Whitehead Lecture

The Whitehead Memorial Lecture

The Whitehead Memorial Lecture is named in honor of Dr. John Boswell Whitehead, a pioneer in electrical insulation and dielectrics and a long-time contributor to the CEIDP.  The Conference opens each year with the Lecture and it is the keynote session of the Conference.  The 2023 Whitehead Memorial Lecture will be given by Prof. Jacques LEWINER, ESCPI Paris-PSL, France. (link)

2023 Whitehead Memorial Lecture: Prof. Jacques LEWINER, ESCPI Paris-PSL, France

TitleSpace charge and polarization in insulators

Abstract—Insulators have held a very important role in the development of Physics and particularly Electromagnetism. Indeed, early “scientists” observed that by rubbing some materials such as amber, they were generating surprising effects which could not be understood with the advancement of science at that time.

It is no surprise that electron is the name for amber in Greek and rashmal is the name for electricity in Hebrew, which is also the name for amber.

During the Middle Ages, progress in understanding the phenomena observed was rather slow. During the 17th century and of course the 18th century, progress was impressive. People found ways of generating this mysterious fluid and even storing it using Leyden jars which are essentially what we would call today capacitors. Maxwell’s equations introduced the relationship between electric field, polarization and space charge. The physicist and mathematician Heaviside, enthused by the Treatise on electricity and magnetism written by Maxwell, reduced the number of Maxwell’s equations to 4 and proposed to call “electret” a permanently polarized insulator in analogy with the “magnet”, which was a material with a permanent magnetic polarization.

The first artificial electret was prepared by Mototaro Eguchi in 1919 using carnauba wax, rosin and bee’s wax. As was progressively discovered, this permanent polarization may be due to dipole orientation, ionic conduction or space charge injection. For many years, electrets were a curiosity and large-scale applications only began with the arrival of new insulating materials, polymers, properties of which could be controlled both from the mechanical and the electrical point of view, for instance polyolefins and among them fluorinated polyolefins.

These new materials allowed for the development of electrets-based sensors and particularly the electret microphone.

In parallel, the development of electric systems and grids required insulators with high breakdown strengths. Progressively, initial natural materials were replaced by synthetic materials: oils and polymers. During more recent years, electric cables have had a very significant growth. Indeed, electricity is increasingly transported through grids at high voltage in order to reduce losses. When this is possible, aerial lines are used and insulators are only placed to isolate the conductor from the electric poles. When various elements of a global network need to be connected to one another and those elements are separated by oceans, aerial lines can no longer be used, and it is necessary to use underwater high-voltage cables. If the distances are great, AC voltage can no longer be used because of the associated losses. In such cases, it is necessary to use DC cables. However, the insulator is then subjected to permanent applied fields, which may lead to the buildup of a polarization or of space charge. If this buildup does not stabilize in time, the electric field generated by this polarization or space charge adds up to the applied electric field and may lead to breakdown.

In order to select an appropriate material as an insulator, and to optimize the interfaces between conductors and insulators, much work has been devoted to the measurement of this space charge. Pioneering work was made by peeling off the insulator and measuring the charge contained in each slice. However, this method was essentially destructive and very unreliable on the validity of the results because of the effect of the interaction between the charges themselves and the cutting tool and of the friction of the cutting tool in the insulator.

Another idea was proposed to use the diffusion of heat. Indeed, during this process, the insulator is slightly deformed because of dilatation, resulting in a variation of the charges induced on the electrodes. It is possible to relate the variation of this charge either by a current measurement when the system is connected to a low impedance load or to the voltage variation when a high impedance load is used. This technique requires a deconvolution process in order to obtain the field or charge distribution and in many cases, the unicity of the solution is difficult to prove. Moreover, this diffusion process is well suited for thin insulators, which is not the case in high voltage devices.

In 1976, it was proposed to use, instead of a diffusion process, the propagation of a pressure wave (Pressure Wave Propagation method or PWP method). Many implementations of this method have been made initially using shockwaves, then laser induced pressure pulses (LIPP), voltage steps to create the wave (Pulsed Electro-Acoustic method or PEA method), or piezoelectric transducers (PIPP). This has allowed the evolution of space charge distribution under applied stress to be measured. It was shown in 1996 that there was a strong correlation between space charge buildup and breakdown. This method is now used to optimize the material and the interfaces, for instance for cable manufacturing.

Other applications of insulators and their polarization are now under development in novel fields, such as Biology and microfluidics. Various phenomena can be applied, such as electrophoresis when dealing with charged particles or dielectrophoresis when dealing with uncharged particles such as particles or cells at microscale. Examples will be given of such new applications.

To summarize, insulators have been known since antiquity. Their applications have been directly associated with the development of electricity but now, new fields of applications are appearing, for instance in Biology.

Short Bio (link): Jacques Lewiner is a Physicist and Inventor. He is Professor and Honorary Scientific Director of École Supérieure de Physique et de Chimie Industrielles (ESPCI Paris-PSL). He was also Dean of Innovation and Entrepreneurship of PSL – Research University Paris from 2012 to 2018. Jacques Lewiner has carried out basic and applied research in various domains of Physics. In 1973 he was nominated Professor in charge of the Electromagnetism Chair at ESPCI Paris. In 1980 Georges Charpak (Nobel Prize, 1992) joined his laboratory. In 1987 the Director of ESPCI, Pierre-Gilles de Gennes (Nobel Prize, 1991), asked him to become Scientific Director of ESPCI Paris, a position he kept until 2001. He has published more than 80 scientific papers in International Journals and has presented at numerous international conferences. In parallel with his research, Jacques Lewiner has been prolific in developing applications of his work. He has filed more than 700 patent applications in France and foreign countries resulting in over 71 patents granted by the European Patent Office and 78 patents granted by the US Patent Office. A significant number of these patents have been either licensed or sold. In 2018, he was one of the three nominees for the European Inventor Award for Lifetime Achievement awarded by the European Patent Office. Jacques Lewiner has participated in the creation of various technology-oriented startup companies. Some of these companies have experienced a strong, and in some case, spectacular growth. Jacques Lewiner is active in building bridges between research and industry. He has created structures to help researchers overcome the difficulties of technology transfer.