Electroplating/ Electrodeposition

Transcription

1 Electroplating/ Electrodeposition Wei Yan ABC s of Electrochemistry 03/22/2012

2 OUTLINE Introduction Electroplating Setup Importance of Electrodeposition Electrochemistry Fundamentals Factors affecting the Electrodeposits Conclusions Ohio University - Avionics Engineering Center 2

3 Introduction Definition 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: 3

4 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 Electroplating Setup Electrolytic Cell Reference: 4

5 Table =The metals can be plated Part of Periodic Table. Metals inside the frame can be electrodeposited from aqueous solutions 5

6 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, Applications Electrodeposition of Metals in Catalyst Synthesis 6

7 Electrochemistry Fundamentals A typical electroplating setup for plating copper one mistake? Anode Cathode Anode Cathode Reference: 7

8 Electrochemistry Fundamentals Anode and Cathode Cathode: Cathode Reduction reaction Anode: Anode Oxidation reaction 8

9 Electrochemistry Fundamentals Faraday s Laws of Electrolysis 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. 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 9

10 Electrochemistry Fundamentals Current Efficiency, and Current Distribution Current efficiency = 100 * W Act /W Theo W act is the weight of metal deposited or dissolved W Theo is the corresponding weight to be expected from Faraday s laws if there is no side reaction. Cathode efficiency Anode efficiency Current distribution 10

11 Electrochemistry Fundamentals Overpotential and overvoltage 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. Reference : 11

12 Electrochemistry Fundamentals Overpotential and overvoltage 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.. 12

13 Electrochemistry Fundamentals How to determine the potential of deposition Fig. Typical voltammogram for a single-metal deposition Reference: 13

14 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 14

15 Electrochemistry Fundamentals How to determine the potential of deposition E < V, cathodic deposition -0.75V <E< -0.3V double layer response E > -0.3V anodic dissolution 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)

16 Electrochemistry Fundamentals How to determine the potential of deposition E < -1.00V Fe and Ni codeposition -1.00V <E< -0.6 V Ni only 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)

17 Factors affecting the Electrodeposits Ohio University - Avionics Engineering Center 17

18 Factors affecting the Electrodeposits Surface Preparation Physical & chemical nature of substrate surface Deposition temperature Deposition current / deposition potential Bath composition Power supply current waveform 18

19 Factors affecting the Electrodeposits Surface Preparation 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: 19

20 Factors affecting the Electrodeposits Deposition current Current density of deposition can make an influence on (1) Current efficiency (2) Thickness of deposit (3) Structure of deposit (4) Composition of deposit 20

21 Factors affecting the Electrodeposits Deposition current- Example Reference: M.J.Rahman, S.R.Sen. J. Mechanical. Engineering. 40(1)

22 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 Bath composition and 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. 22

23 Factors affecting the Electrodeposits Bath composition and additives- Example without boric acid with boric acid Apart from a buffer, boric acid can inhibit zinc deposition and promote nickel discharge Reference: M.J.Rahman, S.R.Sen. J. Mechanical. Engineering. 40(1)

24 Factors affecting the Electrodeposits Power supply current waveform 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. 24

25 Factors affecting the Electrodeposits Power supply current waveform 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 Reference: M.S. Chandrasekar, M. Pushpavanam. Electrochim Acta 53 (2008)

26 Factors affecting the Electrodeposits Reference: M.S. Chandrasekar, M. Pushpavanam. Electrochim Acta 53 (2008) Power supply current waveform 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 26

27 Electrochemistry Fundamentals E < -1.00V Fe and Ni codeposition -1.00V <E< -0.6 V Ni only 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)

28 Conclusions Ohio University - Avionics Engineering Center 28

29 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. 29

30 Questions?