US3464785A - Process for producing ordered complex perovskite-type crystals with high dielectric constants - Google Patents

Description

Sept. 2, 1969 s. F. GALASSO 3,464,785

PROCESS FOR PRODUCING ORDERED COMPLEX PEROVSKITE-TYPE CRYSTALS WITH HIGH DIELECTRIC CONSTANTS Filed Dec. 14, 1965 2 Sheets-Sheet l l IIIHH I Hllllll I I IIHH] I I I lllll] l IIHHII 106 EKJ/JT/V/T/ f p l nnuq I IHHH] IIIIHH] Z20 fi & 600 00 /006 /Zfl0 TEMPf/PA 7.065 G l l I Sept, 2, 1969 Filed Dec. 14, 1965 s. F. GALASSO 3,464,785

PROCESS FOR PRODUCING ORDERED COMPLEX PEROVSKITE-TYPE CRYSTALS WITH HIGH DIELECTRIC CONSTANTS 2 Sheets-Sheet 2 United States Patent O W PROCESS FOR PRODUCING ORDERED COMPLEX PEROVSKITE-TYPE CRYSTALS WITH HIGH DI- ELECTRIC CONSTANTS Salvatore F. Galasso, Manchester, Conn., assignor to United Aircraft Corporation, East Hartford, Conn., a

corporation of Delaware Filed Dec. 14, 1965, Ser. No. 513,735 Int. Cl. C01g 35/00; Bld 9/00; H01]: 3/02 US. CI. 23-51 3 Claims ABSTRACT OF THE DISCLOSURE Perovskite-type single crystals of the formula 'o.3a o.s7) s wherein A is barium or strontium and B is magnesium, calcium, zinc, nickel or cobalt are prepared by mixing powders of a compound yielding the oxide of A with Ta O and the B metal oxide and then mixing the powders with a flux of the formula AF wherein A is as defined above. The resulting mixture is heated to a temperature above the melting point of the flux and after holding for at least 0.5 hours is gradually cooled to effect crystal formation.

This invention relates in general to materials characterized by high, temperature-stable dielectric constants. It contemplates the production of ordered complex perovskite-type crystals with dielectric constants which are relatively unaffected thermally up to temperatures as high as 800 C.

In the electronics industry the more common capacitor compounds which are dielectnically temperaturestable, exhibit dielectric constants on the order of approximately 10. Those materials, including the ferroelectrics such as barium titanate, which possess dielectric constants several orders of magnitude larger than the usual capacitor compounds, are known to be temperature sensitive. A need exists in the industry for capacitor insulators displaying both high dielectric constants and reasonable thermal stability, particularly in the higher temperature ranges.

Accordingly, it is a fundamental object of this invention to provide materials exhibiting high electrical resistivity properties over a wide temperature range.

A further object of this invention is to provide ordered complex perovskite-type crystals characterized by high dielectric constants which are substantially independent of their environment up to temperatures of 800 C.

An additional object is to provide methods of producing crystals of high electrical resistivity from compounds identified by the formula A(B B )O particularly those characterized by the formula A(B Ta )O These and other objects and advantages of this invention will be set forth in the following description or will be evident therefrom or from practice of the invention.

FIGURE 1 is a graph of log resistivity versus temperature for a representative Ba(Mg Ta )O single crystal.

FIGURE 2 is a graph of dielectric constant versus temperature for a Ba(Mg Ta )O single crystal.

The perovskite structure is recognized in the art as that adopted by many ABOg-type compounds in which large A ions and oxygen ions form close-packed layers with small B ions in the octahedral holes between the oxygen ions. Investigations have been made of a large number of new compounds of the general formula A(B' B" )O where B and B are two different elements in the octahedrally coordinated cation position of the perovskite structure. Experiments have confirmed that many com- Patented Sept. 2, 1969 pounds indentified by the formula A(-B' B )O form in an ordered perovskite structure. Inorganic Chemistry, 2, 482 (1963). Complete ordering has been found in those materials of the A(B' Ta )-O type, which can :best be described as a hexagonal unit cell containing three close-packed BaO layers with the B and Ta ions arranged in the octahedral holes.

Compounds were prepared with the divalent B ions selected to obtain compounds with differences ranging from 0.01 to 0.52 in the radii of the B and Ta ions, and where A was a barium or strontium ion. The structural data for the A(B Ta )O compounds prepared, including cell sizes, ionic radii of the divalent B ions, and the difference in ionic radii of the B ions is presented in Table I.

TABLE I [Structure data for A(B' Ta 0 compounds] Diff. in ionic Ionic radii of radii of B Compound Cell size, A. B ion, Ax ions, A.

go.as au.e1) s 0. 67 0. 0 0.s3 0.01) 0a 69 01 (G u.aa -o.e1)Pa 73 05 0.aaT8a.c-1) 03 74 06 o.asTao.e1)Oa 12 (Cdu.aaTao.o1)0a (1:4.167 97 29 M maa M'DOa 99 31 b0.aa o.o1)Oa a=4.250 1. 20 52 goas o.u1) 03 0. 67 01 S (N u.aaTao.o1)Oa 69 01 '(C 0.3a 0.67)O3 .-{Z:;$ .73 .05 s 0.asTau.l7)O3 {g:g;gg .74 .06 S (Cao.aaTao.o1)0s 99 31 Single crystals of the general type A(B Ta )O' were prepared and were found to possess unique electrical properties. As representative of these unique properties, reference is made to FIGURES 1 and 2 in the drawings. Single crystals of the compound,

for example, where A is a barium ion and B is a magnesium ion, exhibit an electrical resistivity greater than 10 ohm/cm. at C., maintaining a resistivity of more than 10 ohm/cm. up to 800 C. Further, as best seen in FIGURE 2, this crystal is characterized by a dielectric constant of approximately 500 which is relatively independent of temperature over the above-mentioned temperature range. This was completely unexpected inasmuch as similar electrical resistivity measurements in powder compacts of substantially the same composition did not display equivalent electrical properties. While the exact reasons for the superior electrical properties of the crystal are not known, a comparison of the respective X-ray patterns associated with the various crystals showed that the perovskite pseudo-cells were less distorted in the crystal. It is postulated that the presence of a small amount of fluoride ion in the crystals produced, the fluoride ion being present in the flux in which the crystals were grown, is a contributing factor.

The preferred technique for growing crystals of the A(B Ta )O' type is as follows, the production of the Ba(Mg Ta )O crystals being specified for the sake of simplicity:

Reagent grade BaCO Ta O and MgO were dry-mixed in the approximate proportions to obtain stoichiometric compositions with the amount of fluoride flux as indicated in- Table II; The fluoride flux, BaF was selected in the preparation of the-Ba(B' Ta )O3 type crystals, instead of the more commonly used KF, PbO- or P130- PbF fluxes, in order to minimize the amount of cation impurities introduced into the' crystals..Wliile BaCOgwas used in the preparation of the crystalsdiscussed, other compounds; such as Ba'NO from which barium oxide may be derived, are use'able in the process. Similarly, although MgO was used in the preparation of the Ba(Mg Ta )0 crystals,- in other instances the diva? lent metal oxide, BO, was used; corresponding to the B ion which: was to be included in the compound prepared. The carbonatesg,nitrates ancl other sources of B'O are also useable in the preparation of these compounds.

The mixtures were placed in 50ml. covered platinum crucibles and heated to a temperature approximately 100 C. above the meltingpointof. the flux. Soaking at temperature was'efiec'tedfor periods ranging from 0.5 to 8.5 hours, depending on the composition of the mixture. Upon completion of soaking, the-crucibles and contained samples were gradually cooled over a temperature range of approximately 400 CI. and. the resulting crystals-Were mechanically extracted from the flux. The specific soaking temperatures, soaking times, sample Weights, flux rates and cooling rates are given in' Table IT. While the tabled conditions represent the best of numerous experiments designed to increase the size of the crystals, it will be. understood that these conditions are representative only. Appropriate variations in these parameters are well within Crystals prepared in accordance with the abovedescribed' techniqueswere'nearly cubic in shape and varied in color from yellow to" green. Selected crystals from each batch were analyzed using X-ray diffraction tech-- niques and photographs: were taken using a 57.3 min; radius Philips X-ray' camera and" high intensity copper Ka radiation, and the presence of ordering and cell sizes were determined.

For the purpose of investigating. the electrical resistance and capacitance propertiesv of the crystals, gold electrodes were placed on two faces of the largest crystals and the specimens were-mounted-between opposing platinum electrodes in a resistance-heated furnace. The DC. resistance of each crystal was then measured as a function of temperature by. monitoring the current while a constant voltage was applied across the faces. Capacitance measure ments were obtained with a Boonton Model 74C-58 capacitance bridge using a frequency of 100 kc. In the electrical measurements made, no hysteresis eifect was uncovered".

The high resistivities andlarge dielectric constants of these crystals, which were seen to be essentially invariant with temperature, make them attractive as dielectrics in electronic microcircuitry which may be subjected to temperature variations andtemperature extremes.

While the present invention has been d'escribedin connection with several preferred embodiments and conditions, no limitation is intended thereby except as defined in the following claims.

What is claimed is:

1. The method of preparing ordered complex perovskite-type crystals of the general formula holding the mixture at the soaking temperature for at least 0.5 hour;

gradually cooling thev mixture to effect crystal formation;

and extracting the crystals from the flux.

2. The method of preparing ordered complex perovskite-type crystals identified by the formula having a high tempe'rature stabl'e dielectric constant comprising:

mixing powders consisting of I B'aCO ,Ta O and MgO in the proportions to obtain a stoichiometric composition according to the formula Ba(Mg Ta )O' mixing the powders with a BaF flux;

heating the mixture in a covered platinum container to a soaking temperature" above the melting" point of the flux;

holding the mixture at the" soaking temperature for gradually cooling the mixture to effect c'rystal'forn'iation;

and extracting the crystals from the flux.

3'. The method of claim 2 in which the mixture is heated to a soaking temperature approximately C. above the melting point' of the flux, and held at the soaking temperature for approximately 8.5 hours, and the mixture is gradually cooled over a temperature range'of approximately 400 C. to effect crystal formation.

References Cited UNITED STATES PATENTS HERBERT T. CARTER, Primary Examiner US. Cl. XJR. 25263.5

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