In dc-plating, constant current is used, and the rate of arrival of metal ions
depends on their diffusion coefficient (electrode-to-part spacing and agitation).
• The pulse current (PC) and pulse-reverse current (PRC) are used to get the
deposit with reduced porosity, and finer grains , and to minimize the use of
additives and , contamination, etc.
• The morphology of some metal and alloy deposits were found to be superior to the
dc-plated deposits.
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Power supply current waveform
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Factors affecting the Electrodeposits
• Characteristics of Pulse Currents (PC)
• PC, where all the pulses are in one
direction (with no polarity)
Each pulse consists of an ON-time (TON)
during which potential /current is applied,
and an OFF-time (TOFF) during which
zero current is applied
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Power supply current waveform
Reference: M.S. Chandrasekar, M. Pushpavanam. Electrochim Acta 53 (2008)
3313–3322
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Factors affecting the Electrodeposits
• Characteristics of PRC
• PRC, where anodic and cathodic pulses
are mixed.
• To characterize a train of pulse-reverse
current waveform, key parameters need
to be known.
(1) The cathodic peak pulse current
density (Ic); The cathodic pulse length
(Tc)
(2) The anodic current density (IAA); The
anodic pulse time (TAA)
(3) IA = (Ic x Tc + IAA TAA) / (Tc + TAA)
average current density
(4) T is the cycle time
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Power supply current waveform
Reference: M.S. Chandrasekar, M.
Pushpavanam. Electrochim Acta 53 (2008)
3313–3322
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Electrochemistry Fundamentals
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Fig. CV for Ni-Fe codeposition in the plating bath containing 0.06M NiCl2 and
0.06M FeCl2 at 25C.
Reference: Chi-chang Hu and Allen Bai, J.Electrochem.Soc. 149(615) 2002
E < -1.00V Fe and Ni
codeposition
-1.00V <E< -0.6 V Ni only
deposition
28 Ohio University - Avionics Engineering Center
Conclusions
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Conclusions
1. “Electroplating/ Electrodeposition: historically old,
technologically advanced” D.Gabe said.
2. The purpose of electroplating is to produce a qualified coating
with the desirable attributes. Each kind of coating needs a
special electroplating process with a given application.
3. It is easy to get the coatings of metals, but it is not easy to get
the coatings with good quality. Key points are those factors
affecting the electrodeposits, which is crucial for the final
deposits.
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Questions?