Sensor measures rapid temperature changes in fluid streams. Sensor withstands contacts with various corrosive fluids, high fluid-flow rates, and turbulences caused by rapid changes in flow rates. Capacitor is part of resonant bridge circuit which produces ac voltage that is proportional to temperature

Gatewood, J. R.

Maserjian, J.

English

NASA Technical Reports Server

R 
August 1974	
-	 B74-10100 
NASA TECH BRIEF 
•	 NASA Pasadena Office 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Thin-Film Temperature Sensor 
E
A thin-film capacitor sensor has been developed for 
measuring the rapid temperature changes in fluid streams. 
The sensor withstands contacts with various corrosive 
fluids, high fluid-flow rates, and the turbulences caused 
by rapid changes in flow rates. 
The heart of the sensor is a temperature-dependent 
thin-film capacitor. Its temperature coefficient a is 
defined by
I dC 
C dT 
-
	 (1) 
where C is the capacitance and T is the temperature in 
degrees Kelvin. Thin dielectric films are generally known 
to have a large a.
The capacitor is part of a resonant bridge circuit 
(Figure 1), which produces an ac voltage that is 
proportional to temperature. The output voltage V for a 
given temperature change AT is described by 
V = VaQ AT	 (2) 
where V0 is the initial amplitude of the harmonic voltage 
applied across the capacitive element. In Figure 1, the 
capacitive element is represented in the dashed box by 
its parallel equivalent capacitance C and resistance R. 
If low-loss coils L are used, the circuit quality factor 
Q is determined by the capacitive elements, namely 
Q = wRC	 (3) 
To Differential Low
Noise Amplifier
Thin-Film 
Capacitive 
Bolometers 
Figure 1. Resonant Bridge Circuit 
.
	
(continued overleaf) 
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https://ntrs.nasa.gov/search.jsp?R=19740000100 2020-03-17T14:05:06+00:00Z
AI 203
 Coating 
Top Contact Al Strip 
Approximately
400.0 nm
Epoxy Seal
- Soldered Lead 
- Ag Dot 
- Si02 Insulation 
Ta205
 Anodic Film 
Ta Foil 
Bottom Contact Al Dot
Thermoplastic Film 
Zr02 40.0-nm Layer	 Polymer on Entire 
Surface 
Al 203 5.0-nm Layers 
Figure 2. Temperature Sensor Structure 
In this case, Q remains nearly constant for a wide range 
of frequencies w, because R is inversely proportional to 
co. The important parameters, therefore, that determine 
the temperature sensitivity of the sensor are a and Q, as 
indicated in equation 2. The resonant bridge circuit 
produces an output voltage larger by the factor Q than 
the voltage from a simple capacitance bridge without 
coils. 
The sensor consists of a tantalum foil supporting a 
thin-film capacitive element, as shown in Figure 2. 
The foil is 99.8-percent-pure tantalum approximately 
0.02 5 mm (0.001 in.) thick. The capacitive element is 
formed by the vapor deposition of successive layers of 
aluminum, aluminum oxide (Al 2 0 3), zirconium oxide 
(Zr0 2 ), silicon dioxide (Si0 2 ), and aluminum. The 
aluminum strips form the capacitor electrodes, and the 
zirconium oxide with the aluminum oxide layers form 
the dielectric. In addition, a circular window approxi-
mately 0.38 mm (0.015 in.) in diameter is etched 
through the tantalum foil to the anodic film (Ta 2 03). 
The entire sensor is coated with a thermoplastic polymer, 
which provides chemical protection and electrical in-
sulation.
Note: 
Requests for further information may be directed to: 
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: TSP74-10100 
Patent status: 
This invention has been patented by NASA (U.S. 
Patent No. 3,676,754). Inquiries concerning nonex-
clusive or exclusive license for its commercial develop-
ment should be addressed to: 
Patent Counsel 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Source: Joseph Maserjian and
John R. Gatewood of 
Caltech/JPL
under contract to
NASA Pasadena Office
(NPO-1 1775) 
B74-10100	 Category 01, 03,04,08
40


Thin-film temperature sensor

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