CN1384914A - Self-compensated ceramic strain gage for use at high temperature - Google Patents

Abstract

A self-compensating strain gauge sensor having a substantially zero temperature coefficient of resistance (TCR) comprising a broadband semiconductor and a compensating metal acting as a series resistor, based on the resistivities of the semiconductor and metal and the temperature range over which the sensor operates is disclosed , determine the dimensions of the semiconductor and metal so that the TCR is zero.

Description

Self-compensating ceramic strain gauges for use at high temperatures

background of the invention

1. Field of Invention

The invention relates to a thin film strain gauge.

2. Description of related fields

It is often necessary to accurately measure static and dynamic strain at high temperatures to determine the instability and life of various structural systems, especially advanced aerospace propulsion systems. Conventional strain gauges are typically applied to static and rotating assemblies for this measurement, and their use is often limited due to their intrusive nature, severe temperature limitations, and difficulty in adhering.

Thin-film strain sensors are especially suitable for use in gas turbine environments because they do not adversely affect gas flow over component surfaces and require no adhesives or cements for bonding. Typically, thin film strain gauges are deposited directly on the surface of the component using RF sputtering or other known thin film deposition techniques, resulting in direct contact with the deformed surface. In general, the piezoresistive response of a strain gage, or instrument sensitivity (g), is the finite change in resistance of the sensing element when strained, which can be determined by (a) the dimensional change of the active strain element and/or (b) the active strain Caused by changes in the resistivity (ρ) of the element. Additionally, active strain elements used in high temperature static strain gages must exhibit low temperature coefficient of resistance (TCR) and drift rate (DR) such that the thermally induced apparent strain is negligible compared to the actual mechanically applied strain .

One material of choice for high temperature thin film strain gauges is a broadband semiconductor such as indium tin oxide (ITO) because of its excellent electrical and chemical stability at high temperatures and its greater instrument sensitivity. When used alone, it is generally limited by the inherently high temperature coefficient of resistance (TCR) of many semiconductors. However, using a metal such as platinum as a thin-film resistor in series with an active indium tin oxide (ITO) strain element, as described in this paper, lowers the resistance temperature of a self-compensating ITO strain sensor Coefficient (TCR).

However, the proper combination of materials, patterns, and dimensions to produce a strain gauge with a predetermined temperature coefficient of resistance is essentially an empirical method, ie, trial and error.

With the present invention, if the operating temperature range of the sensor and the resistivity of the material at the working temperature and the reference temperature are known, the temperature coefficient of resistance of the high temperature strain gauge can be automatically determined.

In general terms, the present invention includes a self-compensating strain gauge sensor having an automatically determined temperature coefficient of resistance, including a substantially zero temperature coefficient of resistance. The sensor includes a broadband semiconductor deposited on a substrate. A metal deposited on the substrate and electrically connected to the semiconductor acts as a series resistor, the length, width and thickness of the semiconductor and metal being determined according to their resistivities at the selected operating and reference temperatures selected, and the temperature coefficient of resistance of the sensor can be automatically determined.

The semiconductor may be selected from silicon carbide, aluminum nitride, zinc oxide, gallium nitride, indium nitride, scandium nitride, titanium nitride, chromium nitride, zirconium nitride, boron carbide, diamond, titanium carbide, tantalum carbide , zirconium carbide, gallium phosphide, aluminum gallium nitride, aluminum oxide doped zinc oxide, cadmium telluride, cadmium selenide, cadmium sulfide, mercury cadmium telluride, zinc selenide, zinc telluride, magnesium telluride, oxide Tin, indium oxide, manganate containing iron oxide - manganese oxide, iron oxide - zinc chromium oxide, iron oxide - magnesium chromium oxide, ruthenium oxide, nickel oxide doped with lithium, tantalum nitride, indium tin oxide - gallium oxide - Tin oxide and mixtures thereof.

The metal resistors may be selected from platinum, rhodium, palladium, gold, chromium, rhenium, iridium, tungsten, molybdenum, nickel, cobalt, aluminum, copper, tantalum, platinum-rhodium alloys, and mixtures thereof.

A particularly preferred semiconductor is indium tin oxide and a particularly preferred metal is platinum.

Brief description of the drawings

Figure 1 is a diagram of a sensor design;

Fig. 2 is the analog circuit designed in Fig. 1;

Figure 3 is a diagram of another sensor design;

Figure 4 is a plot of the resistance (signal) of the sensor of Figure 1 as a function of temperature.

Description of better examples

discuss

In order to establish suitable design rules for self-compensating strain gauges, a temperature coefficient of resistance (TCR) model for self-compensating strain sensors is first established. The TCR model of an ITO sensor with a platinum auto-compensation circuit was established using the following method:

TCR _Compensation = (R _{Compensation, f} - R _{Compensation, 0)} / (R _{Compensation, 0} × ΔT) (1) where, R _{Compensation, f} is the sensor resistance compensated at a specific temperature, R _{Compensation, 0} is the compensation at the reference temperature The sensor resistance, ΔT is the temperature difference,

R _{Compensation, f} = R _{Pt, f} + R _{ITO, f} (2)

R _{compensation, 0} = R _{Pt, 0} + R _{ITO, 0} (3) Substituting equations (2) and (3) into equation (1), the resulting TCR _compensation is:

TCR _compensation = ((R _{Pt, f} + R _{ITO, f} ) - (R _{Pt, 0} + R _{ITO, 0} ))/((R _{Pt, 0} + R _{ITO, 0} ) × ΔT) (4)

The resistance R is related to the resistivity (ρ), which is a constant at a specific temperature,

R=ρ×L/(w×t) (5) where L, w and t are the length, width and thickness of the sensor film. Substituting R into Equation (4) for TCR _compensation , the format equation of the final TCR _compensation model is obtained.

TCR _compensation = (Δρ _Pt ×A _Pt +Δρ _ITO ×A _ITO )/((ρ _Pt , 0×A _Pt +ρ _ITO,0 ×A _ITO )×ΔT) (6) where,

Δρ _Pt = ρ _{Pt, f} - ρ _{Pt, 0} (7)

_ΔρITO = ρITO _{, f} - _{ρITO, 0} (8)

A _Pt ＝ L _Pt /(w _Pt ×t _Pt ) (9)

A _ITO =L _ITO /(w _ITO ×t _ITO ) (10) ρ _Pt,f , ρ _Pt,0 , ρ _ITO,f , ρ _ITO,0 are the resistivities of platinum and ITO at the working temperature and the reference temperature. In equation (6), all resistivities and ΔT are constant, Δρ _Pt >0, Δρ _ITO <0. Different lengths (L), widths (w) and thicknesses (t) of ITO and Pt can be designed so that the temperature coefficient of resistance of the self-compensating ITO-Pt sensor film is 0.

It can also be seen from equation (4) that the temperature coefficient of resistance of the automatic compensation sensor is also related to the temperature coefficient of resistance of Pt and ITO.

TCR _compensation = ((R _{Pt, f} + R _{ITO, f} ) - (R _{Pt, 0} + R _{ITO, 0} ))/((R _{Pt, 0} + R _{ITO, 0} ) × ΔT) (4) TCR _compensation = {[(R _Pt,f -R _Pt,0 )/(R _Pt,0 ×R _ITO,0 ×ΔT)]+[(R _ITO,f -R _ITO,0) /(R _Pt,0 ×R _{ITO ,0} ×ΔT)]}×B where, B=(R _Pt,0 ×R _ITO,0 )/(R _Pt,0 +R _ITO,0 )

Simplifying this equation, the temperature coefficient of resistance of the self-compensated sensor is related to the constant temperature coefficient of resistance of Pt and ITO.

TCR _compensation = (TCR _Pt × R _{Pt, 0} + TCR _ITO × R _{ITO, 0} )/(R _{Pt, 0} + R _{ITO, 0} ) (11)

The above mathematical expressions can be solved using commercially available software (such as Mat Lab or Math Cad software installed in a personal computer).

Preparation of the sensor

Self-compensating ITO sensors were fabricated by sputtering Pt and ITO films followed by patterning. A sensor as an example of the present invention is shown in FIG. 1 . The illustrated sensor 10 generally includes a broadband semiconductor 12 (such as ITO) and metal (such as Pt) compensation circuitry 14 deposited on a substrate S. As shown in FIG. To obtain experimental data, four platinum connection plates 16a, 16b, 16c and 16d were used.

The self-compensating sensor 10 is modeled as a circuit consisting of resistors as shown in FIG. 2 . Ch1 can measure the resistance of the whole sensor, Ch2 can be used to measure the resistance of Pt, Ch4 can be used to measure the resistance of ITO part, Ch3 and Ch5 can be used to measure the contact resistance between Pt and ITO.

Indium tin oxide (ITO) films were formed by radio frequency reactive sputtering at low temperature using a Model 822 MRC sputtering device. A high-density target ₍ diameter 12.7 cm) with a nominal composition of 90 wt% _In2O3 and 10 wt% _SnO2 was used in all depositions. The oxygen partial pressure was 30%, and the RF power density was maintained at 2.4 W/cm ² during each sputtering process, with a total pressure of 9 mTorr. Alumina constant strain beams (beams) were cut from rectangular plates (Coors Ceramics - 99.9% purity) using laser cutting techniques. 4 μm high-purity alumina was sputter-coated on this constant strain bar, followed by deposition of ITO strain gauges. Spin-cast 4 μm ITO first, and then spin-coat a layer of 2 μm thick positive photoresist on the ITO thin film coating. After exposure and development, the ITO film is etched with concentrated hydrochloric acid to form the final device structure. A sputtered platinum film (1.1 μm thick) was used to form ohmic contacts to the active ITO strained elements.

The above sensor dimensions are for experiments to demonstrate the basic principles and mathematical models. Those of ordinary skill in the art will recognize that, based on the description of the present invention, industrially feasible sensors that are at least an order of magnitude smaller in size can be fabricated using microelectronic fabrication techniques employed in the art.

Referring to FIG. 3, the sensor 20 includes a broadband semiconductor 22 and a metal compensation circuit 24 on a substrate S. Referring to FIG. In this design, the G(-) of the semiconductor is the largest and the G(+) of the metal is the smallest. Also shown are metal webs 26a and 26b. A monitor (not shown) is connected to the connection board 26 when the sensor is required to display a reading.

High temperature strain device

惠普数字式万用表和Keithley恒流电源采用四线法监测相应的电阻变化。将高精度LVDT、万用表和恒流电源连接在一块I/O板上并与带IEEE488接口的IBM PC相连。使用Lab Windows软件进行数据采集。Strain measurements were performed using a cantilever bending fixture made of machinable zirconium phosphate ceramic. A solid alumina rod is connected between the alumina constant strain bar and the linear variable differential transducer (LVDT) to measure the deflection of the strain bar. use

The HP digital multimeter and the Keithley constant current power supply adopt the four-wire method to monitor the corresponding resistance change. Connect high-precision LVDT, multimeter and constant current power supply on one I/O board and connect it with IBM PC with IEEE488 interface. Data acquisition was performed using Lab Windows software.

High temperature strain test results

To evaluate the piezoresistance performance of active ITO strain gauges used over a wide temperature range, it is important to characterize the electrical response as a function of temperature. The electrical response of the ITO film formed in 30% oxygen plasma as described above and thermal cycling in air up to a temperature of 1200° C. were observed.

Broadband semiconductors exhibit a single TCR or two or more TCRs over a specific temperature range. It should be understood that when two linear TCRs occur in two different temperature ranges within that particular temperature range, the sensor can be fabricated for each different temperature range in order to measure strain in that particular temperature range.

It is known that for ITO films, there are two different TCRs depending on the temperature: a linear response of -210ppm/°C can be observed when T>800°C, and a TCR of -2170ppm/°C can be observed when T>800°C. Recently, ITO with a single TCR of -300 to -1500 ppm/°C was measured.

Example

In this embodiment, a four-wire method connected to the connection board 16 is used. This method is well known to those of ordinary skill in the art. Sensors were prepared and tested as described above. Four heating and cooling cycles were tested and the results are shown in Figure 4 and below. After the first heating, the change in resistance with temperature is almost the same in four cycles, so it has good reproducibility.

Table 1 temperature(℃) Ch1 resistance (Ω) Ch2 resistance (Ω) Ch3 resistance (Ω) Ch4 resistance (Ω) Ch5 resistance (Ω) 1200 437 379 5 40 12 30 442 160 26 225 31

TCR _compensation =(R _{compensation, f} -R _{compensation, 0} )/(R _{compensation, 0} ×ΔT)=(437-442)/(437×1170)=-9.8(ppm/℃)

TCR _Pt = (R _{Pt, f} -R _{Pt, 0} )/(R _{Pt, 0} ×ΔT) = (379-160)/(160 × 1170) = +1169 (ppm/°C)

TCR _ITO ＝(R _ITO,f -R _ITO,0 )/(R _ITO,0 ×ΔT)＝(40-225)/(225×1170)＝-702(ppm/℃)

Resistivity of Pt

at 30°C

ρ _{Pt, 0} = R _{Pt, 0} × (w × t)/L = 160 × (0.6mm × 0.8 × 10 ^-3 )/500 = 1.535 × 10 ^-4 (Ω·m)

at 1200°C

ρ _{Pt, f} = R _{Pt, f} × (w × t)/L = 379 × (0.6mm × 0.8 × 10 ^-3 )/500 = 3.639 × 10 ^-4 (Ω·m)

Resistivity of ITO

at 30°C

ρ _{ITO, 0} = RITO _{, 0} × (w × t)/L = 225 × (5 × 4.4 × 10 ^-3 )/60 = 8.25 × 10 ^-2 (Ω·m)

at 1200°C

ρ _{ITO, f} = RITO _{, f} × (w × t)/L = 40 × (5 × 4.4 × 10 ^-3 )/60 = 1.498 × 10 ^-2 (Ω·m)

By equation (6)

TCR _compensation = (Δρ _Pt ×A _Pt +Δρ _ITO ×A _ITO )/((ρ _Pt,0 ×A _Pt +ρ _ITO,0 ×A _ITO )×ΔT)(6) where,

Δρ _Pt = ρ _{Pt, f} - ρ _{Pt, 0} = (3.639-1.535)×10 ^-4 (Ω·m)

_ΔρITO = ρITO _{, f} - _ρITO,0 = (1.498-8.25)×10 ^-2 (Ω·m)

A _Pt = L _Pt /(w _Pt ×t _Pt ) = 500/(0.6×0.8×10 ^-3 )(mm ^-1 )

A _ITO ＝L _ITO /(w _ITO ×t _ITO )＝75/(5mm×4.4×10 ^-3 )(mm ^-1 )

TCR _compensation = (Δρ _Pt ×A _Pt +Δρ _ITO ×A _ITO )/((ρ _Pt,0 ×A _Pt +ρ _ITO,0 ×A _ITO )×ΔT)=-21.32(ppm/℃)

By equation (11)

TCR _compensation = (TCR _Pt × R _{Pt, 0} + TCR _ITO × R _{ITO, 0} )/(R _{Pt, 0} + R _{ITO, 0} )

TCR _Compensation ＝-23.5(ppm/℃)

The results of auto-compensated sensors are as follows. The sensor was thermally cycled to 1200°C. Experimental data shows that the TCR of the automatic compensation meter is almost 0 (0ppm/°C±20ppm/°C) in the temperature range from room temperature to 1200°C. The second cycle (30-1200℃) The third cycle (30-1200℃) The fourth cycle (30-1200℃) The fifth cycle (30-1200℃) TCR of Pt resistor 1206.6 1179.8 1183.5 1171.4 TCR of ITO Resistor -701.3 -688.4 -705.6 -699.5 Automatically compensates the TCR of the meter -17.0 -24.1 -17.1 -10.8 TCR calculated from equation (6) -27.5 -16.8 -28.6 -21.4 TCR calculated from equation (11) -29.0 -11.8 -30.5 -23.5

The dimensions of the platinum resistor 14 are 0.6 mm×500 mm×0.8 μm thick, and the dimensions of the ITO sensor 12 connected in series with the platinum resistor are 5 mm×60 mm×4.4 μm thick. These dimensions correspond to the width x length x thickness of each resistor, and the results in the table are obtained from these specific dimensions.

The room temperature resistance can be obtained from Figure 4, which is about 240Ω for ITO resistors and about 160Ω for platinum resistors.

Self-compensating resistors can be used in any electronic device (ie, thermistors, temperature sensors, RTDs, etc.) that requires control of the temperature-dependent TCR.

The above description is limited to specific examples of the invention. It will, however, be apparent that various changes and modifications may be made to the invention in order to obtain some or all of the advantages of the invention. It is therefore the intention in the appended claims to cover all such changes and modifications as are within the spirit and scope of the invention.

Claims (8)

1. high temperature film strain gauged sensor, the temperature-coefficient of electrical resistance that it is characterized in that described sensor is zero substantially, this sensor comprises broadband semiconductor and compensation metal, and the girth of each semiconductor and metal and the size of thickness make that the temperature-coefficient of electrical resistance of this sensor is zero substantially in 20-1200 ℃ temperature range.

2. sensor as claimed in claim 1 is characterized in that described width semiconductor is selected from silit, aluminium nitride, zinc paste, gallium nitride, indium nitride, scandium nitride, titanium nitride, chromium nitride, zirconium nitride, boron carbide, adamas, titanium carbide, tantalum carbide, zirconium carbide, gallium phosphide, aluminium gallium nitride alloy, the zinc paste of doped aluminium, cadmium telluride, cadmium selenide, cadmium sulfide, cadmium mercury telluride, zinc selenide, zinc telluridse, tellurium magnesium, tin oxide, indium oxide, manganate-the manganese oxide that contains iron oxide, iron oxide-oxidation zinc chrome, iron oxide-magnesium oxide chromium, ruthenium-oxide, the nickel oxide of elements doped lithium, tantalum nitride, tin indium oxide-gallium oxide-tin oxide.

3. sensor as claimed in claim 1 is characterized in that described metal is selected from platinum, rhodium, palladium, gold, chromium, rhenium, iridium, tungsten, molybdenum, nickel, cobalt, aluminium, copper and tantalum.

4. sensor as claimed in claim 1 is characterized in that described semiconductor is a tin indium oxide, and described metal is a platinum.

5. the preparation method of a high temperature film gage probe, it comprises:

The temperature range of determination sensor operation;

Be determined at the semi-conductive resistivity of width under at least two temperature spots in the described temperature range, and the resistivity of metal under at least two temperature spots in described temperature range;

The temperature-coefficient of electrical resistance that mensuration will obtain to design, girth that this broadband semiconductor and metal are essential and thickness; With

Form the high temperature film gage probe.

6. sensor as claimed in claim 5 is characterized in that described width semiconductor is selected from silit, aluminium nitride, zinc paste, gallium nitride, indium nitride, scandium nitride, titanium nitride, chromium nitride, zirconium nitride, boron carbide, adamas, titanium carbide, tantalum carbide, zirconium carbide, gallium phosphide, aluminium gallium nitride alloy, the zinc paste of doped aluminium, cadmium telluride, cadmium selenide, cadmium sulfide, cadmium mercury telluride, zinc selenide, zinc telluridse, tellurium magnesium, tin oxide, indium oxide, manganate-the manganese oxide that contains iron oxide, iron oxide-oxidation zinc chrome, iron oxide-magnesium oxide chromium, ruthenium-oxide, the nickel oxide of elements doped lithium, tantalum nitride, tin indium oxide-gallium oxide-tin oxide.

7. sensor as claimed in claim 5 is characterized in that described metal is selected from platinum, rhodium, palladium, gold, chromium, rhenium, iridium, tungsten, molybdenum, nickel, cobalt, aluminium, copper and tantalum.

8. sensor as claimed in claim 5 is characterized in that described semiconductor is a tin indium oxide, and described metal is a platinum.

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