FR2805254A1 - MICROBALANCE WITH PIEZOELECTRIC MATERIAL IN A LIQUID ENVIRONMENT - Google Patents

Abstract

The invention concerns piezoelectric material microbalances operating in liquid medium. Said microbalances comprise a piezoelectric material segment (1) which serves as resonator and which is provided on the two opposite faces with an electrode (2, 3). Said piezoelectric material is lithium tantalate.

Description

The <SEP> present <SEP> invention <SEP> concerns <SEP><SEP> microbalances
<tb> piezoelectric <SEP> material <SEP> operating <SEP> in <SEP> medium <SEP> liquid.
<tb> From <SEP> such <SEP> devices <SEP> include <SEP> a <SEP> blade <SEP> in <SEP> material
<tb> piezoelectric <SEP> making <SEP> office <SEP> of <SEP> resonator, <SEP> equipped <SEP> on
<tb> two <SEP> opposite <SEP> sides <SEP> of a <SEP> electrode <SEP> and <SEP> setting <SEP> in <SEP><SEP> contact with
<tb> a <SEP> medium <SEP> liquid <SEP> and <SEP> are <SEP> used <SEP> for <SEP> measure <SEP> the <SEP> variation
<tb><SEP> electroacoustic <SEP><SEP> properties of the <SEP> resonator, <SEP> in a row
<tb> to <SEP> the appearance <SEP> of a <SEP> phenomenon <SEP> that <SEP> one <SEP> wishes <SEP> to put <SEP> into
<tb> evidence <SEP> and <SEP> quantify.
<tb> such <SEP><SEP> phenomenon appears <SEP> when <SEP> of <SEP><SEP> metric <SEP> of
<tb> different <SEP><SEP> physical-chemical <SEP> quantities such as <SEP> than <SEP> density,
<tb> conductivity, <SEP> viscosity <SEP> or <SEP> mass <SEP> in <SEP> of <SEP> aqueous solutions <SEP>
<tb> and / or <SEP> liquid SEP> organic media.
<tb> This <SEP> technique <SEP> of <SEP> metric <SEP> is <SEP> used <SEP> in <SEP> different
<tb> domains <SEP> such <SEP> as <SEP> the <SEP> biochemistry, <SEP> electrochemistry, <SEP> the <SEP> domain
<tb> biomedical, <SEP> for <SEP><SEP> study of <SEP><SEP> corrosion, <SEP> of <SEP> scaling,
<tb> etc. <SEP> setting <SEP> in <SEP>#people<SEP> different <SEP> methods <SEP> in particular
<tb> electrodeposition, <SEP> electrolysis, <SEP> ion insertion <SEP>, <SEP> la
<tb> chromatography.
<tb> various <SEP><SEP> types of <SEP><SEP> microbalances <SEP> distinguish <SEP> with <SEP> the
<tb> techniques <SEP> of <SEP> measure <SEP> set <SEP> in <SEP>#people<SEP> in <SEP> function <SEP> of
<tb> SEP applications considered.
<tb> can <SEP> thus <SEP> parse <SEP> impedance <SEP> or <SEP> admittance <SEP> of <SEP>
<tb> piezoelectric <SEP> blade, <SEP> or <SEP> well <SEP><SEP> response <SEP> transient <SEP> to <SEP> of
<tb> pulses <SEP> of <SEP> the <SEP> slide, <SEP> or <SEP> well <SEP> again, <SEP> with <SEP><SEP> microbalances
<tb> of the <SEP> type <SEP> to <SEP> oscillator <SEP> of which <SEP> the <SEP> resonator <SEP> is <SEP> constituted <SEP> by <SEP> la
<tb> piezoelectric <SEP> blade, <SEP> on <SEP> measurement <SEP> the <SEP><SEP> variation of <SEP><SEP> frequency
<tb> of <SEP> resonance <SEP> of <SEP><SEP><SEP> slide due to <SEP> at <SEP> phenomenon <SEP> to <SEP> to study <SEP> via
<tb> measure <SEP> of <SEP> the <SEP><SEP> variation of <SEP><SEP> frequency of the <SEP><SEP> oscillator whose
<tb> the <SEP> oscillations <SEP> are <SEP> maintained <SEP> electrically.
<tb> In <SEP> the <SEP> microbalances <SEP> in <SEP> medium <SEP> liquid, <SEP> it <SEP> n '
<tb> practically <SEP> used <SEP> as <SEP> piezoelectric <SEP> material <SEP> than <SEP>
<tb> crystal <SEP> of <SEP> quartz.
<tb> or, <SEP> it <SEP> is <SEP> well <SEP> known <SEP> than <SEP> the <SEP> running <SEP> of a
<tb><SEP> microbalance consisting <SEP> of a <SEP><SEP> oscillator including <SEP> the <SEP> resonator
<tb> piezoelectric <SEP> put <SEP> in <SEP> contact <SEP> with <SEP> a <SEP> liquid <SEP> is <SEP> a
<tb> crystal <SEP> of <SEP> quartz, <SEP> raises <SEP> various <SEP> difficulties <SEP> that <SEP> one <SEP> can
<tb> summarize <SEP> as <SEP> follows - the oscillator oscillation is maintained, when the quartz crystal, being in contact with air, is immersed or even simply put in contact with a tiny drop of liquid, is difficult and usually requires a manual adjustment, either gain, or agree resonant circuit, or even the addition of an automatic gain control loop, - the more the liquid is viscous, the more the maintenance of 1 oscillation is difficult or even impossible, - the oscillator ceases to function as soon as the frequency variation tends to become important, as is the case for example during a high mass deposition with microbalances intended for measurement purposes. mass deposition, - the oscillator electronics is generally not broadband and must be adapted to the resonant frequency the quartz crystal blade, which requires manipulation and prohibits any compactness of the microbalance, - a certain number of oscillators do not offer a ground electrode for the quartz plate, - the relation between the oscillation frequency variation and the phenomenon responsible for these variations does not depend only on the variations of the parameters of this plate.

Moreover, the sealing around the quartz blade, to bring into contact with the liquid only one face of the quartz, is always provided by glue or O-rings which largely encroach on this quartz blade, resulting in a deterioration of the coefficient of quality of the resonator.

The present invention aims to overcome at least some of the disadvantages mentioned above and to improve the performance and flexibility of use of known devices by proposing a piezoelectric resonator microbalance other than the quartz crystal usually used in these devices.

To this end, the subject of the invention is a microbalance with a piezoelectric material operating in a liquid medium, of the type comprising a piezoelectric material blade acting as a resonator and provided on both opposite sides with an electrode, characterized in that said piezoelectric material is lithium tantalate.

Measurements made with such a microbalance have firstly shown that the liquid lithium immersion degradation of the lithium tantalate resonator quality coefficient is less than that of the quartz crystal resonator usually used in known microbalances, and secondly that the of the Q.Cm/Co term of the higher lithium tantalate resonator in a liquid medium to that of the quartz crystal resonator. In this expression Q.Cm/Co is the quality coefficient of the resonator defined as the ratio of the energy stored on the energy dissipated per cycle, multiplied by 2-n, Cm is the motional capacity and Co the inter-electrode capacitance of the resonator.

These properties of lithium tantalate with respect to quartz thus make it particularly advantageous to use it in a microbalance in a liquid medium because they translate into increased performances for this microbalance.

Lithium tantalate also has the advantage of a higher dielectric constant than quartz. This results in an interelectrode static capacitance of the lithium tantalate resonator also greater than that offered with quartz for the same sensitive surface in contact with the liquid to be studied, with the consequence of a lower sensitivity to parallel parasitic connection capacitances.

In addition, this higher dielectric constant provides the advantage that at equal static capacity the lithium tantalate resonator can reduce the minimum sensitive surface of the blade to be contacted with liquid to be studied in comparison with a quartz blade, and therefore reduce the amount of liquid needed for the measurement. In case of expensive liquids, this advantage is particularly interesting.

According to another characteristic of the device of invention, the blade of piezoelectric material is fixed in a removable manner by pinching its periphery by two continuous lips of electrically insulating material and chemically neutral, arranged opposite on both sides of the relatively mobile blade.

Such a mount allows easy assembly and disassembly of the blade, ensures a good seal while minimizing the encroachment of the lips on the blade which limits degradation of the coefficient of quality of the resonator. This frame automatically adapts to blades of various thicknesses, including blades of thickness less than 0.06 mm allowing the use of resonators with high resonance frequency in fundamental mode and also in partial mode.

According to yet another feature of the device of the invention, the microbalance may comprise an oscillator whose resonator is constituted by the piezoelectric material blade, said oscillator comprising, in addition, a broadband amplifier without a selective circuit, one of the electrodes. of the resonator being grounded. Advantageously, such an amplifier may be constituted by a commercial integrated circuit type Burr Brown OPA 660 reference or any other commercial circuit performing the same function or even a specific application integrated circuit (ASIC) incorporating all the electronics necessary for functioning of this microbalance.

Such an oscillator makes it possible to use resonators, in this case blades made of piezoelectric material whatever their nature, provided the term Q.Cm / Co is sufficient, of various fundamental frequencies without any modification or adjustment. . In addition, the small footprint these types of amplifier allows the realization of compact microbalance by placing the amplifier near resonator, thus minimizing the parallel parasitic capacitances of connection to this resonator.

The simplicity of such a device finally makes microbalances low cost. Other features and advantages will emerge from the following description of microbalances according to the invention, a description given by way of example only and with reference to the appended drawings in which FIG. 1 is a diagrammatic view in vertical axial section of FIG. a frame of a microbalance according to the invention; FIG. 2 illustrates the frame of FIG. 1 in an example of use in a microbalance in a liquid medium; FIG. 3 represents an electric equivalent dipole of a piezoelectric resonator around a resonant frequency; FIG. 4 represents the block diagram of an oscillator according to the invention; FIG. 5 represents the diagram of an oscillator according to the invention incorporating an integrated circuit of the Burr Brown type; FIG. 6 represents the diagram of an oscillator according to the invention incorporating an application specific integrated circuit (ASIC), and FIG. 7 illustrates an example of an oscillator according to the invention for a partial mode.

FIG. 1 illustrates the principle of mounting a microbalance according to the invention, characterized in the first place by a blade 1 made of lithium tantalate, in the form of a circular disk, provided on each of the opposite faces with an electrode, respectively 2 and 3, with its electrical connection conductor, respectively 4 and 5, consisting for example of a gold wire very thin work of typically 0, mm thick. In the embodiment described, the blade is a circular disk, but any other shape can be used. According to another characteristic of the device of the invention, the blade 1 is fixed by pinching its circular periphery by two circular lips coaxial with the blade 1, pressed, one (6) on the upper face of the blade and the other (7) on the underside. The two lips are arranged opposite, have a V-shaped section and consist of an electrically insulating material and chemically neutral, in particular a plastic material. The lips press each wire 4, 5 on the electrode 2, 3 considered.

The lower lip 7 is integral with a fixed annular plate 8, while the upper lip 6 is secured to an inner cylinder 9 engageable in a cylindrical housing 10 of enlarged diameter of the plate 8, so that both lips 6, 7 have their contact edges vis-à-vis.

The inner cylinder 9 is pressed against the blade 1 by means of an external nut 11 movable parallel to the axis 12 of the frame by virtue of an internal thread engaged with a thread formed on the outer periphery of the annular plate 8.

The assembly of the plate 8 and the inner cylinder 9 may be made of an electrically insulating plastic material and neutral chemically compatible with the liquid medium in which the mount is likely to be immersed.

By virtue of its system for fixing the piezoelectric blade and its electrical connection conductors, the whole of the plate 8 makes it possible to reduce parasitic capacitances and to minimize the degradation of the coefficient of quality of the resonator due to the introduction of this blade in the mount.

The lithium tantalate blade 1 is an example in X crystallographic section and has a higher dielectric constant than that of a conventional quartz blade, for example in AT section.

The use according to the invention of lithium tantalate to constitute the resonator 1 has unexpectedly revealed that the degradation by liquid immersion of the quality coefficient of this resonator was much less than that of a quartz crystal resonator .

It is known that the ratio Cm / Co, where, as illustrated by FIG. 3 representing the equivalent diagram of a resonator around a resonance frequency, Cm is the emotional capacity of said resonator and Co its inter-electrode capacitance, is higher for lithium tantalate (typically 0.1) than for quartz (typically 0.00625). In Figure 3, Rm and Lm respectively represent the motional resistance and inductance.

It is also known that the dielectric constant of lithium tantalate (typically 37) is greater than that of quartz (typically 4), which implies for a lithium tantalate blade, with equal geometry, a higher Co capacity than that a quartz plate, the Cm / Co ratio nevertheless remains higher for lithium tantalate than for quartz because the Cm motional capacity of lithium tantalate is also higher.

It is also known that the quality coefficient Q which is defined as the product 2n by the ratio of the energy stored on the energy dissipated per cycle, is higher in the open air for quartz than for lithium tantalate. (in the open typically 50000 for the first and 2000 for the second).

Finally, it is known that during the immersion in a liquid of a quartz resonator, the coefficient of quality drops. Experimental analyzes of the electro acoustic impedance of resonators have found that the quality coefficient Q of a resonator of lithium tantalate fell much less during an immersion (typically from 2000 to more than 1000) than that of a quartz crystal resonator (typically 50000 to less than 2000). On the other hand, these immersion measurements made it possible to determine that the Q.Cm/Co term of tantalate became higher in liquid medium (typically 2.6) than that of quartz (typically 1.4), unlike in the case of the free air where said term Q.Cm/Co is higher for quartz by taking the typical values of Q and Cm / Co indicated above.

The advantage of using lithium tantalate for a liquid-based microbalance stems from these better quality coefficient and Q.Cm/Co behaviors, which substantially reduce the limitations of use recalled in the preamble of the this description.

I1 <SEP> is <SEP> to <SEP> note <SEP> incidentally <SEP> that <SEP> the <SEP> tantalate <SEP> of <SEP> lithium
<tb> offers <SEP> a <SEP> inertia <SEP> chemical <SEP> as <SEP> good <SEP> as <SEP> that <SEP> of <SEP> quartz. In addition, a material with a high dielectric constant, such as lithium tantalate, has the advantage that, even with the given capacitance C 0, the minimum sensitive surface to be brought into contact with the liquid to be studied is lower than that exhibited by quartz in particular. This can be usefully used for expensive liquid measurements which will thus require lower amounts of liquid.

FIG. 2 illustrates a frame use of FIG. 1 in a microbalance in a liquid medium.

The plate 8 of the frame is disposed with its axis horizontally in a chamber 13 containing a liquid medium 14.

The plate 8 is sealingly attached to a vertical insulating support 15 immersed in the medium so that the face carrying the electrode 3 of the blade 1 is isolated from the medium 14. The connection son 4 and 5 of the resonator which are on this insulated face are connected, one (4 connected to the electrode 2) to ground and the other (5 connected to the electrode 3) to an oscillator for example (not shown) passing through ducts 16 formed in support 15.

It should be noted that the lithium tantalate blade 1 can be used in any type of microbalance operating in a liquid medium, incorporating or not an oscillator, implementing the various measurement techniques recalled in the preamble for analysis, for example impedance or transient state oscillations free or electrically maintained.

According to another characteristic of the device according to the invention, the microbalance with lithium tantalate blade may comprise an oscillator whose resonator is constituted by said blade, this oscillator further comprising a broadband amplifier, without selective circuit, allowing the grounding of one of the electrodes of the resonator blade.

FIG. 4 shows the block diagram of such an oscillator consisting of a resonator blade 1 made of lithium tantalate, one electrode 2 of which is grounded, the other electrode 3 being connected to a broadband amplifier 17, only with active components, ie without capacity, inductance or any other selective circuit.

This amplifier comprises two current sources 18 and 19 controlled by the voltage (Va-Vb) prevailing between terminals a and b and transconductance Gm. At 20 is shown a load resistor and at 21 a buffer amplifier isolating the terminal a of the output 22 of the amplifier so as not to disturb the potential at a.

Such an amplifier is simple and universal in that it allows the use of resonators (1) of various fundamental frequencies without any modification or adjustment.

The output 22 of the amplifier 17 is connected to a frequency meter (not shown).

The oscillation originates in the resonator blade 1 without intervention in liquid medium, even viscous and is still maintained for large frequency variations, for example for large mass deposits in microbalances for measurements of mass deposition, on a frequency close to that of the series resonance of the resonator 1 whose electrode 2 is grounded.

According to the diagram of FIG. 4, the transconductance Gm of the amplifier 17 is such that 1 / Gm Rm where Rm is the emotional resistance of the resonator 1 as shown in FIG.

As illustrated in FIG. 5, the amplifier 17 may be constituted, for example, by a commercial integrated circuit called Burr Brown reference OPA 660, of which 1 one of the output terminals (6) is connected to a frequency counter ( not shown). This integrated circuit performs the function of the two controlled current sources (18, 19). Inside the circuit case, between the numbered terminals 6, is mounted a buffer amplifier corresponding to the component 21 of FIG.

The amplifier 17 may also be constituted, according to a second example illustrated in FIG. 6, by an electronic integrated circuit made to measure on demand, for example an ASIC (integrated circuit with a specific application) integrating all the elements necessary for the operation of the microbalance, whose output 22 is connected to a frequency meter (not shown). Such a circuit has the advantage over that of FIG. 5 of not requiring any load resistor corresponding to that (20) of amplifier 17 (FIG. 4) or of the circuit of FIG. 5.

Of course, any other integrated circuit of the trade fulfilling the same function as the amplifier could be used.

The small size of the amplifiers of the figures and 6 allows the realization of a particularly compact microbalance by placing the amplifier as close to the resonator 1, thus minimizing the parallel parasitic capacitances of connection to this resonator.

In addition, the simplicity of these circuits makes it possible to manufacture microbalances at low cost.

It finally to note that the addition to such microbalances of a selective circuit allows the use of the resonator 1 in a partial mode, that is to say substantially on harmonics of the fundamental frequency.

FIG. 7 illustrates an example of use of the oscillator of FIG. 4 in partial mode, thanks to the introduction of a selective circuit 23, 24 coupled by a capacitor 25 at the output of the amplifier 17.

A microbalance of the type illustrated in FIG. 4 has the advantage of using resonators of any fundamental resonant frequency since it is simply necessary to change the blade 1.

Claims (6)

R E V N D I C A T IO N S 1. Microbalance piezoelectric material operating in a liquid medium, of the type comprising a blade of piezoelectric material (1) acting as a resonator and provided on both opposite sides of an electrode (2, 3), characterized in that said piezoelectric material is lithium tantalate. 2. Microbalance according to claim 1, characterized in that said piezoelectric material blade (1) is fixed in a detachable manner by gripping around its periphery by two continuous lips (6, 7) of electrically insulating material and chemically neutral, arranged in look on both sides of the blade (1) and relatively mobile. 3. Microbalance according to claim 1 or 2, characterized in that it comprises an oscillator whose resonator is constituted by the blade of piezoelectric material (1), said oscillator further comprising a broadband amplifier (17) without selective circuit, one of the electrodes (2) of the resonator being grounded. 4. Microbalance according to claim 3, characterized in that said oscillator comprises a Burr Brown type amplifier reference OPA 660. 5. Microbalance according to claim 3, characterized in that said oscillator comprises an amplifier produced by a specific application integrated circuit. 6. Microbalance according to one of claims 3 to 5, characterized in that the oscillator further comprises a selective circuit (23 to 25) for partial mode measurements.