Dielectric property of chromium oxide nanoparticles

Dielectric Properties of Cr2O3 Nanoparticles

BY:

Gaurav Kumar Yogesh

Reg. No. CUPB/M.Sc./SBAS/PMS/2013-14/01

Supervisor: Dr. Kamlesh Yadav

(Assistant Professor)

Centre For Physical And Mathematical Sciences

Central university of Punjab, Bathinda

M.sc. physics (weekly seminar)

Content

Introduction

Terminology

Types of polarisation of dielectric

Complex permittivity

Dielectric loss

Dielectric property of Cr2O3 nanoparticles

Applications

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Introduction

Dielectric material is a substance that is a poor conductor of electricity but efficient

supporter of electrostatics field.

Dielectric material has high polarizibility.

Dielectric material is the electrically insulator and polarise in the applied field.

The molecule slightly displaced from the equilibrium and creates an internal field

which decrease the overall field inside the material.

Dielectric properties concern with storage and dissipation of electric and magnetic

energy in material.

Dielectric properties studied in the electronic, optics and solid state physics.

Dielectrics may be sub-divided into two groups :

Non-Polar : which does not possess the permanent dipole moment.

Polar: In which the molecules or atoms possess a permanent dipole moment

which is ordinarily randomly oriented, but which become more or less oriented by

the application of an external electric field.

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Contd.

Polarisation is directly relating to the induced electric field.

P = χeЄo E

Electrical susceptibility of material related to the electrical permittivity

χe = Єr ˗ 1

χe = 0, in case of vacuum

The electric displacement is related to the

polarisation vector as:.

D = Єo E + P = (1 + χe ) Єo E ,

D = Єo Єr E

Where E is average electric field inside the

material.

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Fig. Polarisation of dielectric

in the external field

Terminology

χe is the electric susceptibility of the

material, which measure how easily a

material can polarise.

Єo is the electrical permittivity of

free space.

Є is the electrical permittivity of the

medium

Єr is the relative permittivity of the

medium

P is the polarisation of the material.

E is the average electrical field of the

material.

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Fig. Polarisation of dielectric

inside the external field

Types of polarisation of dielectric

The type of polarization on a microscopic scale is determined by the material.

Most materials exhibit polarization only in the presence of an external field. A

few however show permanent polarization:

Ferroelectric: crystals exhibit spontaneous permanent polarization.

Electrets : become permanently polarized if allowed to solidify in the

presence of a strong electric field.

The type of polarization may be additionally subdivided into the following

categories :

Electronic: a displacement of the electronic cloud w.r.t the nucleus

Ionic: separation of +ve and -ve ions in the crystal.

Orientational: alignment of permanent dipoles (molecules).

Space-charge: free electrons are present, but are prevented from moving by

barriers such as grain boundaries - the electrons "pile up".

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Electronic polarisation

ionic

polarisation

Orientational polarisation

Space charge polarisation

Contd....

Complex Permittivity

Normal material response to the external field depends on the frequency of

applied field.

The frequency dependence material reflects that polarisation is not

instantaneous.

For that reason permittivity is the function of the angular frequency or complex

quantity.

Do e-iωt = є(ω) Eo e-iωt

Response of material at low frequency limit is called the static frequency limit.

At high frequency limit, the complex permittivity commonly referred as the є͚ , at the plasma frequency and above the dielectric behave as the ideal metals, with

electron gas behaviour.

As the frequency increases, measurable phase difference is introduced between

the D & E.

є(ω) = Do / Eo = є e-iδ

δ is the loss angle

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Dielectric Loss

Loss of energy that goes into heating a dielectric material in a varying electricfield.

Capacitor incorporated in an alternating-current circuit is alternately charged anddischarged each half cycle.

During the alternation of polarity of the plates, the charges must be displacedthrough the dielectric first in one direction and then in the other, and overcomingthe opposition that they encounter leads to a production of heat through dielectricloss.

Fig. Dielectric loss in dielectric 9

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Loss Tangent

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It is defined as the ratio of ratio of the imaginary part of the dielectric constant

to the real part.

D denotes dissipation factor.

Q is quality factor.

“quality factor or Q-factor” is used

with respect to an electronic microwave

material.

which is the reciprocal of the loss

tangent. For very low loss materials.

Fig. Loss tangent of dielectric

Dielectric Properties of Cr2O3 Nanoparticls

Capacitance of capacitor can be altered by varying the dielectric properties.

Cr2O3 can also be used as an efficient gate-dielectric-material because it shows

wide band gap, high melting temperature and high oxidation resistance, which is

the essential requirement for a material to be used as gate-oxide materials.

Frequency dependent dielectric constant Єr of Cr2O3 nanoparticles is wide

range of frequency region (100 Hz – 30 MHz)

Value of Єr decreases with increase in frequency.

Higher value of Єr at lower frequency because of

the all kinds polarisation contributes such as dipolar,

Ionic, electronic and space charge.

Fig. Frequency dependent dielectric

constant of Cr2O3 nanoparticles

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Applications of Dielectric Loss

Industrial coatings such as parylen provide a dielectric barrier between the

substrate and its environment.

Mineral oil is used extensively inside electrical transformers as a fluid dielectric

and to assist in cooling.

For drying lumber and other fibrous materials, for preheating plastics before

molding, and for fast jelling and drying of foam rubber.

Piezoelectric materials are another class of very useful dielectrics.

Dielectric fluids with higher dielectric constants, such as electrical grade castor

oil, are often used in high voltage capacitors to help prevent corona discharge

and increase capacitance.

THANK YOU

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