AN ELECTROSTRICTIVE CERAMIC MATERIAL INCLUDING A PROCESS FOR THE PREPARATION THEREOF AND APPLICATIONS THEREFOR

Abstract: This invention relates to a new ceramic electrostrictive compositions having superior electrical properties, a process for the preparation of the new electrostrictive compositions and applications for the new compositions. Specifically, a solid solution composition of lead magnesium niobate - lead titanate - (barium titanate or strontium titanate) has exhibited desirable electrostrictive properties. This composition shows a diminished dependence of dielectric constant on temperature.

Description: -I- AN ELECTROSTRICTIVE CERAMIC MATERIAL INCLUDING A PROCESS FOR THE PREPARATION THEREOF AND APPLICATIONS THEREFOR BACKGROUND OF THE INVENTION 1. Field Of The Invention: This invention relates to new ceramic electrostrictive compositions, a process for the preparation of the compositions into an electrostrictvie material and specific applications for the electrostrictive material. 2. Description Of The Prior Art: The electrostrictive effect relates to a lattice phenomenon in the crystal lattice of materials. Particularly, the electrostrictive effect comes from the direct attraction and repulsion of anions and cations in a crystal lattice, said attraction and repulsion resulting from the application of an external electric field. The attraction and/or repulsion in the crystal lattice results in a physical distortion of the lattice. This lattice distortion causes a displacement or strain in the material. Stated quantitatively, the strain in an electrictive material is proportional to the electrostrictive coefficient multiplied by the square of the electric field. The relationship between strain and polarization and the relationship between strain and dielectric constant are shown below in Equations la and lb, wherein S = strain, Q =electrostrictive coefficient, K = dielectric constant (relative permittivity at the applied field), ∑° = permittivity of free space, P = polarization and E = applied electric field: s = p2 [Equation la] S = Q(K ∑o ) - ~ - [Equation lb] ultrasonic generators (e.g., sonar, ultrasonic cleaning), pick-ups and sensors (e.g. , record players), resonators and filters (e.g., radio, television), and the like. However, on the high temperature side of T Q (i.e, the n B" portion of Figure 1),. the material is para- electric (i.e. , the material is isotropic). When the material is parael ectri c, it typically has a cubic crystal structure. It is the paraelectrie phase of particular ceramic compositions which is the focus of t i s appli cati on. Lead magnesium niobate (hereinafter referred to as PMN) was discovered to be erroelectric as early as 1961. In approximately 1980, the elec ros tr i cti on pheno¬ menon was discovered to exist in PMN. However, pure PMN has a T c which occurs at approximately -20°C. Moreover, the dielectric constant for PMN is very temperature dependent which means that a slight deviation from the temperature at which T Q occurs leads to a large change in the dielectric constant. Thus, attempts have been made to modify the pure PMN system with dopants. For example, PMN has been doped with between 8-10 mole percent lead titanate (hereinafter referred to as PT). By doping PMN wi h PT, thereby resulting in a PMN-PT solid solution, the T c was raised to a hi her temperature and the result¬ ing dielectric constant for the PMN-PT solid solution was higher than the dielectric constant of pure PMN. Thus, the strain which can- be developed in the PMJ-PT system is higher than the strain which can result in the pure PMN system because the dielectric constant for the PMN-PT system is higher at TQ, Accordingly, because strain is proportional to the square of the dielectric constant in an electrostrictive composition, a small change in dielectric constant can have a dramatic impact on the resultant strain in the material. Thus, PMN doped with 8- 10 mole percent of PT results in a material which has a Q closer to room temperature. This shifting of T is very important for the following reasons. If an electrostrictive material is to be used at room temperature, it is desirable for that material to exhibit a T Q at or near room temperature. This permits maximization of the dielectric constant and thus maxi- mϊzation of strain. However, for many applications a constant environmental temperature, such as room tempera¬ ture, cannot be assured. Thus, in reference to Figure 1, it is clear that for known electrostrictive materials the dielectric constant is largely dependent upon te per- ature. Accordingly, all previously known applications for electrostrictive materials have been restricted to environments which have a substantially constant temperature. However, if it is desired to use elec¬ trostrictive materials for applications in which constant environmental temperatures cannot be assured, then the known prior art compositions are inadequate. For example, in reference to Figure 1, if the environmental temperature to which the electrostrictive material is exposed changes as little as 20°C, the dielectric constant could be reduced in value by 1/3- 1/2. This would be undesirable from an application per¬ spective because such changes in dielectric constant would result in fluctuating measurements. Alternatively, the calibration or control devices connected to the elec- trostrictive material would have to be very complex to compensate for such fluctuations in dielectric constant, otherwise, any measurements taken would be unreliable. In other words, all known electrostrictive compositions suffer from the drawback that even if a high dielectric constant can be achieved for a particular temperature, if the environmental temperature varies slightly from T Q , then, the dielectric constant also varies resulting in an unreliable system. There are also known various manu acturing processes for the formation of an electrostrictive ceramic material. Such processes include the conven- tional mixing of oxides. However, as is the case in many el ectrocerami e compositions, many investigations continue in an effort to determine an optimum process for forming electrostrictive ceramic materials. SUMMARY OF THE INVENTION The present invention has been developed in view of the foregoing and to overcome the deficiencies of the prior art. It is an object of the invention to provide a novel electrostrictive ceramic composition which maxi¬ mizes dielectric constant and minimizes the dependence of the dielectric constant on temperature at a temperature near the Curie Point Temperature (i.e. , TQ). It is a further object of the invention to provide a fabrication process for reliably obtaining the new electrostrictive material. Inves igations have focussed upon the lead magnesium niobate-lead titanate (PIVN-PT) solid solution system which is doped with small weight percentages of barium titanate (hereinafter referred to as BT) and/or strontium titanate (hereinafter referred to as ST). The P1VN-PT- (BT,ST) solid solution system has been found to have a TQ between 0° and 60 β C depending upon the precise composition of the system and, more importantly, shows a decreased dependence of the dielectric constant on temperature. Thus, for temperatures near TQ, the dielec¬ tric constant does not vary as much as it does in known electrostrictive compositions. Moreover, in the novel PMN-PT-(BT,ST) compos i- tion, within the temperature range of 0°C to 60 β C, the material remains substantially completely par ael ect r i c. It is important for PMN-PT- (BT, ST) to be paraelectric in the temperature range of 0 β to 60 β C because if the mater¬ i l was ferroelectric rather than paralectric, any appli- cation of an external electric field would result in at least a partial poling of the material (i.e., an orienta¬ tion of domains within the material). If poling were to occur, when an external field was switched, domain walls would have to move in accordance with the applied field. Such domain wall movement causes friction in the mater¬ ial. This friction is not desirable because it leads to hysteresis in the material. Hysteresis is not a problem in PMN-PT- (BT, ST), because the material remains paraelec- tric end domain orientation does not occur to any signif¬ icant extent. BRIEF DESCRIPTION OF THE DRAWINGS T e invention is described in detail with reference to the accompanying drawings, wherein: Figure 1 is a representative curve of the dependence of dielectric constant on temperature for a typical prior art pi ezoelectri c-electrostri cti ve mater- ial; Figure 2A is a ternary diagram showing the preferred and more preferred composition ranges of the present invention; Figure 2B is a ternary diagram showing the preferred, more preferred and most preferred composition ranges of the present invention; Figure 3 shows the effect of doping a 0.88 PMN - 0.12 PT system with 2% BT and 4% BT; Figure 4 shows the effect of doping a 0.88 PMN - 0.12 PT system with 2% ST and 4% ST; Figure 5 shows a super impos i ti on of each of the 1kHz dopant curves shown in Figures 3 and 4 and compares same with undoped 0.88 PMN - 0.12 PT; Figure 6 shows the effect of doping a 0.88 PRfrϊ- 0.12 PT system made with high purity oxides with 2% BT and compares same with undoped 0.88 PM3 - 0.12 PT made with high purity oxides; Figure 7 shows the loss tangent data at 1 kHz as a function of temperature for undoped 0.88 PMϊ-0.12 PT and 0.88 PMN-0.12 PT doped with 2% BT and 4% BT; Figure 8 shows the loss tangent data at 1 kHz as a function of temperature for undoped 0.88 PMN-0.12 PT and 0.88 PMN- 0.12 PT doped with 2% ST and 4% ST; Figure 9 shows the loss tangent data at 1 kHz as a function of temperature for undoped 0.88 PIV_N-0.12 PT 5 made with high purity oxides and 0.88 PMN-0.12 PT made with high purity oxides and doped with 2% BT; Figure 10 shows the relationship between strain and applied electric field at room temperature for: undoped PMN, undoped 0.88 PMN - 0.12 PT, and 0.88 PMN- 10 0.12 PT doped with 2% BT and with 4% BT; Figure 11 shows the relationship between strain and applied electric field at room temperature for: undoped PMN, undoped 0.88 PMN-0.12 PT, and 0.88 PMN - 0.12 PT doped with 2% ST and 4% ST. l~ Figure 12 shows the relationship between strain and applied electric field at room temperature for: undoped 0.88 PMN-0.12PT made with high purity oxi es and 0.88 PMN-0.12 PT made with high purity oxides and doped wi th 2% BT; 0 Figure 13A shows an X- ay diffraction pattern taken of a pellet surface having the composition 0.98 (0.88 PMN - 0.12 PT) - 0.02 BT; Figure 13B shows an X-ray diffraction pattern taken from a pellet surface having the composition 0.96 (0.88 PMN - 0.12 PT) - .04 BT; 5 Figure 14A shows an X-ray diffraction pattern taken from a pellet surface having the composition 0.98 (0.88 PMN - .12 PT) - 0.02 ST; and Figure 14B shows an X- ray diffraction pattern taken from a pellet surface hav¬ ing the composition 0.96 (.88 PMN - .12 PT) - .04 ST; 0 Figure 15 shows an X-ray diffraction pattern taken of a pellet surface having the composition 0.98 (0.88 PMN-0.12PT)-0.02 BT made with high purity oxides. DETAILED DESCRIPTION OF THE INVENTION The solid solution system of PMN-PT-(BT,ST) was examined in an effort to obtain a desirable electrostrictive composition. Table A illustrates the preferred, more preferred, and most preferred compositional ranges of the present invention. Figure 2A shows the broad, preferred, and more preferred composition ranges of the present invention, while Figure 2B shows the preferred, more preferred, and most preferred ranges. TABLE A COMPOSITIONAL RANGES (IN WEIGHT PERCENT) choosing an electrostrictive composition, there is a trade-offf which must occur in the selection process. For example, when either of the dopants of BT or ST are added to the PMN-PT system, these dopants will reduce the dielectric constant. It is desirable for the dielectric constant to be reduced as little as possible so that the strain properties of the material are not sacrificed. However,it has been discovered that addition of these same dopants desirably broadens out or flattens the peak which occurs at TQ. Still further, it has been unexpectedly discovered that the addition of BT or ST to PMN-PT actually lowers T c (i.e., shifts TQ to a lower temperature). This is quite unexpected because an artisan of ordinary skill would expect the addition of BT or ST to the PMN-PT system to follow normal mixing rules. Specifically, each of BT and ST has a TQ which is higher than T c for PMN-PT. Thus, if this system followed the normal mixing rule, one would expect TQ to be shifted to a higher temperature rather than a lower temperature. This unexpected shift in TQ has not yet been explained. It is clear that the balancing or trade-off of the various electrical properties in an electrostrictive material is an important issue. -9- TABLE la Compo- si ti on Dens i ty % Net No. Formula ( g/ cc) Wt Loss 1 0.88 PMN - 0.12 PT 7.12 1.8 2 0,98(0.88 P_vN-0.12PT)-0.02 BT 7.27 0.3 3 0.96(0.88 PMN-0.12PT)-0.04 BT 7.49 2.2 4 0.98(0. ' 88 PMN-0.12PT)-0.02 ST 7.50 1.7 5 0.96(0.88 PMN-0.12PT)-0.04 ST 7.43 3.0 The ceramic compositions recited in Table la were obtained by the following procedure. Starting powders of Pb(Mgl/3Nb2/3 )0 3 (Cabot-Lot 47 "wet cake" purchased Dec. 1986) a fine grain size PbTi0 3 (Cabot) and Puratronic grade (99.999 + %) PbO and Tiθ2 were mixed together with either BaC0 3 or SrC0 in stoi chi ometr ic proportions. Either barium carbonate or stronium carbonate was utilized depending upon which of barium or stronium was desired to be used as a dopant. The "Lot 47 wet cake" was purchased from Cabot in December 1986 and it contains lead, magnesium and niobium compounds made by a solution precipita ion process, said compounds being present in near stoi chi ometri c propor¬ tions to obtain the desired final composition. The amount of "wet cake" used has to be adjusted for the loss of non-oxide constituents which burn-off during calcin¬ ing. Therefore, adjustments must be made to the mixture to compensate for such burn-off. For example, after the wet cake has been calcined, only 68.62 weight % oxi es remain. Thus, for every 100 grams of "wet cake", 68.62 grams of oxides will result after calcining (i.e. , 68.62 grams of PNN material). The resultant powder mixture is present in stoi chi ometr i c proportions. This mixture is then wet milled for approximately 30 hours in water for homogeni zat i on, with Darvon C being added as a di s- persant. The approximate distribution of starting mater¬ ials for the wet milling step is 38 wt % powder mixture, 51 wt % 2 πm diameter Zr0 2 grinding balls, 2 wt % Darvon C and 9 wt % water. The wet mixture was then dried and calcined in platinum crucibles for approximately 3 hours at approximately 750?C. The calcined powder which results has a particle size averaging approximately 1 micron in size. This particle size is due primarily to the size of the starting powder particles. After calcin¬ ing, a second wet milling step occurs. This second wet milling occurs with 1/2 inch Zr0 2 grinding balls and the wet milling is performed in freon. This second wet mill- ing also occurs for approximately 30 hours. After the second wet milling step, PVA binder (Air Products #205) was dissolved in distilled water to form a liquid mix¬ ture. The calcined powder was then added to the liquid mixture. The powder-binder mixture was stirred on a hot plate until all the water had evaporated. The hot plate was held at a temperature of approximately 95-130°C, preferably above 100°C, and the elapsed time of exposure on the hot plate was approximately 6 hours. The dried mass was then ground to - 100 mesh by using an agate mortar and die pressed under a pressure of about 5000 psi to form a compact. The compacts were then i sos tat i cal ly pressed at about 45,000 psi to form pellets. The iso- statically pressed pellets were placed in covered alumina crucibles packed in a powder of a substantially similar chemical composition to the pellets. This step is important because when the pellet-powder mixture was heated in the closed alumina crucibles, the atmosphere created in the alumina crucibles was similar in chemical composition to the chemical composition of the pellets, thereby preventing volatilization of elements such as Pb, and thus preventing deviation of the ceramic from stoichiometry. The alumina crucibles were then heated for binder burnout at a heating rate of approximately 40 C C per hour until a temperature of 600 β C was reached. The pellets were then heated at 900 β C per hour until a temperature of 980 β C was reached. The pellets were main¬ tained at approximately 980°C for about 3 hours followed by a cooling of the furnace. The cooling of the furnace occurs for about 3-5 hours. Another embodiment of the present invention substitutes high purity oxides for the "wet cake" start- ing mater i als. (Hereinafter, an * preceding a composition signifies that the composition was prepared using hi h purity oxides instead of "wet cake" star ti ng mater i al s) . TABLE lb Compo- sition Density % Net No. Formula (q/ccl Wt Loss 6 *0.88 PMN - 0.12 PT 7.82 1.8 7 *0.98 (0.88 PMN - 0.12 PT)-0.02 BT 7.66 1.7 8 *0.96 (0.88 PMN - 0.12 PT)-0.04 BT 7.82 1.5 9 *0.98 (0.88 PMN - 0.12 PT)-0.02 ST 7.85 1.7 10 *0.96 (0.88 PMN - 0.12 PT)-0.04 ST 7.79 1.7 The ceramic compositions recited in Table lb were obtained by the following procedure. Starting powders of MgO and Nb2θg were mixed together and then wet milled in freon using substantially the same milling conditions as applied to the "wet cake" materials. The wet mixture was then dried and calcined in platinum crucibles for approx¬ imately 4 hours at approxima ely 1000 β C, to for M N 2θ g . This high temperature calcining step enhances the reac¬ tion between MgO and Nb 2 0 5 . The calcined M Nb Og powder is then mixed with starting powders of Ti0 2 , PbO and either BaC0 or SrC0 in stoi chi omet r i c proportions. This mixture is then wet milled in freon using substan¬ tially the same milling conditions as described above. The resultant mixture is then dried and calcined in plat¬ inum crucibles for approximately 4 hours at approximately 800 β C. The calcined powder which results has a particle size averaging approximately 1 micron in size. This particle size is due primarily to the size of the start¬ ing powder particles. After calcining, a third wet mill¬ ing step occurs. This third wet milling occurs with 1/2 inch Zr0 2 grinding balls and the wet milling is performed in freon. This third wet milling also occurs for approx¬ imately 30 hours. After the third wet milling step, PVA binder (Air Products #205) was dissolved in distilled water to form a liquid mixture. The calcined powder was then added to the liquid mixture. The powder-binder mixture was stirred on a hot plate until all the water had evaporated. The hot plate was held at a temperature of approximately 95-130 β C, preferably above 100 β C, and the elapsed time of exposure on the hot plate was approx- imately 6 hours. The dried mass was then ground to -100 mesh by using an agate mortar and die pressed under a pressure of about 5000 psi to form .a compact. The compacts were then i sos tat i cal ly pressed at about 45,000 psi to form pellets. The isos tati cal ly pressed pellets were placed in covered alumina crucibles packed in a powder of a substantially similar chemical composition to the pellets. The alumina crucibles were then heated for binder burnout at a heating rate of approximately 40°C per hour until a temperature of 600 β C was reached. The pellets were then heated at 900 β C per hour until a temperature of 1150 β C was reached. The pellets were maintained at approximately 1150 β C for about 3 hours followed by a cooling of the furnace. The cooling of the furnace occurs for about 3-5 hours. In addition, further compositions within the scope of the invention were prepared. These compositions are set forth in Table lc. TABLE lc Composition No. Formula 11 * 0.975(0.846 PMN-0.154 PT)-0.025 BT 12 * 0.975(0.872 PMN-0.128 PT)-0.025 BT 13 * 0.975(0.897 PMN-0.103 PT)-0.025 BT 14 * 0.975(0.923 PMN-0.077 PT)-0.025 BT 15 * 0.95(0.895 PMN-0.105 PT)- 0.05 BT 16 * 0.95(0.921 PMN-0.079 PT)- 0.05 BT 17 * 0.95(0.947 PMN-0.053 PT)- 0.05 BT 18 * 0.90(0.625 PMN-0.375 PT)- 0.10 BT 19 * 0.90(0.722 PMN-0.278 PT)- 0.10 BT 20 * 0.90(0.806 PMN-0.194 PT)- 0.10 BT 21 * 0.975(0.846 PMN-0.154 PT)-0.025 ST 22 * 0.975(0.872 PMN-0.128 PT)-0.025 ST 23 * 0.975(0.897 PMN-0.103 PT)-0.025 ST 24 * 0.975(0.923 PMN-0.077 PT)-0.025 ST 25 * 0.95(0.895 PMN-0.105 PT)- 0.05 ST 26 * 0.95(0.921 PMN-0.079 PT)- 0.05 ST The range of densities for the doped materials made with "wet cake" was 7.27-7.50 g/cc (theoretical = 8.1 g/cc). It was determined that the weight loss during sintering averaged approximately 1.8 weight %. X-ray diffraction analysis of each of the compositions verified that a 100% peroyskite phase was present. The presence of a 100% perovskite phase is shown clearly in Figures 13A and 13B for BT doping; and in Figures 14A and 14B for ST doping. The use of high purity oxides instead of the "wet cake" resulted in an increased density of 7.66 g/cc for PMN-PT doped with 2% BT. In general, compositions of the present invention made with high purity oxides may range in density form about 7.6 g/cc to about 8.0 g/cc. The presence of a 100% perovskite phase is shown clearly in Figure 15 for 2% BT doping of *0.88 PMN-0.12 PT. The fired pellets of material containing "wet cake" and material containing high purity oxides were then formed into discs having two parallel flat surfaces, (i.e., if any warpage occurred during firing, grinding is performed on the pellets to create two opposite parallel- ly located flat surfaces). After taking the density measurements, the flat surface of the discs were gold sputtered to create electrodes thereon. The edges of each pellet were masked so that gold did not sputter thereon. Silver paint was then painted on top of the gold sputtered layer. After the silver paint had dried, measurement of the electrical properties of the material could then be made. To determine the electrical properties of the discs as a function of temperature, the silver painted discs were placed in a Delta Designs Environmental Chamber. The environmental chamber is a closed system which isolates the discs from the external atmosphere and varies the temperature of the internal atmosphere of the chamber between -70 * C (using liquid nitrogen) and 315°C, in order to achieve appropriate measurement temperatures. The electrical properties of compositions of the present invention are shown in Table 2, which also clearly shows that barium titanate addition to the "wet-cake" compositions reduces TQ by • approximately 5.3'C/mole % of BT. There is also a reduction in the dielectric constant of approximately 1169/mole % of BT. Strontium titanate additions also showed a similar effect of decreasing T Q , however, the decrease was 8.3 * C/mole % of ST. Also, there was a decrease in the dielectric constant of 1422/mole % of ST. Also, attention is drawn to the ΔT . values which correspond to the "relaxor effect" in the electro¬ strictive materials. Par icularly, T c for each of the samples increases by a particular amount as the frequency is increased between 1 and 100 kHz. Table 2 also shows the effect of substituting high purity oxides for the "wet cake" starting materials. The increased dielectric constant in compositions 7-10, compared to compositions 2-5, can be partially attributed to the increased density resulting from the use of high purity oxide starting materials. Strontium titanate and barium tiianate were selected as dopants because the effect of "A" site sub¬ stitution in the perovskite structure was desired to be studied. Particularly, Pb 2+ has an ionic radius of 1.20A. Sr 2+ has an ionic radius of 1.13A and Ba 2+ has an e ionic radius of 1.35A. Accordingly, the strontium and barium readily substitute into the Pb 2+ sites (A sites) in the AB0 perovskite structure. In addition to the reduction of T c , Figures 3 and 4 graphically display the change in dielectric con¬ stant as a function of temperature with the additions of BT and ST. Particularly, in reference to Figure 3, it is clear that the dielectric constant decreases for each of the three frequencies tested, namely, 1 kHz, 100 kHz and 1000 kHz, as the amount of BT which is added to PMN-PT increases. However, the peak broadening which occurs with the addition of 2% BT (i.e., 2 wt % BT), and which occurs even more dramatically with the addition of 4% BT (i.e. , 4 wt % BT) is very important. It is this peak broadening which results in a hi gher ' temper atur e stabil¬ ity for the dielectric constant and thus, will result in an increased range of use temperatures for devices made using compositions 2-3. Thus, for the first time, an electrostrictive composition has been discovered which has an improved temperature stability relative to all known electrostrictive compositions. Consequently, the potential applications in which electrostrictive devices could be used has been dramatically increased. Figure 4 shows a similar graphic depiction of the dependence of dielectric constant on temperature with ST being added instead of BT. However, the general trend for the addition of ST to the PΛN-PT solid solution system is similar to that shown in Figure 3. Particular¬ ly, T shifts to lower temperatures and decreases with the addition of ST at 2%, and decreases further with the addition of 4% ST. Likewise, the addition of ST also results in a peak broadening at TQ. It should be noted -17- In addition to the reduction of TQ, Figures 3 and 4 graphically display the change in dielectric con¬ stant as a function of temperature with the additions of BT and ST. Particularly, in reference to Figure 3, it is clear that the dielectric constant decreases for each of the three frequencies tested, namely, 1 l&z, 100 kHz and 1000 kHz, as the amount of BT which is added to PMN-PT increases. However, the peak broadening which occurs with the addition of 2% BT (i.e. , 2 wt % BT), and which occurs even more dramatically with the addition of 4% BT (i.e., 4 wt % BT) is very important. It is this peak broadening which results in a higher temperature stabil¬ ity for the dielectric constant and thus, will result in an increased range of use temperatures for devices made using compositions 2-3. Thus, for the first time, an electrostrictive composition has been discovered which has an improved temperature stability relative to all known electrostrictive compositions. Consequently, the potential applications in which electrostrictive devices could be used has been dramatically increased. Figure 4 shows a similar graphic depiction of the dependence of dielectric constant on temperature with ST being added instead of BT. However, the general trend for the addition of ST to the PIVH-PT solid solution system is similar to that shown in Figure 3. Particular¬ ly, TQ shifts to lower temperatures and l^ aχ decreases with the addition of ST at 2%, and decreases further with the addition of 4% ST. Likewise, the addition of ST also results in a peak broadening at TQ. It should be noted that temperature measurements for ST substitution in the PMN-PT solid solution system occurred only as high as 50°C because any further temperature measurements were deemed unnecessary. Figure 5 shows a direct comparison of the dependence of dielectric constant on temperature at a frequency of 1 kHz for undoped 0.88PMN-0.12PT and the dependence of the dielectric constant on temperature lor that temperature measurements for ST substiiutioh in the PMN-PT solid solution system occurred only as high as 50 β C because any further temperature measurements were deemed unnecessary. Figure 5 shows a direct comparison of the dependence of dielectric constant on temperature at a frequency of 1 kHz for undoped 0.88PMN-0.12PT and the dependence of the dielectric constant on temperature for each of compositions 2-5 (i.e. , the curves of Figures 3 and 4 are superimposed on each other to create new Figure 5). It is observed that the addition of 2% ST and 2% BT produce a similar dependence of dielectric constant with temperature. However, the addition of 4% ST substantial¬ ly reduces the dielectric constant when compared to the addition of 4% BT. The precise reasons for this differ¬ ence are not yet understood. However, as a general statement, it is clear that with dopant levels of either BT or ST being as small as 2%, peak broadening occurs. Moreover, it is clear that with 2% dopant levels, TQ shifts to a lower temperature. As previously discussed, the shift in TQ to a lower temperature is completely unexpected because an artisan of ordinary skill would normally expect the addition of dopants such as ST and BT to cause a shift in TQ to a higher temperature and not a shift to a lower temperature. The precise mechanism for the shift in TQ is not yet understood. Figure 6 shows a direct comparison of the dependence of dielectric constant ' on temperature at a frequency of 1 kHz for undoped *0.88 PMN-0.12 PT and the dependence of dielectric constant on temperature for *0.88 PMN-0.12 PT doped with 2% BT. It is observed that the use of high purity oxides instead of "wet cake" increase the dielectric constant and shifts Tc to a lower temperature. Table 3a shows the percent deviation of the dielectric constant from a room temperature value of the dielectric constant for compositions 1-7 at 1 kHz. TABLE 3 a Temperature Stability of PMN-PT- (BT, ST) Percent Deviation of Dielectric Dieleetr i c Cons tant Compo- Constant at (at 1 KHZ) Relative si tion 0 β C/25 β C7 to 25 β C Over a Range No. Formula Tc(T*C)/50°C of 0 to 50 C C 1 0.88PMN-0.12PT 4983/10,875/ (undoped) 18,968(53.8)/ -54 to +72 18713 2 +.02 BT ' 7537/12, 706/ 16,259(44.2)/ -41 to +28 15932 3 +.04 BT 9300/13,840/ 14,291(33)/ -33 to +3 13030 4 +.02 ST 9163/15,413/ 17,544(37)/ -41 to +14 15,827 5 +.04 ST 10,445/13,170/ 13280(20.6)/ -21 to + 0.8 10,732 6 0.88 PMN-0.12 PT 7318/13,574/ -46 to + 72 23,387(54.3)/ 22,987 7 + 0.02 BT 8888/13,531/ -34 to + 10 16701(42.2)/ 14717 Table 3a displays illustrative quantatative values which show the superior temperature stability of the claimed electrostrictive composition. The present invention should not be interpreted as limited to the specific values displayed in Table 3a. Table 3b shows the percent deviation of dielectric constant from a room temperature value of the dielectric constant for compositions 8-27 at 1 kHz. Figures 7-9 show the loss tangent data as a function of temperature for compositions 1-7 at 1 kHz. It is clear from each of these Figures that the loss in each of compositions 2-5 and 7 is completely acceptable TABLE 3b Temperature Stability of PMN-PT-(BT,ST) Percent Deviation of Dielectric Constant (at 1 khz) 10 15 20 within the desired use range of 0-60 β C. Accordingly, losses are not a problem in the new electrostrictive compos i ti ons. Table 4 shows strain at 1 megavolt per meter for various compositions of the present invention. Figure 10 graphically displays the relationship between strain and applied field at room temperature. It is clear from Figure 10 that composition 2, which corre¬ sponds to 2% BT being added to the 0.88 PIVW-0.12 PT system, exhibited superior strain results relative to composition 3, which corresponds to doping 0.88PIW-0.12PT -22- with 4% BT. The strain data was obtained by connecting the silver painted pellets to an LVDT gauge (linear vari¬ able differential transducer) and thereafter applying an electric field to each sample. The electric field was varied between 0 and slightly greater than 2.5 megavolts/ meter. All measurements of strain as a function of elec¬ tric field were performed at room temperature. It is also noted in Fig. 11, that the relationships for strain versus field for compositions 4 and 5 (ST doping) are similar to those discussed above (i.e., 2% ST doped 0.88PMN-0.12PT yield superior strain results, as compared to 4% ST doped 0.88PMN-0.12PT). Figure 12 graphically displays the relationship between strain and applied field at room temperature for composition 7. It is observed that the use of high purity oxides instead of "wet cake" enhances the strain versus electric field characteristic of *0.88 PMN-0.12 PT doped with 2% BT. Referring to Equation lb, changes in electro- strictive strain due to variation in temperature are related to changes in the dielectric constant (K), which is the only temperature dependent variable in the right side of the equation. Increased additions of BT and ST result in reduced temperature dependence on K, i.e., strain, as evidenced by broader Curie peaks for those materials. Figures 13A and 13B are X-ray diffraction patterns taken from surfaces of pellets corresponding in composition to compositions 2 and 3, respectively. It is clear from Figures 13A and 13B that a 100% perovskite phase resulted. Likewise, Figures 14A and 14B are X-ray diffraction patterns taken from surfaces of pellets corresponding in composition to compositions 4 and 5, respecti vely. These Figures also show that a 100% perovskite phase was produced. Figure 15 is an X-ray diffraction pattern taken from the surface of a pellet corresponding in composition to composition 7. This Figure also shows that a 100% perovskite phase was pro¬ duced. Due to the discovery of these novel electro¬ strictive composi ions, new applications for electro- strictors can now be explored. Specifically, as previously discussed, known electrostrictive compositions could not be utilized for applications which involved an exposure to a range of temperatures due to the severe dependence of the dielectric constant on temperature. The peak broadening which has been discovered and which is due to the addition of BT or ST to the PMN-PT system, now permits electrostri ctors to be used for a lar e range of appl i cat i ons . The new electrostrictive compositions can be used for all prior art applications such as dot matrix printers, microelectronic measurement devices, and the like. Additionally, the new electrostrictive composi¬ tions can be utilized in many different areas never before possible, due primarily to the absence of hystere¬ sis in the new compositions and the peak broadening. Thus, great accuracy (i.e., reliability) of the composi¬ tions for various applications, even when exposed to varying temperature ranges, is to be expected. Such applications include the use of the new elec ros rictive compositions as actuators in active sonar, underwater vehicle guidance and robotics. Many of these applications involve the use of the electrostrictive device in combination with a motion amplifier. The dis¬ placement in a small electrostrictive device, by itself, is typically not large enough (e.g. a few tens of microns) to result in an appreciable displacement of another object. Accordingly, it is sometimes necessary to connect electrostrictive actuators to appropriate motion amplifiers. Stated in greater detail, electrostrictive actuators can be used in active sonar devices because they generate a very accurate sound wave (i.e. , due to the absence of hysteresis). Electrostrictive actuators can also be used for underwater vehicle guidance, such as in torpedos. Speci ically, screws and gears are typical¬ ly being used to control the rudders of torpedos. The screws and gears are inherently not accurate and are quite heavy. Thus, electros tri cive actuators, when coupled with appropriate motion amplifiers, could provide for very accurate control of rudders in a torpedo. It is also foreseeable that electrostrictive actuators could be used in robotics. Specific ways that electrostrictive actuators could be used in robotics include the control of movement such as the fingers or a wrist of a robot arm; the actuators can be used in combination with a hydraulic system for opening and closing valves to con¬ trol flow of fluid therethrough; and the actuator can be used as tactile feedback sensors (i.e. , electros rictive actuators may be used to provide tactile stimulation of a controllers hand, or finger, to create a sensation relat¬ ing to how hard a robot hand is gripping an object). Additionally, electrostrictive actuators may be used to control the opening and closing of valves thereby replac¬ ing less accurate solenoids. It is also foreseeable that electrostri ctors could be used for measuring electrical properties of electrical chips. For example, it is some¬ times necessary to move a measuring device a small amount (a few tens of an augstrom to a few microns) to check for various electrical properties in a formed chip. Such movements could be controlled by an electrostrictive actuator. This list of potential applications should not be considered to be totally inclusive, but rather, repre- sentative of uses for electrostrictive compositions. While the present invention has been disclosed in its preferred embodiments, it is to be understood that the invention is not limited to the precise disclosure contained herein, but may otherwise be embodied with various changes, modifications and improvements which may occur to those skilled in the art, without departing from the scope of the invention as defined in the appended claims.

Claims: -26- WHAT IS CLAIMED IS 1. An electrostrictive ceramic composition comprising a solid solution consisting essentially of magnesium niobate, lead titanate and at least one dopant selected from the group consisting of barium titanate and strontium titanate, said composition being defined by the following formula: (W) Pb (Mg 1/3 Nb 2/3 )O 3 *(X)PbTiO 3 - (Y)Z, wherein 55 wt.%<.Wi.97χ, 2.5 wt.%^X<_45%, Z is at least one material selected from the group con¬ sisting of barium titanate and strontium titanate. 2. The composition of claim 1, wherein Z con¬ sists of barium titanate. 3. The composition of claim 1, wherein Z con¬ sists of strontium titanate. 4. An electrostrictive ceramic composition comprising a solid solution consisting essentially of lead magnesium niobate, lead titanate and at least one dopant selected from the group consisting of barium titanate and strontium titanate, said composition being defined by the following formula (W) Pb (Mg 1/3 Nb 2/3 )0 3 * (X)PbTi0 3 - (Y)Z, wherein 82 wt.% ^94 wt.%, 4 wt.%χXll8 wt.% 0.5 wt.% _Yi 6 wt.%, and Z is at least one material selected from the group con¬ sisting of barium titanate and strontium titanate. 5. The composition of claim 4, wherein Z con¬ sists of barium titanate. 6. The composition of claim 4, wherein Z con- sists of strontiim titanate. 7. An electrostrictive ceramic composition comprising a solid solution consisiting essentially of lead magnesium niobate, lead titanate and at least one dopant selecteα from the group consisting of barium titanate and strontium titanate, said composition being defined by the following formula (W) Pb (Mg 1/3 Nb 2/3 )0 3 " (X)PbTi0 3 * (Y)Z, wherein 84.5 wt.%<W_<86.2 wt.%, 11.5 wt.%^X_< 11.8 wt.%, 2.0 wt.%<.Y_< 4.0 wt.%, and Z is at least one material selected from the group con¬ sisting of barium titanate and strontium titanate. 8. The composition of claim 7, wherein Z con¬ sists of barium titanate. 9. The composition of claim 7, wherein Z con- sists of strontium titanate. 10. The composition of claim 8, wherein W = 86.2 wt.%, X = 11.8 wt.%, and Y = 2.0 wt.%. 11. The composition of claim 8, wherein W = 84.5 wt.%, X = 11.5 wt.%, and Y = 4.0 wt.%. 12. The compos i ti on of cl aim 9 , wher e i n W = .86. 2 wt.%, X = 11. 8 wt .%, an d Y = 2. 0 wt.%. 13. The compos i ti on of cl aim 9 , wher e i n W = 84. 5 wt.%, X s 1 1. 5 wt.%, and Y = 4. 0 wt.%. 14. A sintered electrostrictive ceramic composition consisting essentially of a solid solution of 96-98 weight percent of (0.88 lead magnesium niobate - 0.12 lead titanate) and 2-4 weight percent of a dopant selected from the group consisting of - barium titanate and strontium titanate, said sintered electrostrictive -28- ceramic composition having a crystal phase of approximately 100% perovskite. 15. The composition of claim 14, wherein said composition exhibits a temperature dependency of a dielectric constant of said composition as shown in graphs 2, 3, a and b of Figure 5. 16. The composition of claim 14, wherein a dielectric constant of said composition is within -41% to +28% of a 25 β C dielectric constant value of said compos ϊ- tion over a temperature range of 0 β C to 50 β C. 17. The composition of claim 14, wherein a dielectric constant of said composition is within -41% to +14% of a 25 β C dielectric constant value of said composi¬ tion over a temperature range of 0 β C to 50 β C. 18. The composition of claim 14, wherein a dielectric constant of said composition is within -33% to +3% of a 25 β C dielectric constant value of said composi¬ tion over a temperature range of 0 C C to 50 β C. 19. The composition of claim 14, wherein a dielectric constant of said composition is within -21% to +0.8% of a 25 β C dielectric constant value of said compo¬ sition over a temperature range of 0 β C to 50°C. 20. The composition of claim 14, wherein the composition has a maximum dielectric constant of approximately 13,280-17,544. 21. The composition of claim 14, wherein the sintered composition has a density of approximately 7.27- 7.50 g/cc. 22. The composition of claim 14, wherein the sintered composition has a maximum Curie temperature of approximately 20.6 β C-44.2 β C. 23. A sintered electrostrictive composition consisting essentially of a solid solution of 98.0-99.5 weight percent of (0.88 lead magnesium niobate - 0.12 lead titanate ) and 0.5-2 weight percent of a dopant consisting of barium titanate, said sintered electostrictive ceramic composition having a crystal phase of approximately 100% perovskite. -29- 24. The composition of claim 23, wherein said composition exhibits a temperature dependency of a dielectric constant of said composition as shown in graph A of Fi gure 6. 25. The composition of claim 23, wherein a dielectric constant of said composition is within - 34% to + 10% of a 25 β C dielectric constant value of said composition over a temperature range of 0°C to 50°C. 26. The composition of claim 23, wherein the composition has a maximim dielectric constant of approximately 16,701. 27. The composition of claim 23, wherein the sintered composition has an average density of approximately 7.66 g/cc. 28. The composition of claim 23, wherein the sintered composition has a maximum Curie temperature of approximately 42.2 β C. 29. A method of making an electrostrictive material comprising: mixing together powders of Pb(Mg j / 3 Nb 2 / 3 )0 , PbTi0 , PbO, Ti0 2 and at least one dopant material selected from the group consisting of SrC0 and BaC0 in desired stoi chi ometri c proportions to form a mixture; wet milling said mixture; drying and calcining said mixture at 800°C for 4 hours to form a calcined powder; wet milling said calcined powder; forming a binder-distilled water mixture; adding said calcined powder to said binder- distilled water mixture to form a slurry; evaporating all water from the slurry to form a dried mass; grinding the dried mass to -100 mesh pow¬ der; dry pressing said -100 mesh powder to form compacts; i sos ta i cal 1 y pressing said compacts to form pellets; packing the pellets into a powder having substantially the same chemical composition as the pellets; and firing the pellets. 30. The method of claim 29, wherein the powders are mixed in appropriate proportions to result in a solid solution of an electrostrictive ceramic composition com¬ prising a solid solution of 55-97 weight percent of lead magnesium niobate, 2.5-45 weight percent of lead titanate and 0.5-20 weight percent of a dopant selected from the group consisting of barium titanate and stron¬ tium titanate. 31. The method of claim 29, wherein said wet milling of said mixture occurs for approximately 30 hours. 32. The method of claim 29, wherein said wet milling of said mixture is effected by combining about 38 weight percent of said mixture, 51 weight percent of zirconia grinding balls, 2 weight percent Darvon C and 9 weight percent H 2 0. 33. The method of claim 29, wherein said zir¬ conia grinding balls have a diameter of about 2 mm. 34. The method of claim 29, wherein said wet milling of said calcined powder is effected by combining said calcined powder with freon and about 1/2 inch diame¬ ter zirconia grinding balls. 35. The method of claim 29, wherein said firing of the pellets occurs by heating the pellets at 40 β C/hour until reaching a temperature of about 600 β C and there- after heating the pellets at 900°C/hour until reaching a temperature of about 980 β C. 36. The method of claim 29, wherein the pellets are fired at a temperature of about 980°C for about 3 hours. 37. A sintered electrostrictive ceramic pro¬ duced according to the method of claim 29, wherein the sintered electrostrictive ceramic has a density of approximately 7.27-7.50 g/ cm 3 and a Curie temperature of approximately 20.6-44.2°C. 38. A sintered electrostrictive ceramic composition produced according to the method of claim 29, wherein the sintered electostrictive ceramic consists essentially of solid solution of 96-98 weight percent of (0.88 lead magnesium niobate - 0.12 lead -titanate) and 2-4 wt % of a dopant selected from the group consisting of barium titanate and strontium titanate, said sintered electrostrictive ceramic composition having a maximum dielectric constant of approximately 13280-17544. 39. The composition of claim 38, wherein a dielectric constant of said composition is within -41% to +28% of a 25 β C dielectric constant value of said composi¬ tion over a temperature range of 0 β C to 50°C. 40. The composition of claim 38, wherein a dielectric constant of said composition is within -41% to +14% of a 25 β C dielectric constant value of said composi¬ tion over a temperature range of 0 β C to 50 β C. 41. The composition of claim 38, wherein a dielectric constant of said composition is within -33% to +3% of a 25 β C dielectric constant value of said composi¬ tion over a temperature range of 0 β C to 50 β C. 42. The composition of claim 38, wherein a dielectric constant of said composition is within -21% +0.8% of a 25 β C dielectric constant value of said compo¬ sition over a temperature range of 0 β C to 50 β C. 43. The sintered electrostrictive ceramic com¬ position of claim 38, wherein the sintered composition has a density of approximately 7.27-7.50 g/ cm . 44. A method of making an electrostrictive material comprising: mixing together powders of MgO and b 2 Og in desired stoi chi ometr i c proportions to form a mixture; wet milling said mixture; drying and calcining said mixture to form a calcined powder; mixing together said calcined powder, with starting powders of Ti0 2> PbO and at least one dopant material selected from the group cnosisting of BaC0 and SrC0 3 in desired stoi ehiometric proportions to form a second mixture; wet milling said second mixture; drying and calcining said second mixture to form a second calcined powder; wet milling said second calcined powder; forming a binder-distilled water mixture; adding said second calcined powder to said binder-distilled water mixture to form a slurry; evaporating all water from the slurry to form a dried mass; grinding the dried mass to form a -100 mesh powder; dry pressing said -100 mesh powder to form compacts; i sos tati cally pressing said compacts to form pellets; packing the pellets into a powder having substantially the same chemical composition as the pellets; and firing the pellets. 45. The method of claim 44 , wherein the powders are mixed in appropriate proportions to result in a solid solution of an electrostrictive composition comprising a solid solution of 55-97 weight percent of lead magne¬ sium niobate, 2.5-45 weight percent of lead titanate and 0.5-20 weight percent of a dopant selected from the group consisting of barium titanate and strontium titanate. 46, The method of claim 44 , wherein said wet milling of said mixture occurs for approximately 40 hours. .47. The method of claim 44, wherein said wet milling of said calcined powder is effected by combining said calcined powder with freon And about 1/2 inch diameter zirconia grinding balls. 48. The method of claim 44, wherein said firing of the pellets occurs by heating the pellets at 40 β C/hour until reaching a temperature of about 1150°C. 49. The method of claim 44, wherein the pellets are fired at a temperature of about 1150°C for about 3 hours. 50. A sintered electostrictive ceramic composition produced according to the method of claim 44, wherein the sintered electrostrictive ceramic comprises a solid solution of 96-98 weight percent of (0.88 lead magnesium niobate -0.12 lead titanate) and 2-4% of a dopant selected from barium titanate and strontium titanate.

Inventors: BAILEY ALEX EUGENE, RITTER ANDREW PAUL, SUTHERLAND AUDREY ELIZABETH, WINZER STEPHEN RANDOLPH, BAILEY, ALEX, EUGENE, RITTER, ANDREW, PAUL, SUTHERLAND, AUDREY, ELIZABETH, WINZER, STEPHEN, RANDOLPH
Applicants: MARTIN MARIETTA CORP, MARTIN MARIETTA CORPORATION
Application year: 1988
Main classification: H01L41
IPC codes: C04B35, H01L41