Pyroelectric and ferroelectric materials

6.9.4 Pyroelectric and ferroelectric materials

Some materials, associated with low crystal symmetry, are observed to acquire an electric charge when heated; this is known as pyroelectricity. Because of the low symmetry, the centre of gravity of the positive and negative charges in the unit cell are separated producing a permanent dipole moment. Moreover, alignment of individual dipoles leads to an overall dipole moment which is non-zero for the crystal. Pyroelectric materials are used as detectors of electromagnetic radiation in a wide band from ultraviolet to microwave, in radiometers and in thermometers sensitive to changes of temperature as

small as 6 ð 10 6 °

C. Pyroelectric TV camera tubes

have also been developed for long-wavelength infrared imaging and are useful in providing visibility through

Figure 6.36 Hysteresis loop for ferroelectric materials, smoke. Typical materials are strontium barium niobate

showing the influence of electric field E on polarization P .

The physical properties of materials 195

6.10 Optical properties

the visible spectrum. Colouring of glasses and ceram- ics is produced by addition of transition metal impu-

6.10.1 Reflection, absorption and

rities which have unfilled d-shells. The photons easily

transmission effects

interact with these ions and are absorbed; Cr 3C gives 2C The optical properties of a material are related to the 2C green, Mn yellow and Co blue-violet colouring.

interaction of the material with electromagnetic radi- In photochromic sunglasses the energy of light ation, particularly visible light. The electromagnetic

quanta is used to produce changes in the ionic structure C spectrum is shown in Figure 5.1 from which it can be

of the glass. The glass contains silver ⊲Ag ⊳ ions as a

dopant which are trapped in the disordered glass net- waves down to 10 14 m for -rays and the correspond-

4 m for radio

work of silicon and oxygen ions: these are excited by ing photon energies vary from 10 10 eV to 10 8 eV. high-energy quanta (photons) and change to metallic

Photons incident on a material may be reflected, silver, causing the glass to darken (i.e. light energy is absorbed). With a reduction in light intensity, the silver

absorbed or transmitted. Whether a photon is absorbed atoms re-ionize. These processes take a small period of or transmitted by a material depends on the energy gap

time relying on absorption and non-absorption of light. between the valency and conduction bands and the

energy of the photon. The band structure for metals has no gap and so photons of almost any energy are

6.10.2 Optical fibres

absorbed by exciting electrons from the valency band Modern communication systems make use of the abil- into a higher energy level in the conduction band.

ity of optical fibres to transmit light signals over large Metals are thus opaque to all electromagnetic radiation

distances. Optical guidance by a fibre is produced (see from radio waves, through the infrared, the visible to

Figure 6.37) if a core fibre of refractive index n 1 is the ultraviolet, but are transparent to high-energy X-

surrounded by a cladding of slightly lower index n 2 rays and -rays. Much of the absorbed radiation is

such that total internal reflection occurs confining the reemitted as radiation of the same wavelength (i.e.

rays to the core; typically the core is about 100 µ m reflected). Metals are both opaque and reflective and 2 and n 1 n 2 ³ 10 . With such a simple optical fibre,

it is the wavelength distribution of the reflected light, interference occurs between different modes leading which we see, that determines the colour of the metal.

to a smearing of the signals. Later designs use a Thus copper and gold reflect only a certain range of

core in which the refractive index is graded, parabol- wavelengths and absorb the remaining photons, i.e.

ically, between the core axis and the interface with copper reflects the longer-wavelength red light and

the cladding. This design enables modulated signals to absorbs the shorter-wavelength blue. Aluminium and

maintain their coherency. In vitreous silica, the refrac- silver are highly reflective over the complete range of

tive index can be modified by additions of dopants such the visible spectrum and appear silvery.

as P 2 O 5 , GeO 2 which raise it and B 2 O 5 and F which Because of the gaps in their band structure non-

lower it. Cables are sheathed to give strength and envi- metals may be transparent. Thus if the photons have

ronmental protection; PE and PVC are commonly used insufficient energy to excite electrons in the mate-

for limited fire-hazard conditions. rial to a higher energy level, they may be transmitted

6.10.3 Lasers

rather than absorbed and the material is transparent. In high-purity ceramics and polymers, the energy gap

A laser (Light Amplification by Stimulated Emission is large and these materials are transparent to visible

of Radiation) is a powerful source of coherent light light. In semiconductors, electrons can be excited into

(i.e. monochromatic and all in phase). The original acceptor levels or out of donor levels and phonons hav-

laser material, still used, is a single crystal rod of ing sufficient energy to produce these transitions will

ruby, i.e. Al 2 O 3 containing dopant Cr 3C ions in solid

be absorbed. Semiconductors are therefore opaque to solution. Nowadays, lasers can be solid, liquid or

gaseous materials and ceramics, glasses and semicon- structure is influenced by crystallinity and hence mate-

short wavelengths and transparent to long. 1 The band

ductors. In all cases, electrons of the laser material rials such as glasses and polymers may be transparent

are excited into a higher energy state by some suitable in the amorphous state but opaque when crystalline.

stimulus (see Figure 6.38). In a device this is produced High-purity non-metallics such as glasses, diamond

by the photons from a flash tube, to give an intense or sapphire ⊲Al 2 O 3 ⊳ are colourless but are changed by

impurities. For example, small additions of Cr 3C ions

⊲ Cr 2 O 3 ⊳ to Al 2 O 3 produces a ruby colour by introduc- ing impurity levels within the band-gap of sapphire which give rise to absorption of specific wavelengths in

1 Figure 5.37b shows a dislocation source in the interior of a silicon crystal observed using infrared light.

Figure 6.37 Optical guidance in a multimode fibre .

196 Modern Physical Metallurgy and Materials Engineering Thus, translucent alumina is used for the arc tube of

high-pressure sodium lamps; a grain size of 25 µ m gives the best balance of translucency and mechanical strength.

Ceramics are also available to transmit electromag- netic radiation with wavelengths which lie below or above the visible range of 400–700 nm (e.g. infrared, microwave, radar, etc.). Typical candidate materials for development include vitreous silica, cordierite glass- ceramics and alumina.

6.10.5 Electro-optic ceramics

Figure 6.38 Schematic diagram of a laser

Certain special ceramics combine electrical and optical properties in a unique manner. Lead lanthanum zirco-

source of light surrounding the rod to concentrate the nium titanate, known as PLZT, is a highly-transparent energy into the laser rod. Alternatively an electrical

ceramic which becomes optically birefringent when discharge in a gas is used. The ends of the laser rod

electrically charged. This phenomenon is utilized as are polished flat and parallel and then silvered such

a switching mechanism in arc welding goggles, giv- that one end is totally-reflecting and the other partially-

ing protection against flash blindness. The PLZT plate transmitting.

is located between two ‘crossed’ sheets of polarizing In the ruby laser, a xenon flash lamp excites the

3C electrons of the Cr material. A small impressed d.c. voltage on the PLZT ions into higher energy states. plate causes it to split the incident light into two rays Some of these excited electrons decay back to their

vibrating in different planes. One of these rays can ground state directly and are not involved in the

pass through the inner polar sheet and enter the eye. A laser process. Other electrons decay into a metastable

sudden flash of light will activate photodiodes in the intermediate state before further emission returns them

goggles, reduce the impressed voltage and cause rapid to the ground state. Some of the electrons in the

darkening of the goggles.

metastable state will be spontaneously emitted after a short (¾ms) rest period. Some of the resultant photons are retained in the rod because of the reflectivity of

Further reading

the silvered ends and stimulate the release of other electrons from the metastable state. Thus one photon

Anderson, J. C., Leaver, K. D., Rawlins, R. D. and Alexan- releases another such that an avalanche of emissions

der, J. M. (1990). Materials Science. Chapman and Hall, is triggered all in phase with the triggering photon. London. The intensity of the light increases as more emissions Braithwaite, N. and Weaver, G. (Eds) (1990). Open Univer-

sity Materials in Action Series . Butterworths, London. are stimulated until a very high intense, coherent,

Cullity, B. D. (1972). Introduction to Magnetic Materials. collimated ‘burst’ of light is transmitted through the

Addison-Wesley, Wokingham. partially-silvered end lasting a few nanoseconds and

Hume-Rothery, W. and Coles, B. R. (1946, 1969). Atomic with considerable intensity.

Theory for Students of Metallurgy . Institute of Metals, London.