Ferroelectric thermometry and pyrometry by active operation

Details Ferroelectric thermometry and pyrometry by active operation Ferroelectric thermometry and pyrometry by active operation

1999-12-03

2002-01-15

Hannaher, Constantine (Department: 2878)

Radiant energy

Invisible radiant energy responsive electric signalling

Infrared responsive

C250S338200

Reexamination Certificate

active

06339221

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to pyroelectric sensors and, more particularly, to a method of determining the polarization state of a pyroelectric element by applying an AC signal to the element and calculating the hysteresis loop switching energy of the element where the spontaneous polarization reversal of the element is used as a time-varying function and the charge integration is directly proportional to the temperature of the element.
2. Discussion of the Related Art
A certain class of sensors make use of ferroelectric materials, and their pyroelectric effect for detection of temperature change. Sensors of this type have a wide range of applications, such as imaging in low visibility conditions, for example, poor weather conditions, night vision, etc. A ferroelectric material is a dielectric material that has a temperature dependent spontaneous electrical polarization in the absence of an externally applied electric field which can change state with the application of a critical field, where the polarization magnitude and direction within the ferroelectric material is identifiable by a hysteresis loop. The orientation of the polarization of the material can be changed by applying a reversing external electric field to the material. The electric dipoles within the material, that identify the orientation of the polarization, change when the external field is applied and in a proper circuit layout produce a hysteresis loop. Since spontaneous polarization is generally temperature dependent, ferroelectric materials can employ the pyroelectric effect for temperature detection purposes.
Any area of the hysteresis loop, either the entire saturated hysteresis area or merely a region of operation anywhere within the full loop, is representative of the switching energy required to change the polarization state of some or all the dipoles which make up the atomic lattice structure of the material at a given temperature for the specific state of excitation. Any change in radiation incident on the ferroelectric material, if absorbed, changes the temperature, and thus changes the associated loop area.
FIG. 1
shows two charge versus voltage hysteresis loops for a particular ferroelectric material at a first temperature T
1
and a second temperature T
2
. If plotted independent of physical dimensions, the magnitude of an externally applied alternating electric field is given on the horizontal axis and polarization, in charge density, is given on the vertical axis. The area of the charge versus voltage hysteresis loop of a ferroelectric material has dimensions of energy, and the loop area is a direct function of its temperature. The magnitude of the polarization changes with a change in the temperature of the ferroelectric material for a given electric field. A careful review of the two hysteresis loops in
FIG. 1
will show that for the two different temperatures T
1
and T
2
, the area within the loop is different. Consequently, an electrical measurement of the change in area anywhere within the major loop is an electrical signal corresponding to the change of the temperature of the material, and thus of the incident infrared radiation. The effect is of a dynamic nature due to the switching between polarization states of the pyroelectric material.
The spontaneous polarization P
s
of a ferroelectric medium is a function of temperature T:
P
s
=P
s
(T)  (1)
The pyroelectric coefficient p,
p=dP
s
/dT,  (2)
is a temperature-rate of electric charge effect, that is often used as a pyroelectric quality factor for judging a particular ferroelectric material. The pyroelectric coefficient p is particularly sensitive in the vicinity of a Curie temperature, which marks a phase transition in the ferroelectric material. However, following a discreet change in temperature T, any external evidence of P
s
, by its associated electric field E
s
, is only a transient phenomenon because of the unavoidable thermally generated free-charge, which rapidly neutralizes the E
s
. To overcome the transient nature of the measurable external evidence of P
s
, all ferroelectric pyrometry to date has been based on the concept of forcing the temperature to become a function of time as:
T=T(t).  (3)
Therefore,
P
s
=P
s
{T(t)}.  (4)
Generally, the temperature T is converted to a time-varying function by mechanically shuttering a window between the heat energy source and the ferroelectric pyroelectric sensor. Unfortunately, the shuttering action rejects essentially one-half of the incident thermal power, which seriously decreases the signal-to-noise ratio. Furthermore, the need for a shutter between the sensor and the thermal source distinctly limits its use to the detection of only radiated heat, particularly infrared radiation.
A ferroelectric unit cell possesses two stable spontaneous polarization states P
s
. This bi-modal condition exists while the selective material remains in a specific temperature range. The spontaneously separated ± bound-charge forms a dipole-moment. This dipole moment can be reversed by an opposing E-field if it is of sufficient magnitude. By locking in a residual orientation, the dipole memorizes the polarity of the most recently applied polarizing-reversing external E-field. This phenomenon is the operating principal of the ferroelectric random access memory (FRAM).
Coulomb's Law suggests that the dipole-moment can be seen to represent stored potential energy, within the static, unexcited unit cell. At the instant of dipole reversal, and the immediate removal of the externally applied E-field, a spontaneous electric field E
s
would instantly appear across two hypothetical parallel plate electrodes positioned at the two surfaces of the cell that are oriented orthogonal to the dipole-moment. However, in a realizable, practical material at non-cryogenic temperatures, the omnipresent thermally-generated free charge will automatically migrate toward the bound charges, and reside in a posture so as to effect total neutralization of the externally observable E
s
field. The E
s
field must experience an exponential decay to zero, as determined by the resistivity times permittivity (&rgr;&egr;) time-constant of the material that can be expressed as:
E
s
=[P
s
&egr;]exp(−
t/&rgr;&egr;
).  (6)
Therefore, it is clear that P
s
in a ferroelectric capacitative structure, generally cannot be directly measured in a static manner, because E
s
as observed at the cell boundaries is a transient phenomenon. Consequently, to try to overcome the time-constant restriction, clever dynamic methods must be employed to effect a reliable measurement of P
s
.
When a ferroelectric material, such as a crystal, ceramic, film, etc., consisting of numerous randomly oriented domains, each consisting of many such self-polarized unit cells, is excited with a time-dependent alternating electric field E, a time-independent display of P
s
vs. E defines a directional hysteresis loop. A necessary condition for there to be any external evidence of P
s
is that the period of the alternating excitation, 1/f must be short compared to the &rgr;&egr; time constant as:
(1/ƒ)<<&rgr;&egr;  (7)
or
ƒ>>(1/&rgr;&egr;),  (8)
to insure that free-charge is denied the time necessary to neutralize the rapidly reversing bound-charge, and the P
s
values remain essentially undiminished from their theoretical values. The area of the P
s
vs. E loop has the units of energy density, i.e.,
 w=Total Energy W/Volume,  (9)
or in other words, energy per unit volume.
Since P
s
=P
s
(T), the area of the time independent loop display of P
s
vs. E is a direct measure of the temperature of the material. The electric displacement D in a ferroelectric material can be expressed as:
D=&egr;E±P
s
=&egr;
0
E+P
elastic
±P
s
,  (10)
where &egr;
0
is the permittivity of free space.

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