PIEZORESISTIVE MATERIAL EXHIBITING AN OPTIMAL GAUGE FACTOR

Abstract

The present invention relates to a multilayer composite material comprising:

a base layer

a metallic layer consisting of an insulating matrix phase and of a metallic particles phase, said metallic particles being distributed in the insulating matrix, wherein a volume fraction being the ratio between the volume of metallic particles and the volume of the metallic layer corresponds to a critical volume fraction *+, with 0<5%, the critical volume fraction * being the volume fraction for which an increase of conductivity of the metallic layer as a function of the volume fraction has a maximum value.

Claims

1. A process for manufacturing a strain gauge comprising the steps of: A1) providing a base layer, B1) providing on the base layer a metallic layer consisting of an insulating matrix phase and of a metallic particles phase, said metallic particles being distributed in the insulating matrix, wherein a volume fraction being the ratio between the volume of metallic particles and the volume of the metallic layer corresponds to a critical volume fraction *+, with 0<5%, * being the volume fraction for which an increase of conductivity of the metallic layer as a function of the volume fraction has a maximum value, and C1) forming ohmic contacts on the metallic layer provided in step b).

2. The process according to claim 1 wherein step B1) comprises the steps of: providing on the base layer a layer comprising a photocatalytic material; contacting the base layer covered by the layer comprising a photocatalytic material with a solution containing metal ions selected from the group consisting of silver, gold, palladium platinum, cobalt and nickel ions, and irradiating the base layer covered by the layer comprising a photocatalytic material with radiation permitting activation of the photocatalytic material, for a time sufficient to have a volume fraction being the ratio between the volume of metallic particles and the volume of the metallic layer corresponding to a critical volume fraction *+, with 0<,5%, * being the critical volume fraction for which an increase of conductivity of the metallic layer as a function of the volume fraction has a maximum value.

3. The process according to claim 2 comprising the steps of: a) depositing by a sol-gel route, on a base layer, a first layer of a material, mesostructured by a templating agent, said material comprising a photocatalytic material and a material containing silica; b) depositing by a sol-gel route, on the first layer, a second layer of a material, mesostructured by a templating agent, said material comprising a material containing silica and being free from photocatalytic material; c) performing a heating treatment of the first and second layers whereby a consolidated coating is obtained; d) optionally calcination of the first and second layers for removing the templating agent, e) contacting the consolidated coating obtained in step c) with a solution containing metal ions selected from the group consisting of silver, gold, palladium platinum, nickel and cobalt ions, and f) irradiating the base layer covered by the consolidated coating with radiation permitting activation of the photocatalytic material, for a time sufficient to have a volume fraction being the ratio between the volume of metallic particles and the volume of the metallic layer corresponding to a critical volume fraction *+, with 0<,<5%, * being the critical volume fraction for which an increase of conductivity of the metallic layer as a function of the volume fraction has a maximum value.

4. The process according to claim 1 wherein step B1) comprises the steps of: b) providing on the base layer a first layer comprising a photocatalytic material; c) contacting the base layer covered by the first layer comprising a photocatalytic material with a solution containing metal ions selected from the group consisting of silver, gold, palladium platinum, cobalt and nickel ions, and d) irradiating the base layer covered by the first layer comprising a photocatalytic material with radiation during a time t e) matching the time t of irradiation with a volume fraction with a calibration curve t=f() f) measuring GF with the following formula $\mathrm{GF}=\frac{.Math..Math.R}{R.Math..Math.0}.Math.\frac{1}{.Math..Math.\mathrm{.Math.}}$ wherein R is the difference of the resistance measured on a sample corresponding to a difference of applied stress on said sample; R0 is the resistance measured on a sample under no stress; and is the strain induced by the applied stress. g) repeating m times the measure of GF for calculating the average of GF according to the general formula $<\mathrm{GF}>=\frac{1}{m}.Math.\underset{j=1}{\overset{m}{.Math.}}.Math.\mathrm{GFj}$ h) measuring the standard deviation of GF measurements with the following formula; $.Math..Math.\mathrm{GF}=\sqrt{\frac{\left[.Math.\left(\mathrm{GFj}\right)-<\mathrm{GF}>\right].Math.2}{m}}$ i) measuring the figure of merit F with the following formula $F=\frac{<\mathrm{GF}>}{.Math..Math.\mathrm{GF}}$ j) measuring dF/d(). k) if dF/d=0; the maximum value of F Fmax is obtained and the irradiation is stopped l) if dF/d>0; irradiating the composite material during a further time and repeating the steps b) to j) until the requirement of step k) is fulfilled.

5. The process according to claim 2, wherein the photocatalytic material is selected from the group of metal oxides consisting of titanium dioxide, zinc oxide, bismuth oxide and vanadium oxide, tungsten oxide, iron oxide, BiFe.sub.2O.sub.3 or a mixture thereof or any solid solutions of thereof

6. The process according to claim 1 wherein step B1) comprises the steps of: a) depositing by a sol-gel route, on the base layer, a first layer of a material, mesostructured by a templating agent, said material comprising a photocatalytic material and a material containing silica and; b) optionally performing a heating treatment of the first layer; c) depositing over the first layer a second layer of a solution containing an organic group and alkaline metal ions, preferably Na.sup.+ ions, more preferably a solution containing sodium acetate, so that a homogeneous film over the first layer is obtained; d) performing a heating treatment of the sample to remove the organic groups by calcination and allow homogeneous diffusion of the alkaline metal ions within the first layer; e) immersing the coating obtained in step d) in a solution containing metal ions selected from the group consisting of silver, gold, palladium, platinum, nickel and cobalt ions, for at least one hour, at a temperature comprised between 15 C. and 90 C. to obtain the complete exchange of the alkaline metal ions by the metal ions; f) rinsing and drying the coating obtained in step e); g) irradiating the layer coating obtained in step f) with radiation permitting activation of the photocatalytic material, preferably the energy of the incident radiation being within the band gap of the photocatalytic material for a time sufficient to reach a volume fraction which is the volume fraction of metallic particles to reach *+, with 0<5%, the critical volume fraction * being the volume fraction for which the obtained multilayered composite undergoes insulator to metal transition.

Description

[0250] The invention will now be described by means of the following Examples

EXAMPLES

Example 1. Manufacturing of the Multilayered Composite Material According to the Invention

[0251] From Step a) to Step c) Formation of the Coating and Treatment of Maturation

[0252] Solution (1) is obtained by mixing the following compounds and heating the resulting mixture at 60 C. under reflux and mixing for 1 hour

[0253] 11 mL of TEOS (tetraethoxysilane) (precursor)

[0254] 11 mL of ethanol (aqueous organic solvent)

[0255] 4.5 mL of HCl at pH=1.25 (catalyst)

[0256] Solution (2) is obtained by dissolving 2.205 g of BASF Pluronic PE6800 in 20 mL of ethanol

[0257] Solution (3) is obtained by addition of 10 mL of solution (1) into solution (2).

[0258] Solution (3) is then filtered with a 450 nm NYLONfilter.

[0259] To 4 mL of filtered solution (3) is added.

[0260] 857 mL of TiO2 S5-300A colloidal suspension (Cm=231 g/L) supplied by Crystal Global.

[0261] The resulting mixture is then mixed.

[0262] This solution is deposited on a soda-lime glass substrate (thickness 20 m) by spin coating (2000 rpm for 1 minute). A substrate coated with a first layer is thus obtained and is kept under a humid atmosphere (HR=65%) using a saturated solution of magnesium acetate for 30 min.

[0263] A further layer is then deposited onto the first layer using solution (3). Deposition is achieved by spin coating under the same conditions as for the deposition of the first layer, including the storage under controlled humidity atmosphere.

[0264] The first and second layers are called coating.

[0265] A substrate coated with the coating is then heated at 450 C. for 2 hours whereby a consolidated coating is obtained.

[0266] Step d) Contacting the Coating with a Solution Containing Metallic Ions and Step e) Irradiating said Coating to Reach *+0.3%

[0267] A drop of AgNO.sub.3 solution is placed so that it could cover the active area of the coating (i.e region C on FIG. 2). The coating was then irradiated with UV light (wavelength of 312 nm, and Power=1 mW/cm.sup.2) for different times.

[0268] For each time of irradiation, the coating was then rinsed with water and dried before performing a resistance measurement.

[0269] In order to determine *, a calibration curve R=f() has been plotted (see FIG. 3).

[0270] The FIG. 3 shows a very sharp transition (points C, D and E) from a highly resistive region (points A and B) to conducting region (point F) for between 12.5% and 13.5%. With this method, * is estimated around 13%.

[0271] In order to have a more accurate value of *, a calibration curve GF=f() has been plotted (see FIG. 4).

[0272] FIG. 4 clear cut and very sharp peak in GF is observed, with a maximum value (4330) at =*=13.1% (corresponding to a time of irradiation of 15 minutes at a power of 1 mW/cm.sup.2) well above that of bulk or nano-structured silicon.

[0273] From these determined value * obtained from the curve GF=f(), it is possible to manufacture the multilayered composite material of the invention by adding 0.3% to the value of *, corresponding to an illumination time of 15 minutes and 20 seconds.

Example 2: Correlation with the Maximum Value of the Figure of Merit F

[0274] The sample obtained in step d) of Example 1 was irradiated for various times.

[0275] The values of the Gauge Factor, standard deviation of the Gauge Factor and the figure of merit plotted for a well discrete number of well-defined values (corresponding to a given time of irradiationsee above the correlation between the time of irradiation and the volume fraction) are shown in FIG. 5.

[0276] As shown in FIG. 5 (bottom panel), a clear cut and very sharp peak in GF is observed, with a maximum value (4330) at =*13.1% (corresponding to a time of irradiation of 15 minutes at a power of 1 mW/cm.sup.2) well above that of bulk or nano-structured silicon. In other samples, values of up to 12000 have been observed (data not shown). Here the peak of GF occurs close to the 16% expected for 3-dimensional continuum percolation in a randomly distributed network of spheres.

[0277] FIG. 5 (middle panel) shows .sub.GF measured as a function of and, as expected, it does indeed show a maximum at *. Here the measurement of GF is repeated 200 times so that the value of .sub.GF is reliable. .sub.GF is also found to obey a power law of exponent 0.97 (data not shown), close to the expected value near *.

[0278] The figure of merit F (or FOM) versus irradiation time may then be plotted as shown in FIG. 5 (top panel), and a number of important points are of interest on this calibration curve.

[0279] The first point is that FOM (Figure Of Merit) is not maximized at * but rather at *+0.3% i.e. slightly on the conducting, metallic side of the percolation threshold.

[0280] Thus, in this example, Fmax is reached for an irradiation time corresponding to 15 minutes and 20 seconds (i.e for a volume fraction of 13.4%).

[0281] The peak value of FOM determined here is 3 which is approximately 5-10 times higher than the FOM value measured in bulk silicon (see respectively the dashed horizontal line in the top panel of FIG. 4 which was measured separately). Therefore, according to the invention, it is possible to know that an optimized piezoresistive material (having the same material and being prepared by the same way as for the piezoresistive material for which a calibration curve F=f() has been obtained) may be obtained for an illumination time of 15 minutes and 20 seconds at a wavelength of 312 nm at power 1 mW/cm.sup.2.

Annexed Experimental Parts of Examples 1 and 2: Ohmic Contact Preparation for Resistance Measurements, Gauge Factor Measurements, Correlation Between the Volume Fraction of Metallic Particles and the Irradiation Time

[0282] Ohmic Contact Preparation Technique:

[0283] In view of performing the resistance measurements to determine the value of the resistance or the value of the Gauge Factor as a function of , it is necessary to form ohmic contacts on the coating. To this end, the coating is first masked using Kapton mask at the center (region A and the active area of the film shown by region C in FIG. 6).

[0284] The unmasked parts (region B in FIG. 6) are loaded with silver by immersing the coating in an aqueous solution of AgNO.sub.3 0,05 M: isopropanol mixture (1:1 volume ratio).

[0285] Irradiation is then performed with UV light (312 nm, 1 mW/cm2 for 50 minutes) leading to the photocatalytic reduction of Ag+ ions into metallic silver particles. The irradiation time ensures the loading of silver in the coating up to saturation, i.e. about 18 vol %. After rinsing and drying of the coating, the silver loaded patterns form the two conducting terminals as shown schematically in FIG. 6.

[0286] In a further step, the coating of the unexposed area (region A in FIG. 6) (initially under the kapton mask) except the central area (region C in FIG. 6) of the sample (5 mm5 mm square) is removed mechanically by scratching. The remaining central area (C) between the two conducting terminals (B) is the active area of interest, whose electrical properties are measured.

[0287] Finally, the two conducting terminals (B) are connected to external copper wires using silver paints.

[0288] Determination of the Parameters: GF, <GF> and GF The sample with the ohmic contact as shown in figure la is glued at the center of a steel plate as shown schematically in FIG. 1b (length=50 cm, thickness=0.6 mm and width=5cm) with a cyanolit 202 glue.

[0289] A compressive force is applied to the two ends of the steel plate. The plate deforms into a sinusoidal shape of half-wavelength L and amplitude H. As a result, the piezoresistive material undergoes an uni-axial tensile stress.

[0290] The applied stress is modulated (by modulating the force applied to the two ends of the steel plate). The amplitude of the sinusoid between H1 and H2 is then obtained over 200 cycles.

[0291] The DC resistance is measured using a voltage source and an earthed pico-ammeter.

[0292] The current is always kept below 1 A to avoid unwanted fluctuations in the resistance measurements.

[0293] The resistance is measured at each stress level (Height denoted by H1 and H2), named respectively R(H1) and R(H2).

[0294] The Gauge Factor (GF) is measured by taking the ratio of the difference in the resistances in each cycle to the value of the resistance R(H1) and R(H2) measured for the lower (R(H1)) stress in the same cycle and the multiplied by the applied stress difference.

[00007]$\mathrm{GF}=\frac{R\left(H.Math..Math.2\right)-R\left(H.Math..Math.1\right)}{\mathrm{Rav}}\frac{1}{.Math..Math.\mathrm{.Math.}}$

[0295] The measure is then repeated for 200 cycles and the average value of GF is calculated.

[00008]$<\mathrm{GF}>=\frac{1}{m}.Math.\underset{j=1}{\overset{m}{.Math.}}.Math.\mathrm{GFj}$

[0296] With the <GF> value, it is then possible to calculate the standard deviation GF and then to determine F.sub.GF.

[00009]$.Math..Math.\mathrm{GF}=\sqrt{\frac{\left[.Math.\left(\mathrm{GFj}\right)-<\mathrm{GF}>\right].Math.2}{m}}$

[0297] To make the correlation between time of irradiation and volume fraction , the image of the sample at various irradiation times (corresponding to different volume fractions) is shown in FIG. 7). Increasing the silver loading provides an increased absorption of the films which exhibit a yellow to dark brown color.

[0298] In a previous work, an experimental curve was made plotting the absorbance at different wavelength as a function of the silver loading, as determined by chemical analysis. Chemical analysis was achieved by dissolving with nitric acid the silver particles located in a silver loaded film of a known area and thickness.

[0299] The solution is then analyzed for its silver ions content by ICP. The results enable to calculate the initial silver volume fraction in the film.

[0300] Knowing the absorbance versus curve, it is possible to determine the silver loading in a film after measuring its absorption.

[0301] Images of the samples for various irradiation times were taken (maintaining the same focusing and light condition).

[0302] A second curve was experimentally determined, plotting the brightness of the active area (C) as quantified using the imageJ software (FIG. 9) as a function of the irradiation time. The brightness is defined as the mean of the histogram of the distribution of the gray values.

[0303] Using the previously determined absorbance versus volume fraction curve (FIG. 8), one thus may determine the silver loading as a function of the brightness of the active area which is directly linked to the time of irradiation.

Example 3: Comparison Between the Figure of Merit of Commercial Strain Gauges and the Figure of Merit of a Composite According to the Invention

[0304] The figure of merit of two commercial strain gauges (one metallic gauge and one silicium gauge (Si gauge)) was calculated from the average of gauge factor <GF> and standard deviation .sub.GF (which are well-known from the user) with the following formula F=<GF>/.sub.GF.

[0305] The figure of merit of the composite material obtained according to example 1 was calculated and compared with the figures of merit of commercial gauges.

[0306] The results are summarized in the following table.

TABLE-US-00001 G (expected) = <G> .sub.G (for a operating strain of 1*10.sup.4) F = <G>/.sub.G Composite material 1000 330 3 according to example 1 Metallic gauge 2.11 3.51 0.6 Type: KFG-1-120-C1-11. Si gauge 140 200 0.7 Type: SS-060-040-2500-PM

[0307] The above table underlines that the figure of merit of the composite material according to the invention is 6 times higher than the figure of merit of metallic and silicium gauges.

[0308] Therefore, the composite material according to the invention is more sensitive than the commercial strain gauges.

Example 4 : Manufacturing of Multilayer Composite Materials According to the Invention by the In-Situ Process

[0309] Solution (1) is obtained by mixing the following compounds and heating the resulting mixture at 60 C. under reflux and mixing for 1 hour

[0310] 11 mL of TEOS (tetraethoxysilane) (precursor)

[0311] 11 mL of ethanol (aqueous organic solvent)

[0312] 4.5 mL of HCl at pH=1.25 (catalyst)

[0313] Solution (2) is obtained by dissolving 2.205 g of BASF Pluronic PE6800 in 20 mL of ethanol

[0314] Solution (3) is obtained by addition of 10 mL of solution (1) into solution (2).

[0315] Solution (3) is then filtered with a 450 nm NYLONfilter.

[0316] To 4 mL of filtered solution (3) is added.

[0317] 857mL of TiO2 S5-300A colloidal suspension (Cm=231 g/L) supplied by Crystal Global.

[0318] The resulting mixture is then mixed.

[0319] This solution is deposited on a soda-lime glass substrate (thickness 20 m) by spin coating (2000 rpm for 1 minute). A substrate coated with a first layer is thus obtained and is kept under a humid atmosphere (HR=65%) using a saturated solution of magnesium acetate for 30 min.

[0320] Of course, the substrate may also be a silicon wafer instead of soda-lime-glass.

[0321] After deposition, calcination was performed for 2 hours at 450 C. Then 0.4 ml of Na-acetate solution (4 gm in 1-ml ETOH and 2 ml H2O) was spin coated over the porous layer. Calcination was then performed for 2 hours at 450 C. This allows the acetate groups to be removed and the sodium ions to diffuse inside the porous matrix.

[0322] Process of Silver Photoreduction

[0323] Process of photoreduction was achieved in two steps: 1. Metal ion exchange, and 2. Metal ion reduction. Ion exchange process was achieved by immersing the films into a AgNO.sub.3 (4 gm for 100 ml) solution of isopropanol and water (8:2 ratio) at 90 C. for 1 hour. This allows the silver ions to diffuse inside the porous matrix and exchange with the sodium ions. After rinsing with water and drying under blowing nitrogen, the samples were irradiated with UV lamp (312 nm, 4.5 mW/cm.sup.2). Initially the samples turn brown and after 3 hours of irradiation a shining metallic silver colour of the sample is obtained.

[0324] During the photoreduction process, the resistance has been measured continuously. FIG. 11 shows the obtained curve.

[0325] When comparing FIG. 11 and FIG. 3, one can see that with the in-situ process of the invention, a finer tuning is obtained than with the ex-situ process of the invention.

[0326] FIG. 12 shows the TEM image of a cross section of the multilayer composite material obtained by the in-situ process of the invention wherein the base layer is a silicon substrate and FIG. 13 shows a SEM image of a cross section of a multilayer composite material obtained by the in-situ process of the invention wherein the base layer is a glass substrate.

[0327] Indeed, the base layer (substrate) may be made of glass, silicon or any other material. In the present example, the substrate is made of glass.

[0328] As can be seen in FIGS. 12 and 13, the multilayer composite material of the invention comprises a metallic layer which consists of an insulating matrix phase with a metallic particle phase inside the insulating matrix.

[0329] Here, the insulating matrix is silica obtained by the sol-gel process containing some residual alkaline metal ions.

CONCLUSIONS

[0330] The method according to the invention underlines that the best way to use a composite as a strain gage is therefore to use a composite material exhibiting a volume fraction of metallic particles which do not correspond to a maximum value of the Gauge factor (i.e at the percolation threshold) but slightly above said percolation threshold corresponding to a maximum value of the figure of merit Fmax.

[0331] It is shown that because fluctuations in GF diverge at * (i.e. higher standard deviation at this point), use of any composite at the percolation threshold (*) is not recommended. It is shown experimentally that the optimal Ag volume fraction lies less than 1% to the metallic, conducting side of *. In this case a figure-of-merit for a nano-composite strain gage is shown to be 5-10 times larger than the equivalent in commercial strain gages. The optimal exploitation of composites as strain gauges therefore requires the ability to finely control the metallic volume fraction.

[0332] While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.