A Century of Development in Applied Electrostatics ... · Oct 4, 2010 . Topics •Background...
Transcript of A Century of Development in Applied Electrostatics ... · Oct 4, 2010 . Topics •Background...
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A Century of Development in Applied Electrostatics;
Nothing Static Here!
G.S.P. Castle
Dept of Electrical and Computer Engineering
University of Western Ontario,
London, Ontario, Canada G.S.P. Castle, Plenary Lecture IAS Annual Meeting, Houston,
Oct 4, 2010
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Topics
• Background
• What is Electrostatics?
• Particle Charging
• Three Important Developments 20th Century:
1) Electrostatic Precipitation (ESP); 1907
2) Electrostatic Painting ;1930’s
3) Electrophotography (Xerography) ; 1950-59
• The 21st Century
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In the beginning; there was
LIGHTNING Atmospheric Volcanic
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Thales of Greece 624-536 BC
• Philosopher and “Father of Science”
• Showed magnetic force (lodestone)
• Showed electrostatic force (amber/fur)
• Amber in Greek aka “elektron”
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Last 2400 + years Regarded as a scientific curiosity
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Or a force of “tingling” attraction
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Or a Shocking Experience
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Fast Forward
• Majority of us repeated Thales experiment in grade and/or high school (with varying degrees of success)
• Regarded as a quaint scientific curiosity
• Science of electrostatics
generally ignored in university
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Engineering Reality
• Electrostatic processes are basis of many important applications
• Diverse fields;
air pollution control, printing and copying, painting, materials separation, sand paper manufacture etc.
• All came to fruition in the 20th century
• Common feature; involve controlled movement of small particles (mm to nm)
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Why?
1) Compared to gravity, electrostatic Coulomb force dominant for small particles
Charge to mass ratio (Q/M)
Q α surface area α r2
M α volume α r 3
Q/M α 1/r
2) Energy efficient
3) Direction and strength of force controllable by E
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What is Electrostatics? Is it really “static”?
• Cannot be; otherwise no work is possible
• Unfortunately IEEE definition is misleading;
“the branch of science that treats of the electric phenomena associated with electric charges at rest in the frame of reference”
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Formation of “Electrostatics Society of America”
in 1970 • ESA definition;
“the class of phenomena recognized by the presence of electrical charges, either stationary or moving, and the interaction of these charges, this interaction being solely by reason of the charges and their positions and not by reason of their motion”
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• In practical application means electric field effects predominate over magnetic field effects
• In electric circuit terms, ratio of voltage to current is very high i.e. high impedance
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Also should refute common misconception;
need high voltage
• F =QE but E is the gradient of voltage and so depends upon geometry
• Consider a common reference field, the breakdown strength of air
3 MV/m = 30 kV/cm = 3 V/µm
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Key to practical applications in the 20th century?
Reliable charging and power supplies
• Ionic charging; corona discharge
useful for any material
• Induction (conduction) charging; field induced
only works for conductive materials
• Contact (triboelectric) charging; dissimilar materials
occurs in all materials but only practical for cases where at least one is an insulator
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Three Important Developments;
1) Electrostatic Precipitation (ESP)
• Charge particles in air stream using corona discharge
• Collect on grounded plate through Coulombic attraction
-Very simple in concept
-Difficult to translate into practice
-Interdisciplinary problem; complex interaction among electrical, mechanical and chemical properties
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Evolution
• Hohlfield (Germany) 1824
-used corona to clear fog in a bell jar
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Evolution
• O.J. Lodge (U.K.) 1885
-patented process, demonstrated successfully in lab
-installed at a smelter in North Wales
-failed to work!
TWO Reasons;
a) inadequate power supply
b) lead oxide fume
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F.G. Cottrell
• 1st successful installation at sulphuric acid plant south of S.F. in 1907
• Breakthrough due to four factors
-mechanical rectifier
-”pubescent “corona electrode
-negative corona
-heated HV bushing
Also lucky; sulphuric acid mist
Patent 1908
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Preferred method for cleaning large
scale industrial particulate emissions
• By 1960’s η > 99% (wt)
• Currently η > 99.9% (wt) and limits on escaping particles < 10 µm
• Improvements; gas conditioning, corona wire geometry, power supplies, pre-charging, rapping optimization, intermittent and pulse energization
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Current Challenges
• Improve collection for sub-micron particles
• Remove gaseous pollutants along with particles
-non-thermal discharge plasmas
-electron beam reactors
-advanced oxidation techniques
• Issue of effectiveness and energy efficiency
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Three Important Developments;
2) Electrostatic Painting and Coating
• In 1930’s liquid, mechanical atomized spray painting developed
• Ease of application but η =30% common
• Recognition that principle of ESP could improve η
• By 1940’s automated painting lines regularly achieved η > 70%
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Liquid Electrostatic Paint Spraying
• Improved uniformity of coating (space charge) and “wrap around” (Coulomb attraction)
• Led to improvements in atomizers; blade, rotating bell or disc and hydraulic or air; robotic control etc
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Led to Powder Coating
• 1960’s, recognized that solvent not necessary
• Spray paint in form of finely dispersed, electrically insulating, thermoplastic powder
• Charge (corona or triboelelectric)
• Paint attracted to target,
adheres (image force)
fused in oven
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Key advantage; no (VOC) Volatile Organic Compounds
• Widely adopted starting in 1970’s
• Led to major improvements in
a) equipment
b) powder formulations (electrical and chemical properties)
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Features of Powder Coating
• Note electrostatic force is essential to operation
• Currently η >80% and overall η >95% by recycling overspray
• Surface quality and uniformity still needs work
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Three Important Developments;
3)Electrophotography (Xerography)
• ESP and liquid painting significant improvements in technology
• Powder coating (and elimination of solvent) a major improvement in technology
• Demonstration of first electrophotographic image by Chester Carlson in 1938, truly a revolutionary development
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Chester Carlson
• Initially worked from his “kitchen” laboratory
• First to make true dry copies of documents
Bob Gundlach (1926-2010)
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Six Step Process
Most complex combination of electrostatic processes;
-photoconductivity
-corona charging
-triboelectric charging
-coulombic attraction
-image force adhesion
-ionic neutralization
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Early Machines
Model A (1949) XEROX 914 (1959) 3 minute/copy 26 sec/copy
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Commercialization
• 1959; Xerox 914 first black and white plain paper copier
• 1973; first printer with flash lamp exposure
• 1975; first laser printer (IBM) • With competition,
developments “exploded” in 1980’s and 90’s;
-full colour copiers/printers; -many billions of dollars of
commerce -revolutionized business/home
offices -desktop publishing
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Lesson in all this?
• Don’t forget “old” science
• Reliability of good image production is dependent upon consistent tribocharging
• Thank you Thales!
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NEW APPLICATIONS? 21st Century
Look to three main characteristics
1) useful for particles from submicron to millimetre size
2) force increases as distances get smaller
3) energy efficient
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MEMS
(Micro-Electrical Mechanical Systems)
• Revolutionary devices; comparable to effect microprocessors had on computers
• Mass produce integrated sensors, actuators etc
• Interface between computational and physical world
• Fabrication and operation dependent upon electrostatic forces
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Biotechnology
• Many biological processes governed by electrostatic factors
• Wide application for electrophoretic and dielectrophoretic forces
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Ultrafine Particles and Nanotechnology
• Nanometer sized particles finding increasing application in industry
• Characterized by high surface energy which greatly affect properties of ceramics, metals, optical structures and semiconductors
• Fabrications being pushed to atomic dimensions
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Space
• Absence of gravity and presence of vacuum allows upper and lower size range of particles to
be extended
• “Deep Space I” spacecraft
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Thank you very much
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Important Characteristic
• In ESP electrostatic force; basis of process
• In liquid painting electrostatic force improves the process and enhances η and uniformity
• If voltage fails, process still works