JPH0448509Y2 - - Google Patents

Description

[Detailed explanation of the idea]

[Industrial Application Field] The present invention relates to a pyroelectric infrared detector used in intruder alarms, automatic door switches, radiation thermometers, and the like. [Problems to be solved by conventional techniques and ideas] The voltage sensitivity R _v of a pyroelectric infrared detector (hereinafter referred to as a pyroelectric detector) is expressed by the following formula (Reference:
EHPutley, Semiconductors and Semimetals
(edited by Willadson) 5, Academic Press
(1970)). However, η: Emissivity, ω: Chopping angular frequency (=2πf, f: Chopping frequency (Hz), A: Element light-receiving electrode area (cm ² ), G: Thermal radiation (J/sec.)
K), dP _s /dT; pyroelectric coefficient of the element (C/cm ² .K), H; element heat capacity (J/K), τ _r ; time constant (sec) (=
H/G), τ _E ; electrical time constant (sec) (=C, R,
C: equivalent input capacitance (F) of the detector; R: equivalent input resistance (Ω) of the detector). Equation (1) expresses R _v as a function of the modulation angular frequency ω of the incident infrared rays. When equation (1) is plotted with respect to ω, a trapezoidal graph as shown in FIG. 2 is obtained. The left and right hypotenuses of the trapezoidal graph have slopes of -1 and +1, respectively. The bending point where the hypotenuse becomes gentle is determined by ω=1/τ _E or 1/τ _T. 1/
The smaller of τ _E and 1/τ _T becomes point a, and the other becomes point b. In actual pyroelectric infrared detectors, this can exist in either case, but in the case of human body detection.
When PZT (lead zirconate titanate) ceramics are used as pyroelectric elements, there are many cases where τ _E > τ _T. Therefore, point a is determined by 1/τ _E and point b is determined by 1/τ _r . Next, in the case of human body detectors, it is known that the frequency range in which the detector actually operates is broadly in the range of 0.01 to 10 Hz. This is because human movements occur in this frequency range. A structure to increase the sensitivity of pyroelectric detectors in this frequency range
Consider using equation (1). If we rewrite equation (1) and divide it into terms due to thermal factors and electrical factors, we get Since the electrical term can be ignored when τ _E 》τ _r , (1)′
The formula is approximately becomes. Needless to say, in order for R _v to become large, η, A, dP _s /dT must be large;
The emissivity η is limited to 0.95 for those commonly used. The photosensitive area A is set to a certain value when the application is limited. The pyroelectric coefficient dP _s /dT is a material constant of the pyroelectric material, and it should be as large as possible, but since the purpose of this invention is to optimize factors other than the material, Consider using a constant value. If we look at equation (2) with respect to ω, we get In this equation (2), when the infrared modulation frequency ω is sufficiently large, the element capacitance H becomes the dominant factor _Rv .
However, when ω is small, the magnitude of thermal radiation G cannot be ignored. It is precisely in this frequency range that a pyroelectric detector detects a human body. FIG. 3 is a graph showing equation (1) obtained by using G as a parameter and assuming constants other than G appropriately.
As constants η = 0.95, A = 0.02 (cm ² ), dP _s / dT = 4.4
× 10 ^-8 (C/cm ²・K), τ _E = 20sec, H = 2.9 × 10 ^-3
(J/K) was used. As can be seen from Figure 3, f=
The effect of the magnitude of G is small near 10Hz, but the effect becomes larger as the frequency becomes lower. From this result, it is clear that when the detection range is 0.01 to 10 Hz, G must be decreased in order to increase the sensitivity R _v . Thermal radioactivity G indicates the degree of ease with which heat is dissipated from pyroelectrons heated by infrared irradiation. If we consider the heat dissipation from the pyroelectric element in detail, it can be considered as follows. G=g ₁ +g ₂ +g ₃ +g ₄ ...(3) However, g ₁ ; Heat dissipation by conduction from the pyroelectric support, g ₂ ; Heat dissipation by conduction through the atmosphere surrounding the pyroelectric body, g ₃ : Heat dissipation by convection using the atmosphere surrounding the pyroelectric body; g ₄ : Infrared radiation from the surface of the pyroelectric body. Among the above four terms, it has been known that g ₁ makes a large contribution, and various structures have been proposed to reduce this. However, g ₂ , g ₃ , and g ₄ were rarely considered. The present invention investigated this point and came to discover important facts about _g2 . Conventional pyroelectric detector packages have typically been filled with dry nitrogen gas at atmospheric pressure. The purpose of this was to prevent the electrodes deposited on the pyroelectric element chip from oxidizing and to prevent the built-in high resistance from deteriorating due to humidity. However, from the viewpoint of heat dissipation from the above-mentioned pyroelectric element, the properties of this filled gas should have an important meaning. In view of the above points, the present invention aims to provide a pyroelectric detector that has increased sensitivity particularly in the low infrared frequency range by reducing the heat dissipation from the pyroelectric element through thermal conduction using the atmosphere as a medium. purpose. [Means and effects for solving the problems] The pyroelectric detector according to the present invention uses an inert gas having a lower thermal conductivity than nitrogen, specifically argon, xenon, or krypton, in the detector package. A single gas consisting of one type of gas, a mixed gas consisting of two or more types, or a mixed gas consisting of the single gas or mixed gas and nitrogen is sealed. The thermal conductivity values of several gases are shown below.

〔Example〕

Hereinafter, the present invention will be explained in detail using the illustrated embodiments. Figure 1 shows a cross-sectional view of a pyroelectric detector used in an intrusion alarm.
is an alumina substrate 4 fixed to the upper end of multiple lead pins 3 that penetrate through the TO-5 stem 2 and are fixed.
It is fixed thereon with an adhesive via a plurality of supports 5 made of glass or resin. Note that the contact area between the support 5 and the pyroelectric body 1 is kept to the necessary minimum, so that the amount of heat dissipated by heat conduction through the support 5 is minimized. 6 is a stem 2 surrounding the pyroelectric element chip 1;
It is a cap that is fixed to the
A window material 7 on which a 7 μm cut-on bandpass filter that transmits only infrared light emitted by the human body is deposited is attached. In the pyroelectric detector having the above structure, the signal output of the detector is first measured without the cap 6 attached, and then after the cap 6 is attached, various gases are injected into the space 8 inside the cap 6. The signal output was compared with that when encapsulated. The measuring jumping frequencies were set at two points: 0.1 Hz and 1 Hz. Filled gases include argon, xenon, krypton, argon + xenon (1:1) mixed gas, nitrogen + xenon (1+
1) and six types of nitrogen for comparison. The results are shown in Table 1 below.

【table】

[Effect of idea]

As described above, the pyroelectric detector according to the present invention has important advantages in that the sensitivity in the low infrared frequency range is improved and malfunctions due to environmental temperature fluctuations are reduced.

[Brief explanation of the drawing]

Fig. 1 is a cross-sectional view of one embodiment of the pyroelectric detector according to the present invention, Fig. 2 is a graph showing the voltage sensitivity of the pyroelectric detector as a function of the modulation frequency of incident infrared rays, and Fig. 3 is the above-described graph. It is a graph when voltage sensitivity is expressed by using thermal radiation as a parameter and assuming other constants appropriately. 1...Pyroelectric element chip, 2...TO-5 stem, 3...Lead pin, 4...Alumina substrate, 5
...Support, 6...Cap, 7...Window material, 8...
…space.

Claims (1)

[Scope of utility model registration request] In a pyroelectric infrared detector comprising a pyroelectric element enclosed in a hollow package, the space within the package is a single gas consisting of one of argon, xenon, and krypton, or a mixed gas consisting of two or more types, or the single gas. A pyroelectric infrared detector characterized by being filled with a gas or a mixed gas consisting of a mixed gas and nitrogen.