BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be explained in more detail, with reference to the enclosed drawings. There is shown in:
FIG. 1 a state-of-the-art piezoelectric pressure sensor in axial section;
FIG. 2 a section along line A-A of FIG. 1;
FIG. 3 a first variant of a sensor according to the invention in a sectional view as in FIG. 1;
FIG. 4 a section along line B-B of FIG. 3;
FIG. 5 a detail of FIG. 3;
FIG. 6 a second variant of a sensor according to the invention in axial section;
FIG. 7 a section along line C-C of FIG. 6;
FIG. 8 a measuring element of a pressure sensor according to the invention made by multilayer technology;
FIG. 9 a variant of the measuring element of FIG. 8;
FIGS. 10 and 11 a further variant of a measuring element of the pressure sensor in two phases of its manufacture.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The state-of-the-art piezoelectric pressure sensor shown in FIGS. 1 and 2 has four lamella-shaped piezoelectric measuring elements 1 making use of the transversal piezoeffect, which are clamped between a pick-up electrode 2, which is electrically insulated against the housing 4, and a pick-up electrode 3, which is at the potential of the housing. The individual measuring elements 1 are in contact with the pick-up electrodes 2, 3 via their narrow sides 5, 5′, with the pick-up electrode 3 directly adjacent to a membrane 10 located at the pressured side of the housing 4. Force is being applied to the pressure sensor parallel to its axis (indicated by arrow F). In order to position the individual measuring elements 1 relative to each other, spacer sheets 9 are placed between adjacent side faces 11, which spacer sheets 9 may be corrugated to provide a spring effect pressing the individual crystal elements 1 against the interior wall of the housing 4. A coating layer 8 of insulating plastic material is provided between the measuring elements 1 and the housing 4 to avoid insulation loss. An insulating element 7 for the pick-up electrode 2 is provided between the pick-up electrode 2 and the housing 4.
The individual elements of the sensor must be positioned individually and with care, which requires great manipulative effort if the interior diameter of the sensor is small (up to 2 mm).
In the various embodiments of the invention described below identical or corresponding parts have the same reference numbers.
In the first variant of the invention shown in FIGS. 3 to 5 the piezoelectric measuring elements 1 are attached to the pick-up electrode 2 by thermo-compression or soldering with their narrow sides 5 and thus form a compact measuring element stack 6, which can be inserted into the housing 4 with the help of the pick-up electrode 2 and which can be centered without the use of centering means remaining in the housing 4 in such a way that a sufficient air gap is provided between the interior wall of the sensor housing 4 and the individual measuring elements 1. The stack 6 of measuring elements as shown in FIG. 5 may either consist of the pick-up electrode 2 with the attached measuring elements 1 or may additionally be provided with a pick-up electrode 3 on the opposite side, which is attached to the opposite narrow sides 5′ of the measuring elements 1 by thermo-compression or soldering. The piezoelectric measuring elements 1 and/or the pick-up electrodes 2, 3 are provided with a ductile coating on their contact sites (i.e., in the area of the narrow sides 5 and 5′ of the measuring elements 1), which preferably is applied by sputtering.
The ductile coating may for instance consist of gold, a gold alloy or a solder containing silver. If the measuring elements 1 have thin electrode coatings of gold on their side faces 11 and narrow sides 5, 5′, it is only necessary to coat the contact sites of the pick-up electrodes 2, 3 with gold in order to attach the measuring elements 1 to the pick-up electrodes 2, 3 by means of thermo-compression.
The pick-up electrode 2 essentially consists of a disk-shaped contact element 13 and a pin-shaped lead element 14, the piezoelectric measuring elements 1 being attached to the contact element 13, which rests against the housing 4 with an insulating element 7 being interposed. The pin-shaped lead element 14 passes through an opening 15 of the insulating element 7. According to a variant of the invention the pick-up electrode 2 insulated against the housing 4 may essentially be a disk with an aperture and a pin-shaped lead element 14 which is inserted into the aperture from above.
The fixed attachment of the measuring elements 1 as proposed by the invention must ensure reliable assembly, but should also permit movement between the measuring elements 1 and their contact faces at the pick-up electrodes 2, 3 in the assembled, ready-for-use sensor, in order to keep their measuring properties intact and to avoid destruction of the measuring elements 1 at high temperature loads due to differing thermal expansion coefficients of the measuring elements 1 and the pick-up electrodes 2, 3.
Assembly is carried out as follows:
- the measuring elements 1 are coated with a ductile layer on their narrow sides 5, 5′ by means of a sputtering process;
- preferably the pick-up electrodes 2, 3 are also coated by sputtering at least on the contact faces for the measuring elements 1;
- the measuring elements 1 and the pick-up electrodes 2, 3 are aligned relative to each other and fixedly attached outside the pressure sensor. Using a compression force which does not damage the crystal elements 1, and increased temperature, the parts 1 to 3 are fixed and mutually attached by thermo-compression or soldering;
- the resulting measuring element stack 6 is introduced into the interior space 16 of the sensor housing 4 as a single unit and can be aligned by means of a centering device 18, which may be removed later on (for instance after the membrane 10 has been mounted) and may be located between the pin-shaped lead element 14 and a bore 17 in the sensor housing 4, the alignment being such that an air gap is provided between the measuring elements 1 and the interior wall of the housing 4, which ensures the necessary insulation;
- for accurate positioning of the measuring element stack 6 a conical transition surface between the contact element 13 and the lead element 14 in cooperation with the bore 15 in the insulating element 7 may be used.
In the second variant of the invention shown in FIGS. 6 and 7 the piezoelectric measuring elements 1 are attached to the pick-up electrode 2 with a side face 11 each by thermo-compression or soldering, thus making use of the longitudinal piezoeffect. The pick-up electrode 2 with the signal lead 19 may essentially be disk-shaped, with two piezoelectric measuring elements 1 of opposite polarization being placed on opposite sides of the disk-shaped pick-up electrode 2. The disk-shaped pick-up electrode 2 with the attached piezo-electric measuring elements 1 forms a measuring element stack 6 which is easily handled. The pick-up electrode 2 need not necessarily be circular but could also have other shapes (e.g., square or rectangular).
The measuring element stack 6 may additionally comprise one or both pick-up electrodes 3, the measuring elements 1 being attached with their side faces 11′ to the pick-up electrodes 3 by thermo-compression or soldering. One or both of the pick-up electrodes 3 are adjacent to a membrane not shown in the drawings, which is welded to the housing 4. The piezoelectric measuring elements 1 and/or the pick-up electrodes 2, 3 are provided with a ductile coating (for instance gold or a solder containing silver) at their contact sites in the area of the side faces 11, 11′.
In both variants the pick-up electrode 2 may be made of metal, a metal alloy, an electrically conductive ceramic material or a ceramic material with a conductive coating. For high-temperature measurements the piezoelectric measuring elements 1 may be made of high-temperature-resistant material, preferably GaPO4.
According to an advantageous variant of the invention the piezoelectric measuring elements 1 shown in the diverse variants may be configured as multilayer elements consisting of thin piezoelectric layers 20 alternating with thin electrode layers 21 (see FIGS. 8 to 11). Such “thin film stacks” may be manufactured in various ways.
Due to the layered configuration the piezoeffect (for single crystals 1-10 pm/V) is increased for application in pressure sensors, and at the same time other advantages of piezo-electric single crystals may be exploited (low dielectric loss, temperature-stable constants, etc.)
FIG. 8 shows the layered configuration of a piezostack. Since the piezoelectric constant is predetermined by the single crystal, the piezoelectric layers 20 must be oriented in such a way that charge pick-up by the electrode layers 21, 21′ will be possible. Lateral electrode layers 22, 22′ connect layers with charges having the same sign.
The multilayer configuration will multiply the effective area and addition of the charges will result in a substantially increased piezoeffect as compared with single elements (e.g. six-fold in the variant of FIG. 8).
Construction of the Layers
According to a simple variant the piezoelectric plates or layers 20 are separated by metal foils acting as electrode layers 21, 21′, and are stacked one above the other with opposite orientation. The conductive edges, which are electrically of opposite polarity, may be slightly milled before the lateral contacts are applied. The lateral electrodes 22, 22′ may be applied by means of known techniques.
In another variant the electrode layers 21, 21′ may be directly applied on the piezoelectric layers 20 (for instance by sputtering or evaporation techniques).
In FIG. 9 two piezoelectric measuring elements 1 are combined into a multistack. The individual elements 20 must be manufactured first, then they are aligned (with alternating polarity) and welded under pressure with the metal foils 21, 21′.
Very Thin Layers
Piezostacks with very thin piezoelectric layers 21, 21′ are particularly effective. A very advantageous way of manufacturing such stacks is the folding technique as shown in FIGS. 10 and 11. The piezoelectric single crystal layer 20 may for instance be applied on the electrode layer 21′ by epitaxial growth and then covered with an electrode layer 20 (FIG. 20). The required gaps 23 in the individual layers may be made by sputtering or etching processes. The structure is then folded as shown in FIG. 10, resulting in a thin film stack according to FIG. 11.
In all these processes the piezoelectric layer 20 must be oriented (for instance by introducing an oriented structure into the metallic substrate, or by means of an electric field or of mechanical pressure during growth). Epitaxial methods include atomic growth (such as CVD, PVD) or direct, oriented growth (liquid-phase epitaxy, flux growth, etc.)
Patent Application
20070277618
