Triboelectric Generation: Getting Charged

Ryne C. Allen

Desco Industries Inc. (DII), Employee Owned

December 2000

Reproduced with Permission, EE-Evaluation Engineering, November, 2000

 

I Introduction

Have you ever received an unexpected static shock? Itís a nice brisk day as park your car and slide out of the driverís seat only to experience a significant electric shock as you grab the metal door handle. What happened? Why did you just get zapped? First, you got charged up with static electricity when sliding out of the driverís seat. This is a common phenomenon experienced every time when two materials come into contact and then separate and is known as triboelectric charging. Second, when your bodyís skin, being a fair electrical conductor, came into contact with a very good conductor (metal door) your bodyís accumulated charge is quickly discharged (brought to the same potential as the door) which resulted in an electric shock.

Triboelectric charging or tribocharging for short is the most frequently occurring phenomenon for charging materials and yet one of the least understood. Tribocharging affects us every day whenever we move ourselves or other materials around us. This phenomenon is especially a concern to the electronics industry. Static charge build up can lead to detrimental effects to semiconductor devices upon discharge, more specifically electrostatic discharge (ESD).

To control ESD events, it is highly desirable to use materials that are low tribocharging (antistatic) as well as dissipative for the use of handling, packaging, and storing ESD sensitive electronic devices. Understanding triboelectrification may help to choose or design materials that will minimize tribocharging in static sensitive areas.

 

II Triboelectric Series

When two materials with neutrally charged surfaces come into contact (< 4 Å) and then separate, the materials will have undergone tribocharging and now be at a non-neutral surface charge level. The level and polarity this newly acquired surface charge is at is dependent on several factors, but can be relatively answered by viewing the triboelectric series chart [figure 1].

A material such as glass that comes into contact with a vinyl material will acquire a more positive charge because it is near the Ďmore positiveí position in the triboelectric series chart relative to the position of vinyl. Alternately, the vinyl will acquire a more negative charge following the same logic. The fact that these two materials are far apart from each other in the series may result in a larger charge level generated then if the glass came into contact with, say aluminum.

The above list is combined from references: [4, 5, and 6]

FIGURE 1

 

 

 

The triboelectric series is a loose ranking of a materialís polarity when triboelectrically charged with a given material. The ranking of a material may easily change position in the series depending on several factors such as surface roughness, force of contact, work function, charge backflow, charge breakdown (of air), etc. These variables only add to the confusion of understanding the tribocharging mechanism and make the triboelectric series chart a relative comparison of materials and not an exact science.

 

III THEORY

There are several mechanisms that contribute to the resulting charge that is generated by the triboelectric process. There appear to be 4 major factors that have the greatest influence on the triboelectric charging process and they are: surface contact effects; work function; charge back flow; and gas breakdown. The amount that each mechanism influences the net charge is not well understood at this time.

Surface contact effects include the surfaces roughness, contact force, and frictional heating (caused by rubbing), all of which influence the amount of surface area that is in contact with the other material during tribocharging. The greater the surface contact, the greater the resulting net charge may be when two surfaces are separated after contact.

Though surface contact may seem rather intuitive, there are some subtleties that should be elaborated on; one being surface friction, the other surface roughness [figure 2]. When the coefficient of friction between two surfaces increases, this indicates that the surface roughness between the two surfaces may be greater, which results in decreased surface contact. As an illustration, when two surfaces come into contact on a work surface, letís say 1.0 square inch, the actual or physical contact may only be 0.2 square inches because of surface roughness. Now, if you press down onto surface, the contact area may increase to 0.4 square inches, depending on this contact force and again the surface roughness of both surfaces. If both surfaces were polished to an extremely smooth and flat area (micro-polished), the contact area may be further increased to 0.8 square inches. The smoother either surface is, the more contact both surfaces will make with each other resulting in possible increase of the exchange of charges.

 

The actual area of contact is dependent on the surface roughness

Figure 2

 

Surface charge imbalance is related to friction in that both are dependent on the adhesion between to surfaces on the molecular level. Two surfaces may stick together because chemical bonds form on the surface. When surfaces in contact are separated, some bonds may rupture, and any asymmetrical bonds will tend to leave imbalanced charges behind. Which surface bonds rupture is dependent on their work function.

The work function is the property of a materialís ability to hold onto its free electrons (the electrons orbiting the outer most shell of the material). The greater the materials work function, the less likely it is to give up its free electrons during contact (triboelectric generation). The weaker the work function is, the more likely the material will acquire a more positive charge by giving up or loosing some of its free electrons. In general, materials with higher work functions tend to appropriate electrons from materials with lower work functions.

Charge backflow occurs when two materials have been charged possibly from the above mechanisms and are then separated from intimate contact. The backflow of some of this charge imbalance may flow back to the original material reducing to some degree the net charge (charge imbalance) on either surface from tribocharging.

Gas breakdown can occur between two surfaces during separation. The microscopic surface topology of a surface has many peaks and valleys. It is one of these peaks that may have substantial charge that yields a large electric field in a very small area causing corona discharge or the breaking down of the air molecules which were acting as a dielectric (insulator between the two separating surfaces). During this breakdown, charge can be transferred from one surface to the other via the path of the electrified air (plasma). The amount of charge transferred is dependent on the distance of separation and the gas(ses) pressure(s).

 

IV EXAMPLES

Standard cellulose tape is a good example of a material that has a strong surface adhesion and consequential large surface area contact typically resulting in large charge imbalance during unwind or removal. During unwind the contact and separation of the tape to itself is called "contact charging" or "electrification by contact" and has little to do with friction (or rubbing). Another contribution to the large imbalance of the tape during unwind is the difference in materials. The tape film is cellulose and the adhesive may be rubber based. The two are spaced far enough apart in the Triboelectric Series to result in defined polarities, see figure 1. The rubber adhesive will acquire a more positive charge and the cellulose will acquire a more negative charge due to the difference in their work functions as illustrated below [figure 3]. Voltages well over 20 kV are easily measured from this type of tape.

Illustration of tribocharging during unwinding of standard cellulose tape

Figure 3

 

Another good illustration of tribocharging is to use a pair of ESD training paddles, see figure 4. One paddle is typically aluminum and the other acrylic, which are well separated in the Triboelectric Series. When the bottom of both paddles are brought together and rotated (frictional heating) then separated an electrical charge imbalance will exist between the two plates. Using a static field meter or charge plate analyzer you can measure several kilovolts on each paddle. As you can guess the resulting electric field (charge imbalance) on the aluminum paddle tends to be more positive and the acrylic tends to be more negative.

Example of ESD training paddles

Figure 4

The fact that one paddle is very conductive and the other very insulative helps to illustrate the types of materials that may become charged in your ESD safe work area. Even though a surface may be conductive, it can still become charged through triboelectric generation. Only when a conductive surface is tied to ground or other reference point, will it not be a threat by holding a charge imbalance. Controlling charge imbalance is important in ESD control. Conductors can be grounded, but insulators must be controlled by other methods.

 

V SOLUTIONS

When designing an ESD control program there are two simple rules relative to dealing with charging problems. One is to ground all conductors, the other is to either remove or control all insulative charge generators. Grounding can easily be accomplished with various ESD control products [5]. In lieu of abstaining from the use of non-conductive charge generators, controlling them may be essential to the program. Controlling process necessary insulators can be more involved and may require the use of ionization or surface treatment with a topical antistat solution or spray. Neutralizing of charged objects is accomplished by using a balanced output air ionizer. The target is flooded with a multitude of positive and negative air ions which result in a near zero voltage level relative to ground on the charged surface after just a few seconds of exposure (decay time). The decay time is depending on several factors such as surface proximity to the ionization source, surface area, surface capacitance, level of charge imbalance, etc.

 

CONCLUSION

Understanding how materials interact with each other during the triboelectric process can help in the design of a control program that tries to minimize charge imbalance caused by this phenomenon.

Choosing materials that are either close to each other in (work function) the triboelectric series or antistatic will help to minimize potential charge imbalances in an ESD sensitive work area.

 

REFERENCES

  1. ESD Program Management, 2nd Edition, Dangelmayer, G. Theodore, Kluwer Academic Publishers, Boston, MA, 1999
  2. ADV-11.2, The ESD Association, 7900 Turin Road, Bldg. 3, Suite 2, Rome, NY 13440-2069, http://www.eosesd.org
  3. NATURE'S ELECTRICITY, Charles K Adams, 1987 Tab Books, #2769, p63
  4. MIL-HDBK-263B, "Electrostatic Discharge Control Handbook for Protection of Electrical and Electronic Parts, Assemblies and Equipment (Excluding Electrically Initiated Explosive Devices)", Department of Defense, Naval Sea Systems Command, Arlington VG, 1994
  5. "ESD Control Standards: Setting up an ESD control program", Ryne C. Allen, EE-Evaluation Engineering, February, 1999
  6. "Triboelectric Charging of Common Objects", Thomas B. Jones, University of Rochester, http://www.ee.rochester.edu:8080/~jones/demos/charging.html, December, 1999
  7. "TRIBOELECTRIC SERIES", William Beaty, http://www.eskimo.com/~billb/emotor/tribo.txt, 1995

 

 

About the Author

Ryne C. Allen is the technical manager at ESD Systems.com, a division of Desco Industries, Inc. (DII). Previously, he was chief engineer and lab manager at the Plasma Science and Microelectronics Research Laboratory at Northeastern University. Mr. Allen is a NARTE-certified ESD control engineer and the author of 31 published papers and articles. He is an active member of the ESD Association on several standards working groups and secretary and webmaster of the local Northeast Chapter of the ESD Association. He graduated from Northeastern University with B.S.E.E, M.S.E.E., and MBA degrees. ESD Systems, 19 Brigham St., Unit 9, Marlboro, MA 01752-3170, (508) 485-7390, e-mail: ryne@esdsystems.com, URL: http://www.esdsystems.com.