WO2009042629A2

WO2009042629A2 - Flextensional transducer with variable beam pattern and frequency control

Info

Publication number: WO2009042629A2
Application number: PCT/US2008/077431
Authority: WO
Inventors: Grant Adam Morris; Yongkang Gao; Michael Smith
Original assignee: Piezotech, Llc
Priority date: 2007-09-24
Filing date: 2008-09-24
Publication date: 2009-04-02
Also published as: WO2009042629A3

Abstract

A flextensional transducer for use in the instrumentation of an oil well drill bit includes a stack of piezoceramic elements arranged to define a longitudinal axis and to move longitudinally in response to an electric signal, and a plurality of flexible, generally arcuate longitudinal ribs directly or indirectly connected to the piezo stack so that the ends of the ribs move longitudinally in response to the longitudinal movement of the piezo stack. This longitudinal movement of the ends of the ribs causes the middle portions of the ribs to flex radially, thus producing acoustic waves. The transducer is designed to operate in the hostile environment of the drilling bit of an oil well, and operates effectively at temperatures of at least -40°C to 150°C. The transducer provides sound waves at a frequency of about 15 KHz, with a -6dB bandwidth of 5KHz to 25 KHz and at a sound pressure of at least 2 kPa. The piezoceramic elements may be made of a material having a base formula of : PbxSr(1-X)(Mn1/3Sb2/3)(1-y)(ZrzTi1-z)yO3 wherein x is in the range of 0.95 to 0.99; wherein y is in the range of 0.92 to 0.97; and wherein z is in the range of 0.45 to 0.55; with the composition further including one or more dopants selected from the group consisting of : PbO, CeO2, SnO2, Sm2O3, TeO2, MoO3, Nb2O5, SiO2, CuO, CdO, HfO2, Pr2O3, and mixtures thereof.

Description

FLEXTENSIONAL TRANSDUCER WITH VARIABLE BEAM PATTERN AND FREQUENCY CONTROL

REFERENCE TQ RELATED APPLICATIONS This application claims priority from U.S. Provisional Patent

Application No. 60/974,590, the entire contents of which are hereby incorporated herein by reference.

FIELD QF THE INVENTION The present invention relates generally to electro-acoustic transmitters, and more particularly to a flextensional transducer having a variable beam pattern and frequency control.

BACKGROUND QF THE INVENTION The extreme environment encountered by instrumentation used at the bit end of a drill string during the drilling of an oil well presents unique challenges to the providers of such instrumentation. Temperatures may exceed 150°C, and pressures may exceed 15,000 psi. Further, a mix of corrosive drilling fluids surrounds the bit. Since at least the 1980's significant efforts have been applied to the development of low frequency and high power underwater acoustic transmitters for sonar and oceanography applications. Various designs, such as longitudinal transducers (with single or double ends), flexural, flextensional transducers (type I-V), Helmholtz resonators have been proposed and developed.

Flextensional transducers transform the high impedance, small motion of one or more piezoelectric ceramic elements into a low impedance, large flexural motion of the transducer shell, thereby increasing the vibration level of the device while decreasing the tensile stress on the ceramics. Flextensional transducers have become prominent as low frequency, high power and high efficiency under water acoustic transmitters which are commonly used in the 300-3000 Hz frequency range. However, such transducers have not become popular with the oil well detection industry where the transducer environment is much more hostile. In addition to the extreme temperatures that are faced by oil well instruments, the extreme pressures that exist in an oil well downhole reduce the piezoelectric ceramic pre-stress and therefore the available power output.

It can be seen from the above that a need continues to exist for improved transmitter designs for oil-well applications, with variable beam pattern and working frequency control as well as high-pressure compensation mechanism. The present invention addresses that need.

SUMMARY OF THE INVENTION In one aspect of the present invention there is provided a flextensional transducer for use in the instrumentation of an oil well drill bit. The transducer includes a stack of piezoceramic elements arranged to define a longitudinal axis and to move longitudinally in response to an electric signal. The transducer also includes a plurality of flexible, generally arcuate longitudinal ribs. The longitudinal ribs are directly or indirectly connected to the piezo elements so that the ends of the ribs move longitudinally in response to the longitudinal movement of the piezo stack. This longitudinal movement of the ends of the ribs causes the middle portions of the ribs to flex radially in response to the longitudinal movement of the piezo elements, thus producing an acoustic wave pattern.

The transducer is designed to operate in the hostile environment of the drilling bit of an oil well. Accordingly, the transducer operates effectively at temperatures of at least -40⁵C to 150⁵C, and more preferably at temperatures of at least -40⁵C to 175⁵C. In some embodiments the transducer operates effectively at temperatures of up to at least 200⁵C.

The transducer is effective for operating at a frequency of about 15 KHz, with a -6dB bandwidth of 5KHz to 25 KHz. The transducer provides a sound pressure of at least 1 kPa at 1 m when used with a continuous (100%) duty cycle in an environment having a temperature of 175⁵C.

In some preferred embodiments the transducer provides the performance characteristics referenced above in a package measuring less than 2" in diameter, and more preferably in a package measuring no more than 1.7" in diameter.

In the most preferred embodiments the piezoelectric material is a piezoelectric ceramic having a Curie temperature in excess of 260⁰C. Preferably, the piezoelectric material will also have one or more of the following properties: a thickness electromechanical coupling coefficient (k_t) of at least about 0.45; a free dielectric constant (K^τ ₃₃) of at least about 1500; and a mechanical quality factor (Q_m) of at least about 2000. More preferably, the piezo material will have a combination of these electromechanical properties while still having a high Curie temperature (in excess of 260 ⁰C), thus ensuring both high power output and proper functioning in high temperature environments.

In some preferred embodiments the piezoelectric material is a PZT ceramic material having a base formula of:

PbχSr₍i-_X)(Mni/₃Sb2/3)(i-y)(Zr_zTii-_z)_yθ3 wherein x may range from 0.95 to 0.99; wherein y may range from 0.92 to 0.97; and wherein z may range from 0.45 to 0.55.

In one preferred embodiment the piezoelectric material is a PZT ceramic material having a base formula of:

PbχSr(₁-_x)(Mn₁/₃Sb2/3)(i-y)(Zr_zTi₁-_z)yθ3 wherein x is about 0.96; wherein y is about 0.94; and wherein z is about 0.5.

One or more dopants may be included in the piezo ceramic material.

Preferred dopants may be selected from the group consisting of: CeO₂, CuO, PbO, SnO₂, Sm₂O₃, TeO₂, MoO₃, Nb₂O₅, SiO₂, CdO, HfO₂, Pr₂O₃, and mixtures thereof. The dopants are preferably added to the ceramic composition in individual amounts ranging from 0.01 wt% to up to 5.0 wt%. In one preferred embodiment the piezoelectric material is a PZT ceramic material having a base formula of: Pb_xSr₍i-_X)(Mni/₃Sb₂/₃)(i-_y)(Zr_zTii-_z)_yO₃ wherein x is about 0.96; wherein y is about 0.94; and wherein z is about 0.5; and wherein the material further includes dopants in the amounts of: CeO₂ is about 0.4%; CuO is about 1 %; and Nb₂O₅ is about 4%.

In further embodiments the piezoelectric material may be any of the piezoelectric materials disclosed in U.S. Patent Application Serial No. 1 1/374,744, the entire contents of which is hereby incorporated herein by reference.

The flextensional transducer of the present invention may be adapted to provide a monopolar, dipolar or quadrapolar wave pattern with the same piezoelectric stack.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of the flextensional transducer of the present invention according to a first embodiment.

FIG. 2 is a perspective view of the flextensional transducer of FIG. 1 with the longitudinal ribs removed.

FIG. 3 is a perspective view of the flextensional transducer of FIG. 1 with the longitudinal ribs attached.

FIG. 4 is an end view of a flextensional transducer 10, showing the monopolar beam pattern produced by the device.

FIG. 5 is a perspective view of the flextensional transducer of the present invention according to a second embodiment.

FIG. 6 is an end view of another embodiment of flextensional transducer 10, showing the dipolar beam pattern produced by the device.

FIG. 7 is an end view of another embodiment of flextensional transducer 10, showing the quadrapolar beam pattern that may be produced by the device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS For the purposes of promoting an understanding of the principles of the invention, reference will now be made to certain embodiments and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. As indicated above, one aspect of the present invention relates to a flextensional transducer comprising: a) a stack of piezoceramic elements arranged to define a longitudinal axis and to move longitudinally in response to an electric signal; and b) a plurality of flexible longitudinal ribs or bands operationally connected to at least one of said piezo elements in a manner effective to cause the longitudinal ribs/bands to flex radially in response to the longitudinal movement of the piezo elements. In some embodiments the device additionally comprises one or more end pieces, alternatively referred to herein as end rings or band rings, that move longitudinally in response to the longitudinal movement of the piezo stack and cause the longitudinal ribs to flex.

The piezoelectric material used to make the piezoceramic elements may have a Curie temperature in excess of 260⁰C, more preferably in excess of 300⁰C, and most preferably in excess of 320⁰C. Preferably, the piezoelectric material will also exhibit suitable electromechanical properties for use in high power applications. For example, in some embodiments the piezoelectric material will exhibit one or more of the following electromechanical properties: a free dielectric constant (K^τ ₃₃) of between about 1200 and 2000 (including embodiments in which the free dielectric constant (K^τ ₃₃) is between about 1350 and about 1650, and embodiments in which the free dielectric constant (K^τ ₃₃) is at least about 1500); a mechanical Q (thickness) of between about 80 and about 1000 (including embodiments in which the mechanical Q (thickness) is between about 400 and about 1000, and embodiments in which the mechanical Q (thickness) is at least about 670); a mechanical Q (radial) of between about 1500 and 2800 (including embodiments in which the mechanical Q (radial) is at least about 2000); a piezoelectric strain constant (d₃₃) of between about 250 and 450 pC/N (including embodiments in which the piezoelectric strain constant (CI33) is between about 290 and about 350, and embodiments in which the piezoelectric strain constant (d₃₃) is at least about 320 pC/N); a dielectric loss/dissipation factor (D) of between 0.002 and 0.008 (including embodiments in which the dielectric loss/dissipation factor (D) is less than about 0.004); a thickness electromechanical coupling coefficient (k_t) of between 0.45 and 0.7 (including embodiments in which the thickness electromechanical coupling coefficient (k_t) is between about .475 and about .530); a planar coupling coefficient (k_p) of at least about 0.3 (including embodiments in which the planar coupling coefficient (k_p) is at least about 0.5, and embodiments in which the planar coupling coefficient (k_p) is between about 0.58 and about 0.65); a longitudinal coupling coefficient (k₃₃) of at least about 0.7; a transverse coupling coefficient (k₃₁) of at least about 0.27 (including embodiments in which the transverse coupling coefficient (k₃₁) is between about 0.24 and about 0.3); and a Curie temperature of between about 315°C and about 325°C.

In some embodiments the piezo material will have a combination of properties such that the product of the planar coupling coefficient (k_p ²), the free dielectric constant (K^τ ₃₃), and the mechanical Q (Q_m(radial)) value is maximized. In particular, in some embodiments the product (k_p ²)(K^T ₃₃)(Q_m(radial)) is at least about 10⁵, while in other embodiments the product is at least about 5 x 10⁵, and in other embodiments the product is at least about 10⁶. When provided by a material having a Curie temperature in excess of 260 ⁰C, and more preferably by a material having a Curie temperature in excess of 300 ⁰C, this combination may help ensure both high power (sound) output and proper functioning in high temperature environments.

PbχSr₍i-_X)(Mni/₃Sb2/3)(i-y)(Zr_zTii-_z)_yθ3 wherein x may range from 0.95 to 0.99; wherein y may range from 0.92 to 0.97; and wherein z may range from 0.45 to 0.55. In one preferred embodiment the piezoelectric material is a PZT ceramic material having a base formula of:

PbχSr₍i-_X)(Mni/₃Sb2/3)(i-y)(Zr_zTii-_z)_yθ3 wherein x is about 0.96; wherein y is about 0.94; and wherein z is about 0.5.

One or more dopants may be included in the piezo ceramic material.

Preferably, the dopant(s) are included in the material by incorporating into the pre-fired composition one or more dopant precursor materials selected from the group consisting of: CeO₂, CuO, PbO, SnO₂, Sm₂O₃, TeO₂, MoO₃, Nb₂O₅, SiO₂, CdO, HfO₂, Pr₂O₃, and mixtures thereof. These dopant precursors are preferably added to the ceramic composition in individual amounts ranging from 0.01 wt% to up to 5.0 wt%.

In one preferred embodiment the piezoelectric material is a PZT ceramic material having a base formula of: Pb_xSr₍i-_X)(Mni/₃Sb₂/₃)(i-_y)(Zr_zTii-_z)_yO₃ wherein x is about 0.96; wherein y is about 0.94; and wherein z is about 0.5; and wherein the material further includes dopants in the amounts of: CeO₂ is about 0.4%; CuO is about 1 %; and Nb₂O₅ is about 4%.

In one embodiment the piezoelectric material is a PZT ceramic made as described herein from the following relative amounts of starting materials:

PbOo 9θSrOo 0194MnOo oiθSbOo 032ZrOo ₄6θTiOo ₄5θNbOo o3sCuOo oioCeOo oo₄- In further embodiments the piezoelectric material may be any of the piezoelectric materials disclosed in U.S. Patent Application Serial No. 1 1/374,744, the entire contents of which is hereby incorporated herein by reference.

The piezoelectric ceramic compositions used in the present invention preferably have a composite perovskite crystal structure. In some preferred embodiments, the composite perovskite ceramic provides a unique crystal structure as a single-phase ceramic composition. The term "composite perovskite crystal structure," is intended to encompass ceramic compositions exhibiting a unique crystal structure prepared by combining the selected elements in a unique, stoichiometric ratio. In this structure, each element or type of element is located at a crystallographically predictable or determinable site, typically a lattice site within the crystal structure. Consequently, the piezoelectric ceramic materials preferably used in the present invention do not exhibit the same properties normally exhibited by a solid solution of metals, or metal oxides, in a ceramic matrix. Similarly, the preferred piezoelectric ceramic materials may exist as a composite perovskite crystal structure with one or more added dopants which may be located in the interstitial sites of the crystal lattice.

The preferred composition of the present invention can be prepared by selecting metal containing precursors and combining the metal containing precursors in a selected relative ratio to provide the desired stoichiometric composition of Formula 1 above. The above formula can be thought of as the perovskite structure of the ABO₃ type. In this formula type, the stoichiometric ratio of the A type element or component to the B type element or component is 1 :1. In accordance with this construct, the metals Pb and M (where M is either strontium or barium) in Formula 1 above can be represented by the identifier A. Similarly, the identifier B can be represented by the combination of (Mn/Sb) and (Zr/Ti). Consequently for the present invention, the relative molar ratio of the A component, [Pb(Sr/Ba)], to the B component, [(Mn/Sb) and (Zr/Ti)], is about 1 :1.

Within this construct, the relative atomic ratio of Pb to M (either Sr or Ba) can be selected and varied to provide a composition with the desired electromechanical properties. In a preferred embodiment, the relative atomic ratio of Mn to Sb is preselected to be about 1 :2 Mn:Sb. The relative atomic ratio of Zr to Ti can range from 7:13 to 1 1 :9 (ZrTi).

Further, the relative ratio of the (Mn/Sb) component to the (Zr/Ti) component can vary. In a preferred embodiment, the relative ratio of (Mn/Sb) to (Zr/Ti) can be varied or selected to be between 1 :9 and 1 :20.

As noted above, the relative ratios of the metals in the ceramic can be varied to affect the desired electromechanical properties. Preferably, the relative ratios are selected to provide a ceramic composition exhibiting a structure that lies near or at the morphotropic phase boundary (MPB) area. The MPB delineates two solid phases, e.g., a tetragonal phase and a rhombohedral phase, that remain in a near-equilibrium state over a wide temperature range.

The preferred metal precursors for the present invention are selected to be metal oxides or metal carbonates. Preferably, the metal precursors are available as PbO, MgO, Nb₂O₅, ZrO₂, and TiO₂.

Additionally, SrCO₃ and BaCO₃ can be used as the precursors for Sr and Ba. These metal precursors are commercially available from a number of commercial vendors and in various levels of purity. It is preferred that the metal precursors be at least 99.95% pure. In other embodiments, the ceramic of the present invention can include one or more dopant materials. The dopant materials can be selected to modify and enhance the electromechanical properties of the resulting piezoelectric ceramic. Alternatively, one or more of the dopants can be added to the precursors to facilitate and/or ease processing steps to formulate the desired ceramic. The dopants can be added to the present composition in individual amounts up to about 2 percent by weight (wt %) based upon the total weight of the starting, precursor material. If more than one dopant is used, the total amount of the dopants should not exceed 5 wt%. More preferably, the dopants are included in the ceramic compositions in combined amounts between 1 .0 wt% and 4.0 wt% based upon the total weight of the starting, precursor materials. Examples of the dopants for use in the present invention include cerium, cesium, lead, tin, samarium, tellurium, molybdenum, niobium, silicon, copper, cadmium, hafnium, and praseodymium ceramics. More preferably, the dopants are provided by one or more of the following dopant precursors CeC>₂, PbO, SnO₂, Sm₂O₃, TeO₂, MoO₃, Nb₂O₅, SiO₂, CuO, CdO, HfO₂, Pr₂O₃. Some preferred composition include between 0.8 wt% and 1 .2 wt% PbO, based upon the total weight of the starting precursor. Preferred compositions also include 0.2 wt % CeO₂, again, based upon the total weight of the starting precursor. Additional preferred compositions include between 0.05 wt% - 0.25 wt% CuO. Still other preferred composition include 1 .6 wt % Nb2O5.

In other embodiments, one or more different piezoelectric compositions (such as PZT4, PZT8, a composite variety, a single crystal of piezoelectric, and/or a piezoelectric polymer just to name a few non-limiting examples) can be alternatively, or additionally, utilized for the present invention.

The piezoelectric ceramics used in the present invention may be prepared by slurrying the selected powdered metal precursors in a liquid such as water or an alcohol. The suspended powder may be pulverized in a ball mill until the mixed slurry is homogeneous and has a sufficiently small particle size. The resulting pulverized mixture may be dried, preferably in an oven at elevated temperatures between about 100 and 150⁵C. The resulting powder may be thermally treated at temperatures of up to 1000⁵C (or more, in some cases), or calcined, to form the desired perovskite structure. The powder is slowly heated to the selected temperature over a period of time. The heating rate can be varied considering the powder mass, the components in the powder, and the desired application for the final piezoceramic component.

Thereafter, the powder may be held at the selected temperature for several hours. Again, the time period or hold time can be varied depending on the mass, identity, and amount of the components in the powder. Typically the powder is held at the selected temperature for a hold time between 1 and 10 hours, more preferably between 2 and 5 hours, and most preferably for about 3 hours. After this thermal treatment, the powder is allowed to cool to room temperature.

The calcined powder may be re-pulverized in a ball mill as has been described above and then dried. This re-pulverized ceramic may then be blended with a binder to provide a paste with the pulverized ceramic suspended in the paste. Preferred binders would include polyvinyl alcohol (PVA).

The ceramic/binder paste may be molded, pressed, or extruded as desired into a shaped article, alternatively referred to herein as a green article. For example, the shaped article may be molded into the shape of a generally parallel piped block or a round disk or any other desired shape.

The binder may be removed from the article either by heating to evaporate the binder, heating to a higher temperature to decompose the binder or, more preferably, by using a solvent to dissolve the binder material. The solvent can be any desired solvent, preferably an organic solvent, into which the binder material exhibits a suitably high solubility. Typical solvents include water, alcohols, acetone, chloroform, methylene chloride, and other polar organic solvents that exhibit a relatively low boiling point or high vapor pressure. A preferred binder/solvent combination is polyvinyl alcohol (PVA) dissolved in water.

The green article may then be sintered or fired at an elevated temperature range. The green article may be placed in a suitable container such as an alumina crucible and additional (unmolded) ceramic powder is placed around the shaped article during the firing process. The elevated temperature range can be selected to be between 900⁵C and 1350⁵C, more preferably between about 1000⁵C and 1300⁵C and most preferably between 1200⁵C and 1290⁵C. The article can be held at one or more selected temperatures within that temperature range for a time between about 10 and about 25 hours. More preferably, the article is slowly heated through the elevated temperature range at a selected heating rate. The heating rate can be selected by considering the mass or volume of the green article, the constituents in the ceramic, and the desired properties of the piezoceramic article. After the firing process, the article comprising the ferroelectric ceramic can be cooled to ambient temperature. The ceramic article comprising the ferroelectric ceramic may then be poled at about 70 to about 80 V per mil thickness of the article. In one embodiment, the ceramic temperature during poling is selected to be between 100⁰C and 140⁰C.

It is to be appreciated that the electrode deposition and poling can be performed differently than that in the above described in connection with other processes. For example, poling electrode deposition on the ceramic article can be accomplished by sputtering or screen printing processes. Typically, the electrodes are deposited on the opposing faces of the article. In one form, the electrode metallization includes low temperature sputtering of gold or an alloy thereof; however, other deposition processes and/or materials suitable for electrode formation can be utilized in different embodiments.

The ceramic of the article is poled (polarized). Polarization can be accomplished by subjecting the ceramic article to the following regime: (a ) a slow ramp-up to an elevated temperature, (b) a slow ramp-up of a polarizing electric field (voltage) across the electrodes while maintaining the elevated temperature, (c) a slow ramp-down to room temperature while the field is maintained, and (d) a slow ramp down of the electric field while at room temperature. Temperature changes are performed at a rate of about 10⁰C to 100⁰C per minute and voltage changes are gradual to a maximum of about 50-80 volts per mil thickness of material with a dwell time at maximum temperature and voltage of about 5 minutes. Performance parameters of the piezoelectric ceramic are tested after poling. If desired at this stage, or at another stage of the process, the poling electrodes can be removed.

In the context of the present invention the piezoelectric material is used to form piezoceramic elements that can be combined to form a piezo stack that defines a longitudinal axis and moves longitudinally in response to an electric signal. The longitudinal movement of the piezo stack causes one or more longitudinal ribs to flex radially, thus producing acoustic waves of a desired frequency.

The piezo elements in the stack may have an annular shape and may surround a center rod or tube to assure proper alignment of the elements in the stack. The piezo elements may be separated by annular shims which may include leads for applying an electric signal to the individual piezos. The piezos and the shims are sized according to a desired application, with piezos and shims having a diameter of about 1 " being preferred for some embodiments.

As to the other elements of the inventive transducer, the longitudinal ribs are effective for moving radially to create acoustic waves in response to the longitudinal movement of the piezo stack. The longitudinal ribs are operationally connected (either directly or indirectly) to the piezo elements such that when the piezo stack and longitudinal ribs are held together in a tight (compressed) unit, the longitudinal ribs will vibrate with a predictable frequency when a signal is applied to the piezo stack. The frequency of the acoustic waves depends largely on the shape and composition of the longitudinal ribs, so the frequency can be varied without changing the piezo stack.

Preferred longitudinal ribs are made of titanium and are shaped as a thin band of metal as shown in the attached figures. The ribs may bend outward (i.e., be convex when viewed from the outside of the device) or they may bend inward (i.e., be concave when viewed from the outside of the device). In either case, when the ribs are all either convex outward or concave outward they will function together to produce a monopole acoustic wave pattern when a full compliment of ribs surrounding the stack is used (i.e., when the ribs extend substantially the full 360⁵ around the axis).

In some embodiments both convex and concave longitudinal ribs may be used in a single device. When arranged with the concave ribs on one side of the device and the convex ribs on the other side of the device a bipolar wave pattern may be produced.

In one preferred embodiment of the present invention the transducer is designed to operate in the hostile environment of the drilling bit of an oil well. Accordingly, the transducer operates effectively at temperatures of at least -40⁵C to 150⁵C, and more preferably at temperatures of at least - 40⁵C to 175⁵C. In some embodiments the transducer operates effectively at temperatures of at least -40⁵C to 220⁵C, and more preferably at temperatures of at least -40⁵C to 260⁵C.

In one embodiment the transducer is effective for operating at a frequency of about 15 KHz, with a -6dB bandwidth of 5KHz to 25 KHz. In some embodiments the transducer is effective for providing a sound pressure of at least 2 KPa (more preferably at least 3 KPa, and most preferably at least 4 KPa) when operated with a continuous duty cycle for a period of at least two (2) days (more preferably at least ten (10) days, and most preferably at least twenty (20) days) near the bit end of a drill string in an oil well hole having a temperature of at least 175⁵C. In other embodiments the transducer provides a sound pressure of at least 4 kPa (or at least about 132 dB with a reference of 1 microPa / Vrms @ 1000V) at 1 m in water at 25⁵C and I atm. In some preferred embodiments the transducer provides the performance characteristics referenced above in a package measuring less than 2" in diameter, and more preferably in a package measuring no more than 1.7" in diameter. In such embodiments the transducer preferably has a length of between 6 and 10 inches, or alternatively between 8 and 9¹/2 inches.

As previously indicated, the flextensional transducer of the present invention may be adapted to provide a monopolar, dipolar or quadrapolar beam pattern with the same piezoelectric stack.

Referring now to the drawings, Figure 1 shows the flextensional transducer of the present invention according to one embodiment. A plurality of piezoceramic elements 1 1 is provided, with the piezos being arranged in a stack 1 1 a defining a longitudinal axis A. The piezoceramic elements are effective for expanding/contracting longitudinally (along axis A) upon the application of an electric signal. The piezoceramic elements 1 1 are preferably made of a high-Curie temperature piezoelectric material such as described above so that the piezo elements will function effectively in the hostile environment of an oil well. Such piezoelectric materials may produce acoustic waves of the desired sound pressure even when the transducer is sized small enough to fit in a space having a diameter of less than 2" and when used in high temperature/high pressure environments such as described herein. In some embodiments the piezoceramic elements have a diameter of about 1 " and a thickness of about 0.05".

The piezoceramic elements 1 1 are preferably separated by a series of shims 12. The shims may be made of a ferrous alloy such as a nickel- cobalt ferrous alloy, or some other suitable material. In certain preferred embodiments the shims are made of Kovar and have a diameter of about 1 " and a thickness of about 0.004".

In the most preferred embodiments both the piezo elements 1 1 and the shims 12 are annular in shape, and are arranged in a stack on a center rod 13. The number of piezo elements 1 1 and shims 12 may vary according to the power and size parameters of a specific device, with approximately 40-60 (most preferably about 52) piezo elements being included in the stack of one preferred embodiment. Center rod 13 may be hollow to facilitate placement of wires, etc., and may be made of titanium or another material capable of providing appropriate structural support and resisting temperature, corrosion, etc.

An end ring 14 may be provided adjacent one or both ends of the piezo stack, and may be used to connect the piezos to the longitudinal ribs. When end ring 14 is secured under compression adjacent piezo stack 1 1 a, ring 4 will move longitudinally as piezo elements 1 1 expand and contract. End ring 14 advantageously provides a structure for connecting the longitudinal ribs 15 to piezo stack 1 1 a. In some preferred embodiments pins 16 are used to secure longitudinal ribs 15 to ring(s) 14. The ring(s) 14, longitudinal ribs 15, and pins 16 may all be made of titanium or some other suitable material.

Longitudinal ribs 15 are connected to piezo stack 1 1 a, optionally via end rings 14, in a manner effective to cause longitudinal ribs 15 to flex radially upon the longitudinal movement of the piezo elements 1 1 . The longitudinal ribs flex radially at a frequency effective to create sound waves that may be received by receivers to provide information about, for example, the geophysical characteristics of the rock encountered by an oil drill bit.

The shape and dimensions of the longitudinal ribs may be altered to change the frequency of the acoustic waves generated by the device, given a particular piezoceramic stack and power input. In some preferred embodiments a frequency centered at 15 KHz is desired, and can be provided by longitudinal ribs made of titanium and having a length of about 6", a width of about 3/8", and a thickness of about 1/8". In other embodiments different frequencies are obtained by forming the longitudinal ribs such that they have the appropriate dimensions to obtain the desired frequency.

In one preferred embodiment the device includes about eight longitudinal ribs spaced uniformly around the circumference of the device. To make a transducer sized to fit in the instrumentation of a modern drill bit the device will preferably have a diameter of no more than about 2" and a length of no more than about 8", and may include between six and twelve longitudinal ribs. In other embodiments fewer or more longitudinal ribs may be used, and the ribs may be spaced uniformly around the circumference of the device or may be spaced such that significant portions of the circumference are left without ribs.

The ribs are preferably spaced apart by small openings of between 1/16" and ¹Λ". The openings provide a pressure release mechanism, which allows the transducer to work under a hydrostatic pressure up to 200MPa (about 30,000 psi).

A nut 17 may be used to tighten end ring(s) 14 to piezo stack 1 1 a so that the entire construct is under compression. When the construct is under appropriate compression the longitudinal ribs will vibrate at a uniform frequency when an electric signal is applied to the piezo elements. One or more end caps 18, 19 may be used to attach the device to the drill bit assembly of an oil well drilling rig. Leads 19 and 20 provide the electric signal to the device. In one embodiment, the input power is a high power input having a short pulse (preferably one cycle) utilizing a maximum input voltage of about 10,000 Vp impulse (half-sine or square wave). In other embodiments the device may utilize higher or lower input power, and the signal may be presented in single or multiple cycles, including, for example, multiple excitation signals such as "chirps."

The piezoelectric ring stack is preferably pre-stressed via the end metal and through tube. Compression of about 25MPa is preferred for some embodiments tested to date.

As indicated above, the ultrasound beam may spread radially from the longitudinal axis. As shown in Fig. 4, a monopolar wave pattern 40 may be produced by a transducer having ribs around substantially the entire circumference of flextensional transducer 10. When some of the ribs are "flipped" from the convex orientation shown in FIGS. 1 -3 to a concave orientation as shown in FIG. 5, the device may produce a dipoar beam pattern, with the lobes 60a and 60b of the beam being 180- out of phase, as shown in FIG. 6. In this embodiment the ribs on opposite sides of the piezoelectric stack expand and shrink on the same direction, thus producing the dipolar pattern. This is different from the embodiment shown in FIGS. 1 -3 in which the ribs 180° apart move in opposite directions, resulting in the illustrated monopolar beam pattern. The dipolar type transmitter ensures better directional control of the beam pattern. Referring more particularly to the embodiment shown in FIG. 5, device 50 includes a plurality of piezoceramic elements 51 , arranged in a stack defining a longitudinal axis. As with the prior embodiments, the piezoceramic elements are effective for expanding/contracting longitudinally (along axis A) upon the application of an electric signal. The piezoceramic elements are preferably separated by a series of shims 52, with both the piezo elements 51 and the shims 52 preferably being annular in shape and arranged in a stack on a center rod 53. An end ring 54 may be provided adjacent one or both ends of the piezo stack, and may be used to connect the piezos to the longitudinal ribs. A nut 57 may be used to tighten end ring(s) 54 to piezo stack 51 a so that the entire construct is under compression. When the construct is under appropriate compression the longitudinal ribs will vibrate at a uniform frequency when an electric signal is applied to the piezo elements. One or more end caps 58, 59 may be used to attach the device to a drill bit assembly of an oil well drilling rig. In yet another embodiment four sections of ribs may be used to provide a quadrapolar beam pattern with the four lobes 70a, 70b, 70c, and 7Od of the beam being 90° out of phase as shown in FIG. 7. In other embodiments additional multi-polar beam patterns, with the various lobes of the beam being selectably out of phase, may be provided. In the embodiments discussed above, the beam pattern may be varied by manipulating the geometry, etc., of the longitudinal ribs. In other embodiments the beam pattern may be adjusted by "segmenting" the piezo stack to that different portions of the piezo stack drive different longitudinal ribs. For example, a bipolar beam pattern may be achieved by using a piezo stack that comprises two, distinct halves, with each half driving a separate portion of the longitudinal ribs. Similarly, a quadrapolar beam pattern may be achieved by using a piezo stack that comprises four, distinct quarters, with each quarter driving a separate portion of the longitudinal ribs. It is to be appreciated that the first resonance frequency of the transducer may be varied by changing the shape and thickness of the metal ribs. The longer the center part of a metal rib, the lower the first resonance frequency of the rib. By using a different set of metal ribs with the same piezoelectric stack, variable beam patterns and operating frequencies can be achieved. In one preferred embodiment of the present invention there is provided an oil well drilling device having included therein a sonic logging transmitter effective for continuous, long-term use at temperatures of at least about i δO'€ (more preferably at temperatures of 150°C to 175°C) and at an operating pressure of about 100 MPa (internally compensated in oil filled vessel). The sonic logging transmitter may produce acoustic waves at a sound pressure of at least 2 kPa (more preferably at least 3 KPa and most preferably at least 4 KPa), and may have a center frequency of operation of about 15 KHz, a -6 dB bandwidth (transmit mode) of 5 KHz to 25 KHz, and a -20 dB transmit pulse length of < 5 cycles.

In one embodiment, the oil well downhole device comprises: a) a drill string having a drilling end; b) a drill bit assembly at the drilling end of the drill string; and c) a flextensional transducer in the drill bit assembly; wherein said flextensional transducer, comprises: i) a stack of piezoceramic elements arranged to define a longitudinal axis and to move longitudinally in response to an electric signal, said piezoceramic elements having a Curie temperature of at least 260⁵C; and ii) a plurality of flexible longitudinal ribs operationally connected to at least one of said piezoceramic elements such that the longitudinal movement of the piezoceramic element causes the ribs to flex radially; wherein said piezoceramic elements and said longitudinal ribs are selected to provide a flextensional transducer effective for generating sound waves with a sound pressure of at least 4 kPa (measured at 1 m in water at 25⁵C) when the transducer is continuously employed in an environment having a temperature of about 175⁵C.

The oil well drilling device most preferably includes a stack of piezoceramic elements made from a material having a base formula of: PbχSr(₁-_X)(Mn₁/₃Sb2/3)(i-y)(Zr_zTi₁-_z)yθ3 wherein x may range from 0.95 to 0.99; wherein y may range from 0.92 to 0.97; and wherein z may range from 0.45 to 0.55.

In other embodiments the piezoelectric material used in the oil well drilling device may include one or more dopants added by incorporating into the pre-fired piezoceramic composition one or more dopant precursor materials selected from the group consisting of: CeO₂, CuO, PbO, SnO₂, Sm₂O₃, TeO₂, MoO₃, Nb₂O₅, SiO₂, CdO, HfO₂, Pr₂O₃, and mixtures thereof. Such dopant precursor materials are preferably added to the ceramic composition in individual amounts ranging from 0.01 wt% to up to 5.0 wt%. In one embodiment the oil well drilling device discussed above uses a piezoelectric material that is a PZT ceramic made from the following relative amounts of starting materials:

PbO_{0 96}SrOo 0194MnO₀ oieSbOo 032ZrO₀46eTiO₀45eNbO₀03δCuO₀010CeO₀ oo4- As previously indicated, the oil well drilling device may utilize a transducer that operates at a frequency of about 15 KHz, with a -6dB bandwidth of 5KHz to 25 KHz. The transducer is effective even in the high temperature environment faced by an oil well drilling device, where temperatures may exceed 150O, and pressures may exceed 15,000 psi, and where a mix of corrosive drilling fluids may surround the bit.

While the invention has been illustrated and described in detail in the drawings and foregoing description, the same are to be considered as illustrative and not restrictive in character, it being understood that only certain preferred embodiments have been shown and described, and that all changes and modifications that come within the spirit of the invention are desired to be protected.

Claims

CLAIMS What is claimed is:

1 . A flextensional transducer, comprising: a) a stack of piezoceramic elements arranged to define a longitudinal axis and effective to expand and contract longitudinally in response to an electric signal, said piezoceramic elements having a Curie temperature of at least 260⁵C; and b) a plurality of flexible longitudinal ribs operationally connected to at least one of said piezoceramic elements such that the longitudinal movement of the piezoceramic element causes the ribs to flex radially; wherein said piezoceramic elements comprise a PZT ceramic material having a base formula of: PbχM₍i-_X)(Mni/₃Sb2/3)(i-y)(Zr_zTii-_z)_yθ3 wherein M is Sr; wherein x is in the range of 0.95 to 0.99; wherein y is in the range of 0.92 to 0.97; and wherein z is in the range of 0.45 to 0.55; wherein said composition further includes one or more dopants selected from the group consisting of: PbO, CeO₂, SnO₂, Sm₂O₃, TeO₂,

MoO₃, Nb₂O₅, SiO₂, CuO, CdO, HfO₂, Pr₂O₃, and mixtures thereof; and wherein said longitudinal ribs are selected to provide a flextensional transducer effective for generating sound waves with a sound pressure of at least 2 kPa when the transducer is employed with a continuous duty cycle in an environment having a temperature of at least 150⁵C.

2. A flextensional transducer according to claim 1 wherein said PZT ceramic material has a base formula of:

PbχSr₍₁-_x)(Mn₁/₃Sb₂/₃)(i-y₎(Zr_zTi₁-_z)yO₃ wherein x is about 0.96; wherein y is about 0.94; and wherein z is about 0.5; and wherein the material includes dopants in the amounts of: CeO₂ is about 0.4%; CuO is about 1 %; and Nb₂O₅ is about 4%.

3. A flextensional transducer according to claim 1 wherein the piezoelectric material is a PZT ceramic made from the following relative amounts of starting materials: PbO_{0 96}SrOo 0194MnO₀ oieSbOo 032ZrO₀ 46θTiO₀455NbO₀ ossCuOo 010CeO₀ oo₄-

4. A flextensional transducer according to claim 1 and further including at least one end piece that moves longitudinally in response to the longitudinal movement of the piezo stack; wherein said longitudinal ribs are connected to said end piece in a manner effective for causing said longitudinal ribs to flex radially in response to the longitudinal movement of the end piece.

5. A flextensional transducer according to claim 1 wherein at least some of said flexible longitudinal ribs are concave with respect to the longitudinal axis of the transducer.

6. A flextensional transducer according to claim 5 wherein each of said flexible longitudinal ribs is concave with respect to the longitudinal axis of the transducer, thereby allowing the transducer to provide a monopolar beam pattern.

7. A flextensional transducer according to claim 1 wherein at least some of said flexible longitudinal ribs are convex with respect to the longitudinal axis of the transducer.

8. A flextensional transducer according to claim 5, and additionally including at least some flexible longitudinal ribs positioned convex with respect to the longitudinal axis of the transducer; wherein said concave longitudinal ribs and said convex longitudinal ribs are arranged to provide a dipolar wave pattern.

9. A flextensional transducer according to claim 1 wherein said piezoceramic elements and said longitudinal ribs are selected to provide a flextensional transducer effective for generating sound waves at a sound pressure of at least 4 KPa (at 1 m), and having a center frequency of about 15 KHz, a -6dB bandwidth of 5KHz to 25 KHz, and a -20 dB transmit pulse length of < 5 cycles.

10. A flextensional transducer according to claim 1 wherein said transducer includes a central longitudinal cylindrical core; wherein each of said piezoceramic elements is shaped as an annulus; and wherein each of said piezoceramic elements encircles said central longitudinal cylindrical core.

11. A flextensional transducer according to claim 1 wherein said transducer further includes wiring appropriate to provide an electric signal to the piezoceramic elements.

12. A flextensional transducer according to claim 1 wherein said transducer further includes a housing.