METHODS AND CURRENT

DEVELOPMENTS OF PRODUCING

ELECTRON BEAMS AND THEIR USES

R.M.A.D.RATHNAYAKE

S/10/486

Department of Physics, Faculty of Science, University of Peradeniya

2014

i

METHODS AND CURRENT DEVELOPMENTS OF PRODUCING

ELECTRON BEAMS AND THEIR USES

A PROJECT REPORT PRESENTED BY

R.M.A.D.RATHNAYAKE

In partial fulfillment of the requirement

for the award of the degree of

B.Sc. (Physics Special)

of the

Faculty of Science,

University of Peradeniya,

Sri Lanka

2013/2014

ii

ACKNOWLEDGEMENTS

I would like to offer my sincere thanks to Dr. V.A.Senevirathne for her excellent guidance and

constant supervision which led me in the right track. I also want to offer my deepest gratitude to my

parents and family for their love and support. And then of course I’m grateful to all of my colleagues

and academic and nonacademic staff members of the Department of Physics, University of

Peradeniya for their support in various ways.

iii

ABSTRACT

METHODS AND CURRENT DEVELEPMONTS OF PRODUCING

ELECTRON BEAMS AND THEIR USES

Electron beams are an essential type of particle radiation to the functionality of many modern

electrical devices. Cathode ray tubes, X-ray tubes, electron beam sputtering, electron microscope,

electron beam lithography, electron beam welding and gas ionization are just a few devices and

processes which uses electron beams for their operation. Thermionic emission is still the major

electron emission mechanism used to extract electrons from matter, but field electron emitters are like

to replace thermionic emitters almost completely in the near future due to their superior qualities.

Since electrons are negatively charged particles their motion can be influenced by electric and

magnetic fields, and therefore after the extraction of electrons out of the matter these fields are used to

accelerate, collimate, focus and direct the electron beam properly for it to be usable. Among the

modern developments of producing electron beams, carbon nanotube field emitters are the most

outstanding electron emitters discovered so far which have the potential to be used in variety

application where an electron beam is required.

iv

TABLE OF CONTENTS Page No

Title i

Acknowledgements ii

Abstract iii

Table of contents iv

List of figures v

CHAPTER 1: INTRODUCTION

1.1 Introduction 1

1.2 History 2

CHAPTER 2: EMISSION MECHANISMS

2.1 Thermionic emission 4

2.2 Field Electron emission 7

2.2.1 Metal cathode field electon emitters 9

2.2.2 Carbon nanotube field electron emitters 10

2.2.2.1 Geometry of the CNT emitters 11

2.3 Photocathode 12

CHAPTER 3: ELECTRON BEAM MANEUVERING

3.1 Fore on electrons in an electric field 13

3.1.1 Uniform electric fields 13

3.2 Force on electrons in a magnetic field 14

3.3 Usage 15

3.3.1 Cathode ray tube 15

3.3.2 Electron beam sputtering/evaporation 16

3.3.3 Electron microscope 17

CHAPTER 4: CURRENT DEVELOPMENTS

4.1 Application of CNT field emitters 18

4.1.1 Field emission display 18

4.1.2 Miniature X-ray devices 19

4.1.3 Micro satellite thruster neutralizers 20

REFERENCES 22

v

LIST OF FIGURES

Figure No Page No Caption

1 4 A picture showing a filament in a low pressure mercury gas discharge lamp. To

increase electron emission a white thermionic emission mix coating is applied,

visible on the central portion of the coil.

2 5 Schematic representation of thermionic emission

3 7 Plot of anode current vs fillament temperature for a termionic emitting fillament.

4 9 Schematic diagram of a field emitter, the electric field around the tip is encanced to

the point electrons can tunnel through the surface potential barrier.

5 9 Schematic structure of a Spindt cathode.

6 11 A: Diode structure, B: Triode structure.

7 12 Photocathode

8 14 The force on an electron in a uniform electric field between two parallel

conducting plates.

9 14 The path of an electron in a uniform magnetic field, the magnetic field is towards

the paper and the force exerted on the electron is a perpendicular direction to both

velocity and magnetic field as given by Fleming’ left hand rule.

10 15 A CRT which uses electric fields in both focusing and deflecting the electron

beam.

11 15 A CRT that uses magnetic fields in focusing and deflecting electrons.

12 16 Schematic diagram of an electron beam evaporation process.

13 17 The schematic comparison of a magnetic lens and an optical lens.

14 19 Schematic comparison of a CRT tube display and Field Emission display.

15 20 A miniature field emitter X-ray tube with gated a MWNT-2mm

diameter cathode.

16 21 Small colloid thruster prototype (left) featuring a CNT field emission cathode for

beam neutralization (right), developed by Busek Co. Inc.

vi

1

CHAPTER 1

Introduction

1.1 Introduction An electron beam can be defined as the controlled propagation of electrons along a certain direction.

Electron beams are used in many applications of modern technology since they offer us with a high

level of control and ease of extraction. Almost no other type of charged particle radiation can be so

delicately manipulated. Electron beams with different properties are used in devices and processes such

as CRT (cathode ray tube) displays, electron microscopes, microwave generation, vacuum electronics,

electron beam lithography, X-ray generation, gas ionization, ion beam neutralization, electron beam

evaporation and many more. The electron energy, beam intensity, beam diameter, electron source,

control mechanism etc will differ from one to another in these applications according to the

requirements.

In order to produce an electron beam with the desired properties, we need what’s called an electron

gun or an electron emitter. An ‘electron gun’ can be categorized primarily under mechanism of

emission, type of electric field generation and focusing method. Very importantly in almost all the

instances where an electron beam is produced, it’s done inside a vacuumed chamber. Otherwise, loss of

electron energy and decrement in the number of electrons reaching the anode will take place due to the

electron and gas molecules/atoms interactions. This leads to poor efficiency of the emitter. In addition,

having gas molecules inside the emitter may also cause electrical breakdown inside the chamber due to

high strength electric fields present to accelerate electrons.

There emission mechanisms currently used to extract electrons from matter are; thermionic emission,

cold cathode (field electron emission) and photo cathode. The first two mechanisms are the two

prominent emission mechanisms used whereas photo cathode electron emitters have only some specific

uses. Thermionic emission is known as a hot cathode method where the electron emitting cathode is

heated to a very high temperature for electron emission. On the other hand, field electron emission is a

cold cathode method where no heating of the cathode is done but instead a high strength electric field is

used for the electron extraction from cathode. Due to some of the very important qualities accompanied

with field electron emission over thermionic emission such as high power efficiency, small size and fast

switching capability it is predictable that field electron emission may take the place of thermionic

emitters. But yet thermionic electron emitters are the more widely used. With the recent development of

carbon nanotube field emitters which addresses to some of the common problems with metal cathode

field electron emitters, the future of field electron emitters looks more promising.

2

The handling of electron beams in these many applications is mainly done by electric and magnetic

fields since electrons are charged particles of which’s motion is readily influenced by these fields. For

example in CRTs both electrostatic and magnetic deflection are used. Then in electron microscopes, we

find magnetic lenses which are in fact electromagnets which perform the role identical to the glass

lenses in an optical microscope. Likewise, in almost every instance an electron beam is used in a device

a magnetic or an electric field or both are definitely used to control the electron beam.

1.2 History

Electron beams were first known as cathode rays. In 1838, Michael Faraday passed a current through

a rarefied glass tube and he notice an arc of light starting from the cathode and ending from the anode.

In 1857, German physicist Heinrich Geissler obtained an even low pressure of atm and he

observed that a glow filled the tube instead of an arc. These tubes were known as Geissler tubes and the

glow observed were due to the collision of electrons and air molecules. Because of these collisions,

cathode rays going up to the anode were not obtained in the Geissler tubes. After that, by about 1870

British physicist William Crookes and his fellow scientists were able to obtain an even low pressure of

below atm. With decreasing air pressure, the glow was observed to shift eventually towards the

anode leaving a dark space behind. This is because the average distance the electrons travel before

colliding with a gas molecule increases with the decreasing air pressure. In this dark space, electrons

travel without any collisions with the air molecules.

However, until the discovery of electron by J.J.Thomson in 1897 the constituents of the cathode rays

were not known exactly. Thomson measured the charge/mass ratio of cathode rays which reveals that

cathode rays are made of particles. Thomson calculated the mass of these particles and found that it is

1800 times lighter than the Hydrogen atom. Later it was recognized that particles in the cathode rays are

identical to the particles given off by photoelectric and radioactive materials and particles that carry

electrical current in wires.

The gas ionization (aka Cold cathode) method of producing cathode rays used in Crookes tubes was

not reliable enough because it depends a lot on the pressure of the residual air in the gas tubes and

overtime these air molecules are absorbed by the walls of the tube eventually causing the stop of

cathode ray emission. In 1880, Thomas Edison discovered a more reliable method of obtaining more

bright and stable cathode rays.

In this method, the cathode was a filament wire heated red hot by passing a separate current through it

which then releases electrons in to the vacuum by thermionic emission. The first vacuum tubes which

were used in the amplification of electric signals used this hot cathode technique.

3

Nowadays, cathode rays are called as electron beams. The technology of manipulating electron beams

in modern devices such as CTR and electron microscopes were originated from these early cathode ray

tubes. [1]

4

CHAPTER 2

Emission Mechanisms

The major emission mechanisms used for extraction of electrons from matter are,

a) Thermionic Emission (Hot Cathode)

b) Electron Field emission (Cold Cathode)

c) Photocathode

2.1 Thermionic Emission

Thermionic emission is the most common emission mechanism used to extract electrons from matter

to date. It is a mechanism to move electrons in solid state to an adjacent vacuum space. Basically,

what is done is a metal filament is heated to give enough thermal energy to free electrons in the metal

filament to overcome the surface potential barrier or what is known as the ‘work function’ of the

metal. The magnitude of the amount of electron flow is observed to be increasing drastically with the

increasing temperature of the filament.

Figure.01: A picture showing a filament in a low pressure mercury gas discharge lamp. To increase

electron emission a white thermionic emission mix coating is applied, visible on the central portion of

the coil.

The British physicist Owen Williams Richardson (Nobel Prize-1928) has done some significant work

on the field of ‘Thermionic emission’ that later he named it so by himself. He assumed the velocities

of free electrons in the ‘sea’ of free electrons in a metal follow the Maxwell-Boltzmann distribution

[2]. Some electrons in the tail of this distribution will have sufficient energy to overcome the surface

potential barrier so once they escape, they won’t be pulled back into the metal.

5

The minimum amount of energy required for an electron to completely escape from the metal surface

is known as its work function which is a characteristic of the metal. For most of the metals this is in

the order of few electron volts. Therefore, thermionic current can be increased by decreasing the work

function. It has been experimentally proven that this can be achieved by applying various oxide layers

on the filament wire. Barium, Calcium and Strontium oxides can be given as some examples.

Figure.02: Schematic representation of thermionic emission.

The Richardson’s Law for thermionic emission,

(1)

where,

J- emission current density,

T-absolute temperature of the metal,

W-work function of metal,

k-Boltzmann constant

Can be written in the form

-Proportion of electrons that will be reflected back at the emitter surface due to the wave nature of

electrons.

- Correction factor due to the band structure of the emitting material.

6

Here, m and e arethe mass and the charge of elcetrons and h is Plank’s constant.

In a typical electron emiting device the fillament will be held at a negative potential (cathode.). Let’s

suppose this creates a electic field of magnitude F at the emitter surface.

Without this E-field the surface potential barrier seen by an escaping Fermi level electron be

‘W’,which is the work function.The electric field brings down the surface barrier by an amount ΔW

and thus increases the thermionic current. This is known as the Schottky effect (named after Walter

H.Schottky) or the field enhanced thermionic emission [2]. By simply replacinng W in the

Richardson equation by (W − ΔW) we can model this effect.

√

(2)

wher, ε0 is the electic constant.

This equation is moderately accurate for electric field stength lower than about 108 Vm

−1. For higher

electric field strenghts the so called Fowler-Nordheim (FN) tunneling begins to contribute to the

emission current by a significant amount and in this regime field-enhanced thermionic and field

emission can be modeled by the Murphy-Good equation for thermo-field (T-F) emission. For even

higher electric field strengths FN tunneling becomes the dominant electron emission mechanism and

at here the emitter is in the “cold field electron emission regime”, which is the electon emission

mechanism that will be discussed next.

The following diagram gives a logarithmic plot of some experimental values of anode current versus

filament temperature measurements at three constant anode voltages. Black line represents the

Richardson function for work function W = 4.7 eV.[3]

7

Figure.03: Plot of anode current vs fillament temperature for a termionic emitting fillament.

Thermionic electron emitting sources are used mainly in CRT displays (Televisions, Oscilloscopes),

X-ray generating tubes, transmission electron microscopes and in many and most of the instruments

which requires an electron beam. However, there some drawa bakcs in thermionic emitters which

prevents them from using in certain applications. To name a few,

large amount of energy loss by radiation

inertia of the emitter(a thermionic emitting gun is considerably large and therefore cannot be

fitted into some small electronic instruments)

dimentiona changes due to thermal expansion

the outgassing of species that contribute to vacuum degradation

limited life time from thermal cycling.

2.2 Field Electron Emission (cold emission) Field electron emission is considered as one of the best emission mechanisms among alternative

methods of electron generation. Any place where an electron source is required, we could potentially

use a field electron emitting device. Field electron emitters are particularly important when high

efficiency is necessary, such as in space and portable applications. It’s also effective in producing

high current densities and in applications which requires fast switching or in ultra-high frequency

modulation.

8

Field electron emission (also known as electron field emission and field induced electron emission)

is the emission of electrons from a solid/liquid surface into a dielectric medium which is induced by

an electrostatic field. In contrast with thermionic emission, field electron emission doesn’t require any

heating of electron emitting cathode and therefore it is more generally known as ‘cold emission’.

Field emission can provide bright electron beams with extremely high current density (up to

A. at the emitting site [4]).

However, field emission from pure metals require high strength electrostatic fields with potential

gradient greater than 1 GV/m [5] and depends heavily upon the work function of the metal. Such high

electric fields can only be obtained by the phenomenon known as the local field enhancement. This is

the geometrical enhancement of the electric field caused by the concentration of eclectic field lines

near a conducting solid, shaped as a sharp tip with very low radii of curvature. When very sharp

cathodes are used, the macroscopic electric field needed to induce field emission can be reduced to

about fields with potential gradient of few V/µm are sufficient [4].

The physical mechanism of field emission can be explained by using quantum tunneling. A theory of

field emission from bulk materials has been proposed by Ralph Fowler and Lothar Wolfgang

Nordheim. This is known as Fowler-Nordheim tunneling, which is the wave mechanical tunneling of

electrons through the surface of an electron conductor induced by a high electric field.

A family of approximate equations ‘Fowler-Nodheim equations’ is named after them. Even though,

this model strictly describes field emission from metals at 0 k, it can be used to explain field emission

from some other materials as well.

A very simplified version describing the field emission current from a metallic tip is given below.

[4]

(3)

Where,

I-Emission Current

𝛽 - field enhancement factor

V-Applied voltage

a,b-constants

9

𝛽

d-cathode to anode distance

Figure.04: Schematic diagram of a field emitter, the electric field around the tip is encanced to the

point electrons can tunnel through the surface potential barrier.

2.2.1 Metal cathode field electon emitters

From about 1950 and onwards, extensive effort has been taken for the development of field electron

emitters as electron gun sources. For the generation of a bright and a stable electron beam a ‘large

area field emission source’ is necessary which comprices of a high density of individual field

emission sites created on a subtrate (originally silicon).

One of the early field emitting sources is the ‘Spindt array’ which uses a fabrication technique similar

to the development of silicon-integrated-circuit (IC). These contain very sharp miniature

micromachined metal tips (molybdenum) deposited on small cylindraical voids in an oxide film, with

the void covered by a counterelectrode with a central circular aperture.

Gate(metal) 1 µm Micro tip (metal)

Si substrate

Figure.05: Schematic structure of a Spindt cathode.

V

d

10

2.2.2 Carbon nanotube field electron emitters

Field emission relies heavily on having a very low radius of curvature at the emitting tip. Therefore

it’s essential that the emitting tips’ sharpness remains unaltered during the period of operation.

However, micro machined tips have been shown to be very sensitive and sputtering of cations tends to

dull the tip eventually decreasing the quality of the emission. Hence ultra-high vacuum conditions are

required for this technology which is very expensive and inefficient in industrial scale. Therefore,

researchers initiated looking for other materials that could be grown or deposited as thin films with

required field enhancing properties.

At about 1995, carbon nanotubes rose as a very good potential candidate for a field emitting

material. They showed excellent emission characteristics. Especially because of their shape with high

aspect ratio, they were natural field enhancing objects. Now it’s considered that nanotube films have

the largest potential for technical applications due to their excellent emission characteristic and

relatively ease of fabrication which allows scientists to scale them up for mass production.

Field emission has been observed from Single Wall Nanotubes (SWNT), Multiple Wall Nanotubes

(MWNT) and also nanofibers. However it’s considered that well graphitized nanotubes are superior

field emitters than nanofibers due to their larger dimensions and hence low field enhancement. It’s

unclear whether MWNT or SWNT are better. We might expect single wall nanotubes to have low turn

on voltage due to their higher field enhancing ability, but they exist as bundled ropes which results in

lowering this effect.

MWNT are more mechanical robust than single wall nanotubes due to their multi layered structure.

The choice of nanotube type is often dependent upon the fabrication process selected. MWNTS are

the best choice for in situ chemical vapor deposition process whereas SWNT are used in post

processing emitter fabrication. [4]

11

2.2.2.1 Geometry of the CNT emitters

According to the FN model, the electron emission is dependent upon the strength of the

macroscopic electric field. Therefore, by controlling the macroscopic electric field the electron

emission can be controlled. A simple diode structure as shown in the figure can be used and these are

mostly used in research purposes due to their simplicity. The high voltage applied between the

cathode and the anode provides both the extraction and accelerating field. In here, in order to control

electron generation either the high voltage should be regulated which requires complicated power

supplies and also affects the energy of the electrons at the anode, or the cathode to anode distance

should be regulated for which a mechanical controller has to be integrated.

For most of the practical purposes, the triode emitter structure is used. In these the control of the

extraction field on the emitter surface is independent of the accelerating voltage. In triode geometry,

the strength of the extraction field is controlled by the voltage of the transmission grid placed above

CNT film. The voltage on the grid can be reduced by placing it very closely to the CNT film, typically

about few micrometers to few hundred micrometers. This allows the regulation of extraction current

to be done using simple electronics. [4], [6]

A B

Figure.06: A: Diode structure, B: Triode structure.

HV HV

V

Gate

Anode

+

Cathode -

-

12

2.3 Photocathode

A photocathode cathode is a negatively charged electrode coated with a photosensitive compound

placed in a vacuum. When electromagnetic waves of quantum energy greater than a particular

threshold value incident on the cathode, electrons are ejected in to the adjacent vacuum space via the

photoelectric effect. A photocathode is usually coated with alkali metals with very low work function

to ensure photoemission. These emitted electrons are then accelerated towards the anode through the

vacuum. However, photocathodes are seldom used as an emission mechanism in devices which

requires a bright and stable electron beam. This is referred to as ‘low quantum efficiency’.

Photocathodes are vastly used in light detecting devices such as photubes and photo multiplier tubes

where incident electromagnetic radiation is transferred into an electrical signal. The following figure

gives a schematic illustration of the function of a photomultiplier tube. [7]

Figure.07: Photocathode

13

CHAPTER 3

Electron Beam Maneuvering

Electrons are negatively charged particles and thus their motion is influenced by electrostatic and

magnetic fields. Depending on the type of application either one or both of these fields are used.

3.1 Force on electrons in an electric field

The relationship between the electric field strength and electric potential is given by;

Ø (4)

which says that electric field strength E(r) at a point r is equivalent to the negative gradient of the

electric potential Ø(r) at that point which is a scalar field at the same point.

Electric potential (Ø) is defined as the electrical potential energy (U) per unit positive test charge.

(5)

The force on a charged particle in an electric field at a point r (x, y, z) is given by

(6)

Here, q is the charge of the particle

for electrons q = -e = -1.60217657 × 10-19

coulombs

Ø(r) (7)

Force on an electron is in opposite direction to the direction of electric field lines in a particular point.

3.1.1 Uniform Electric Fields

In a uniform electric field, both the direction and magnitude of electric field is constant over the space

considered. The electric field formed between two conducting plates with a constant voltage

difference is approximated as a uniform electric field (ignoring the edge effects). The magnitude of

electric field is then given by;

(8)

where, is the potential difference between two plates and d is the plate separation.

Therefore, magnitude of the force on an electron between two parallel conducting plates with a

potential difference of and plate separation of d;

(9)

In the direction of negative plate to positive plate, i.e. towards the opposite direction of electric field

lines.

14

Figure.08: The force on an electron in a uniform electric field between two parallel conducting plates.

3.2 Force on electrons in a magnetic field

The force on an electron in a magnetic field (B) travelling with an instantaneous velocity (V) is

proportional to the magnetic field strength in that position, component of the velocity vector

perpendicular to the B-field and electric charge.

(10)

This is also known as Lorenz force. This force is always perpendicular to the velocity of the electrons

and magnetic field. If the magnetic field is constant/uniform, the electrons will trace a helical path

with the helical axis being perpendicular to the magnetic field and the speed of the electrons will

remain constant.

Figure.09: The path of an electron in a uniform magnetic field, the magnetic field is towards the paper

and the force exerted on the electron is a perpendicular direction to both velocity and magnetic field

as given by Fleming’ left hand rule.

15

3.3 Usage

3.3.1 Cathode ray tube (CRT)

Electrons generated inside a cathode ray tube (CRT), should be accelerated, focused and deflected to

make the electrons hit the correct phosphor pixel with required intensity and energy.

Typically, a high voltage (about 1 to 20 kV) is used to accelerate electrons to large speeds. Electrons

are focused either by using a focusing anode (electrostatic) or electromagnetic coils (magnetic). Then,

to deflect electrons both the electrostatic deflection plates and electromagnetic coils are used. In large

CRTs where the electron beam has to be deflected by large angles, magnetic deflection coils are used,

like in most televisions. In small CRTs such as is oscilloscopes where the angle of deflection required

is small, electrostatic deflection plates are used. [8]

Figure.10: A CRT which uses electric fields in both focusing and deflecting the electron beam.

Figure.11: A CRT that uses magnetic fields in focusing and deflecting electrons.

16

3.3.2 Electron Beam Sputtering/Evaporation

In electron beam evaporation, a beam of high energy electrons is collided with the anode and it

causes the anode material to move into gaseous phase. Upon precipitation, everything inside the

chamber will be coated with a thin layer of anode material.

Figure.12: Schematic diagram of an electron beam evaporation process.

Electrons generated from the electron source (thermionic or field emitter; thermionic in this case) are

accelerated into high velocities by a high voltage (20-25 kV). The accelerated electrons are made to

follow a circular trajectory by the influence of a magnetic field towards the ingot. The ingot (anode

material) and the filament are kept out of sight to each other to avoid chemical interaction between

ingot material and the filament. An electric field applied through the surface of the ingot is used to

steer the electrons over the surface of the ingot in order to facilitate uniform heating. [9]

17

3.3.3 Electron microscopes

Electrons are used to form images of the samples in electron microscopes as visible light in optical

microscopes. Magnetic lenses are used in electron microscopes to focus electrons onto the sample and

to focus electrons coming out of the sample to form an image similar to the role of convex glass

lenses in optical microscopes.

Figure.13: The schematic comparison of a magnetic lens and an optical lens.

When a current is passed through the copper coils in the magnetic lens, a magnetic field is created in

the bore along its axis. The magnetic field focuses the electron beam bringing the off axis rays to

focus. The focal length of the magnetic lens can be altered by changing the current through the coil.

[10]

18

CHAPTER 4

CURRENT DEVELOPMENTS

4.1 Application of carbon nanotube field emitters

As discussed in the section 2.2- ‘Field electron emission’ under Emission Mechanisms, carbon

nanotube field emitters have the largest potential to be used as an electron source in modern

applications. This is due to some attractive qualities accompanied with them such as;

High energy efficiency over thermionic emitters.

Device size can be reduced since a CNT field emitter is in the scale of only a few

micrometers. I.e. can be also used in handheld devices.

Very robust when compared with micro machined metal tip field emitters against destructive

arcing.

Can obtain very high current densities (>1 A/ ) which is essential in instruments like

SEM, TEM.

There are already some instruments which uses CNT field emitting cathodes such as in handheld

XRD machines and some possible applications are under development.

4.1.1 Field emission display

CRT displays has been largely overseen by technologies such as LCD, Plasma displays and OLED

due to its high power requirement and large size. But CRT displays are superior over these by

qualities such as high image contrast, brightness and large field of view. Since CNT field emitters are

very energy efficient and very small in size, attention was drawn to use CNT field emitter arrays in

place of thermionic emitter in CRT displays. In fact, the initial motive behind the developing of

CNT field emitters were to be used in field emitter displays. The idea is to dedicate a separate CNT

field emitter for each phosphor pixel rather than scanning a single beam of electrons throughout all

the pixels to produce an image. This is commonly referred as ‘pixel addressing’. [11]

19

Figure.14: Schematic comparison of a CRT tube display and Field Emission display.

The main advantage of field emission displays over CRT displays is the very low thickness. Since no

beam scanning is required, electron sources can be placed very close to phosphor pixels. Field emitter

displays can be made for very high resolution with high image quality and their size can be varied

from small handheld devices to large wall mounted public displays. However, this technology is still

under development and hasn’t hit the public market yet.

4.1.2 Miniature X-ray devices

X-ray tubes are used in variety of places such as X-ray imaging in medical, industrial and security

applications; spectroscopic analysis (X-ray fluorescence, X-ray photo electron fluorescence ); X-ray

diffraction etc. In modern applications such as portable X-ray fluorescence analysis systems and

space- deployed X-ray instruments appeal for miniature, energy efficient X-ray tubes which were

restricted by the inefficiency and large size of the thermionic emitter X-ray tubes. X-ray tubes with

field emitters will solve both of these problems. Field emitting X-ray tubes have some other

advantages over thermionic emitters as well. Since the cathode is not heated to high temperatures, the

out gassing of cathode material which contributes to vacuum degradation and contamination of the

target is inhibited. The cold nature of the cathode will prevent the thermal drift of the cathode

allowing finer and more stable electron focusing on to the target. Another main advantage is that the

elimination of the mechanical shutter in systems that require pulse X-ray. By simply switching the

field emitting cathode by controlling the voltage on it, X-ray pulses can be generated with desired

frequency.

20

However, until recently the field emitting cathodes couldn’t be implemented into X-ray tubes since

no metal emitter structure could survive the rough high-voltage operating conditions inside X-ray

tubes. Since CNT field emitters are mechanically, chemically and electrically more robust, after their

development they are being used in the development of miniature X-ray tubes.

Figure.15: A miniature field emitter X-ray tube with gated a MWNT-2mm

diameter cathode. [4]

4.1.3 Microsatellite Thruster Neutralizers

Electric thrusters are used in miniature spacecrafts which are based on ejection of high

velocity positively charged ions or colloidal particles accelerated in an electric field. The

charge of this positive ion beam should be neutralized in order to maintain the electric

neutrality on the space craft. This is done by directing an electron beam towards the outgoing

thrust. Since thermionic emitters are energy inefficient and requires worm up time, which

takes a longer ignition time they put severe constraints over electric thrusts being used as low

power thrusters. Furthermore, the high pressure environment present near the cathode

prevents a normal field emitter being used due to being damaged by sputtering of cations.

CNT field emitters has shown improved resistance to operating in poor vacuum conditions

and they are also very energy efficient. Therefore, CNT field emitting micro satellite thruster

neutralizers promises the production of low power electric thrusters. Experimental level

prototypes have been already made.

21

Figure.16: Small colloid thruster prototype (left) featuring a CNT field emission cathode for beam

neutralization (right), developed by Busek Co. Inc. [4]

Apart from these applications, other potential applications where the use of CNT field emitters are

possible;

Scanning and Tunneling Electron microscopes

Electron beam sputtering

Electron beam lithography

Vacuum microelectronics

Microwave amplifiers

22

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http://en.wikipedia.org/wiki/Thermionic_emission. [Last Accessed 28th November 2014]

[3]. Jones & Langmuir , (1927). ''. In: (ed), The Characteristics of Tungsten Filaments as

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