Archive for July, 2011
This article is provided by my student Mr. Rajinder Singh Virk for the benefit of others.
This test is conducted in day-2. This testing forms an integrated and indispensable part of SSB testing. It gives a complete overview of ones thoughts and his thinking capabilities, it gives a clear view to evaluator about ones personality and the qualities being possessed by the individual.
In this test, all the candidates who have been screened in for stage-2 are called in a hall and a dozier is given to them.
Following are the tests which are done with them:
1.Picture perception test (PPT): More >
Note: This article is provided by my student Mr. Rajinder Singh Virk for the benefit of others. He has recently passed through the selection process. Thanks and all the best to him. I will present this article as given by him :
Day- 1: Screening:
It is a very evaluative and comprehensive test designed to judge your creativity and observational skills. It is the first test and forms the testing stage-1.
In this test, a picture which is blurred enough is displayed and we have to frame a story on it, depending upon your thoughts and observation for it. In this test one gets around 30 seconds to see the picture and 30 minutes to write a story on it. After the story has been written, 10 minutes would be allotted to each candidate by the evaluating officers for discussion and narration of the stories and concluding to a common group story after discussion. After a healthy discussion, a team leader chosen by candidates themselves would narrate the group story.
Points to be kept in mind: More >
Ferroelectric materials and piezoelectric materials can be compared with the help of following points :
a) In piezoelectricity, the crystal is polarized by the application of an external stress, whereas in ferroelectricity the source of polarization is the dipole interaction energy itself.
b) Both the phenomena occur in non-centrosymmetrical crystals which are 20 in number. Piezoelectricity occurs in all the 20 crystals, whereas ferroelectricity only in 10, namely, those which provide a favourable axis of a polarity.
c) All ferroelectrics are therefore piezoelectric, but all piezoelectrics are not ferroelectric. For example: tourmaline is piezoelectric, but not ferroelectric.
d) The piezoelectric coefficient is the ratio of the set-up charge to the stress applied along crystallographic axis. The ferroelectrics have very large piezoelectric coefficients.
ferroelectric materials have following applications:
(a) As Capacitors: Ferroelectric materials can be used to make capacitors.
(b) As Varactors: The value of dielectric constant for ferroelectric materials can be changed depending upon the change in the magnitude and directions of electric field. If it happens, then they can be made to behave like varactors.
(c) As Radio frequency power amplifiers: Change in electric field changes dielectric constant and this is then changes the power supply. Change in the field modulate the frequency and it means that the ferroelectric capacitors can be used as radio frequency power amplifiers.
(d) As computer memories:.Very thin ferroelectric crystals can be
used for crystal memories.
Ferroelectric materials can be classified on the basis of their chemical composition and structure as :
(a) Rochelle salt: The first solid to show ferroelectric properties is Rochelle salt. Rochelle salt is the sodium-potassium salt of tartaric acid (NaKC4H4O6.4H2O). It has the property of being ferroelectric only in the temperature range between -18 C and 23 C, which means it has two transition temperatures.
(b) Dihydrogen phosphates and arsenates: Busch and Scherier discovered ferroelectric properties in KH2PO4 in 1935 which is an example of the dihydrogen phosphate and arsenate of the alkali metals. The spontaneous polarization of this material a function of temperature.
In this case, there is only one Curie temperature = 123 K
(c) Oxygen Octahedron group (Barium Titanate): The best known ferroelectric material is barium titanate (BaTiO3) and it is a representative of the oxygen octahedron group of ferroelectric
material. The reason for this name is that the above the Curie temperature that is at 120 C, BaTiO3 corresponds to the cubic structure.
When the temperature is decreased through the critical temperature of 120 C, the material becomes spontaneously polarized and at the same time the structure changes.
BaTiO3 has two more transition temperatures; one at 5 C, and other at -80 C. There will be a change in the crystal structure of the material associated with there ferroelectric transitions.
In the last article, I have discussed about ferroelectricity and hysteriesis loop. Today we will discuss about the reason of hysteresis loop.
Ferroelectric crystals have regions with uniform polarization called ferroelectric domains.
A single crystal of ferroelectric material may contain number of domains in each of which the polarization and the crystal axes may have different directions. Domains are separated by interfaces called domain walls.
A single domain can be obtained by domain wall motion made possible by the application of electric field. A very strong field could lead to the reversal of the polarization in the domain and it is known as domain switching.
When electric field is applied, then the domain for which the polarization points along the direction of the applied field grow at the expense of other domains for which the polarization points in other directions.
In ferroelectric materials, the polarization P does not vary linearly with electric field E.
Hysteresis curve: The plot of P versus E in which the material is polarized in one direction and then in opposite direction is called the hysteresis curve of the specimen.
When a piece of ferroelectric material initially unpolarized, is subjected to a gradually increasing electric field, the polarization P varies with E.
Remanent polarization or retentivity or remanence: More >
Last time I have discussed about the piezoelectricity. Let us discuss about the applications of piezoelectric effect.
(a) Electro-mechanical transducers.
Piezoelectric effect is used to convert electrical energy into mechanical energy and vice-versa, that is the piezoelectric substances are used as electro-mechanical transducers.
If an electric signal is applied to one end of a quartz rod, the variations in strain generated in the rod in consequence of the effect propagate down the rod, resulting in a mechanical wave, or an acoustic wave on reaching the other end of the rod.
(b) Highly stable oscillators: More >
Definition: When certain crystals become electrically polarized that is electric charges appear on their surface when stressed then this phenomenon is called the piezoelectric effect, or piezoelectricity and the crystals as the piezoelectric crystals.
Examples: Quartz, rochelle salt, tourmaline are piezoelectric substances.
The inverse effect-that these crystals become strained when polarized (i.e. place in an electric field) has also been observed.
Piezoelectric strains are very small, and the corresponding electric fields are very large.
Example. In Quartz a field of 100 V/cm produces a strain of the order of 10-7.
Origin of piezoelectric effect: More >
In the earlier article, I have explained the electronic polarization. Today we will discuss the ionic polarization.
Ionic polarization. As the name suggests, ionic polarization occurs in ionic materials. It occurs when an electric field is applied to an ionic material then cations and anions get displaced in opposite directions giving rise to a net dipole moment.
Example : Polyatomic gases
In absence of electric field E, the distance between the ions is d but in presence of electric field, distance between the ions increases
The dipole moment p for each ion pair is equal to the product of charge on each ion and relative displacement.
That is p = qd
Thus ionic polarization is given as
Pe =n αiE
where αi is constant of proportionality known as ionic polarizability constant.
This polarization is independent of temperature.