Barium oxide-rare earth oxide-titanium dioxide based ceramics exhibiting isotropy

Ceramics resulting from the oxide system barium oxide-rare earth oxide-titanium oxide have been found to exhibit an isotropy with respect to electrical properties such as the temperature coefficient of frequency at the first resonant frequency, dielectric constant, and, to some extent, the loss factor, Q. Such anisotropy effects reproducibility in fabricating ceramic articles for use in the microwave region and in the performance of these articles. Isotropic ceramics from the same ternary oxide system can be made by compacting non-nucleated powders followed by the usual sintering of the green compact. Anisotropic bulk ceramic workpieces can be machined to reproducibiy afford ceramic articles with the appropriate value of the electrical property in question by measuring the components of the electrical property along the three principal axes of the workpiece, and then determining the angles between the principal axes necessary to give a resultant having the preselected value.

BACKGROUND OF THE INVENTION 
Ceramics which are prepared from oxides of the system barium oxide-titanium 
oxide-rare earth oxide, where the rare earth is one of the lanthanum 
series, have found broad use in components used in the microwave region of 
the electromagnetic spectrum, especially as filters. Recently it was 
observed that certain electrical properties, such as the temperature 
coefficient of frequency, T.sub.f, were highly variable, depending upon 
the forming process used and the method of sampling the ceramic for 
electrical testing. Since reproducible electrical properties are a 
prerequisite in providing ceramics for the microwave industry these 
observations were understandably vexing and led to a program whose purpose 
was the elucidation of the origin of the observed variability. To our 
surprise we discovered that barium oxide-rare earth oxide-titanium oxide 
ceramics made via prereacting (calcining) a suitable powder, forming using 
uniaxial pressing, and sintering exhibited anisotropy with respect to the 
electrical properties in question. Although anisotropy in other ceramics 
has been recognized previously, there is no hint or suggestion of 
anisotropy in the class of ceramics under discussion. Once the existence 
of anisotropy was observed as the source of the problem of variability in 
electrical properties, two avenues of addressing the problem became clear. 
One avenue involved the preparation of the same class of ceramics but with 
the anisotropy eliminated. The second avenue accommodates anisotropy by 
first recognizing and measuring its degree, and then machining the 
apparatus along appropriate dimensions to reproducibly and consistently 
furnish ceramic articles reflecting the desired values of selected 
electrical properties. Quite related to the latter is the production of 
ceramic articles having the desired values of selected electrical 
properties maximized or minimized. In turn these are merely specific 
manifestations of the more general characteristic of reproducibly 
obtaining a preselected set of values of electrical properties from a bulk 
ceramic workpiece exhibiting anisotropy with respect to the electrical 
properties of interest. 
SUMMARY OF THE INVENTION 
The purpose of this invention is to reduce or eliminate difficulties 
arising from anisotropy in barium oxide-rare earth oxide-titanium dioxide 
based ceramics with respect to electrical properties in the radiofrequency 
and microwave portion of the electromagnetic spectrum. One embodiment 
comprises the preparation of isotropic ceramics by pressing or extruding 
unreacted, non-nucleated powders of the suitable oxides followed by 
sintering at a temperature in the range of about 1250.degree. to about 
1450.degree. C., most preferably at 1300.degree.-1380.degree. C. Another 
embodiment is a process for machining the desired article from an 
anisotropic bulk ceramic workpiece at that angle, defined by the principal 
axes of the workpiece and components of the anisotropic electrical 
property along each principal axis, so as to give a ceramic article with 
the desired value of the electrical property as measured by a defined 
sampling method.

DESCRIPTION OF THE INVENTION 
It often is the case that the most perplexing and difficult portion of a 
technological problem is the recognition of the underlying cause or nub of 
the problem. Once the basis of the problem has been recognized, it 
frequently is relatively straightforward to devise means to solve the 
problem, to avoid the problem, or in some cases even to utilize some 
features of the problem to achieve new results. Our invention is but 
another in a long line of such instances and can perhaps be best 
appreciated by exemplifying the characteristics of the underlying problem. 
It can be mentioned here that we were sufficiently fortunate to be able to 
incorporate some of the underlying bases of the problems into novel 
methods of fabricating ceramic articles used in the radiofrequency and 
microwave region. 
One class of ceramics which enjoy broad use in components, especially 
filters, which are used in the microwave region is that arising from 
mixtures of BaO:Ln.sub.2 O.sub.3 :TiO.sub.2 which, after reaction, form a 
region of solid solutions extending from mixtures whose mole ratio of 
oxide components is 3:2:9 through 1:1:4 to about 2:2.3:9, where Ln 
represents a member of the lanthanum rare earth series, i.e., elements of 
atomic number 57-71. Within this region the ceramics have relatively high 
values of the dielectric constant, E', of about 60-110 and show a rather 
moderate loss factor, Q, in the range of 1,000-4,000 at frequencies on the 
order of 3 GH.sub.z. Our journey begins with the observation that a powder 
of composition 1:1:4, where the rare earth was samarium, upon calcination 
(prereaction, or presintering) at 1200.degree. C., uniaxial pressing, and 
finally sintering at 1300.degree.-1380.degree. C. could yield a ceramic 
with highly variable T.sub.f depending upon the forming process used and 
on the method of sampling the ceramic for electrical testing. 
EXAMPLE 1 
Five large, dimensionally precise pucks, or cylinders, of dimension 6 cm 
across by 3 cm high were formed from a barium oxide-samarium 
oxide-titanium oxide powder whose components were in the mole ratio of 
1:1:4.5, each at a different uniaxial pressure. The pucks were cut in 
half, and one set of halves was isostatically overpressed at greater than 
100 MPa. All ten pieces were densified at 1330.degree. C. to give 
materials of a density greater than 5.70 g/cc (&lt;2% porosity). Every 
half-cylinder was machined to obtain a minimum of 2 orthogonal sets of 
test parts for T.sub.f measurement using cavity methods and the transverse 
electric first resonant frequency mode, TE.sub.01.delta. (D. Kajfez and P. 
Gullion, "Dielectric Resonators," Artech. House, Norwood, Mass., 1987 
(539)). The results are summarized in. FIG. 1, which shows that the 
ceramics in question exhibit anisotropy in T.sub.f that widens in 
magnitude as uniaxial pressure increases. Isostatically overpressed parts 
retain the anisotropy imposed by the initial uniaxial pressing. Ceramics 
that are isostatically pressed only do not display anisotropy and yield 
T.sub.f .apprxeq.0.+-.1 ppm/.degree.C. Anisotropy in E' was examined using 
the same test pieces and the parallel plate method (E. Courtnay, IEEE 
Trans. Mic. Th. and Tech., MTT-18, 476 (1970)). Small variations near the 
limit of error were observed, so a more sensitive but indirect method was 
developed; refer to FIG. 2. Cubes containing the uniaxial pressing 
direction as a unique 4-fold axis were precisely extracted and machined 
from all half-puck ceramics. These were coupled to a 50 .OMEGA. stripline, 
and the resonant frequency (F.sub.r) was measured at six different cube 
orientations, i.e., each 4-fold axis, (+) and (-) direction, brought to 
the vertical position. If E' is isotropic, F.sub.r should remain constant 
for all cube orientations. Table 1 shows an E' anisotropy of -0.4 units, 
and the unique uniaxial pressing direction is easily discernable. 
TABLE 1 
______________________________________ 
Anisotropy by the Cube Technique 
Axis.sup.1,2 
F.sub.r (MHz) 
______________________________________ 
A-A' 4635.1 
A'-A 4636.5 
B-B' 4646.5 
B'-B 4646.5 
C-C' 4646.5 
C'-C 4646.7 
______________________________________ 
.sup.1. A-A' indicates measurements taken in direction from A to A'- 
.sup.2. The A-A' axis is the pressure axis, i.e., direction in which 
pressure was exerted. 
The extensive data collected show that Sm-ceramics display anisotropy in 
T.sub.f of about 6 ppm/.degree.C. and in E' of 0.4-0.7 units. The 
magnitude of the anisotropy within the 1:1:4-1:1:5-1:1:5 (+2 weight 
percent TiO.sub.2) compositional region examined remains constant provided 
uniaxial pressure also is constant. This indicates that secondary phases 
(e.g. TiO.sub.2) contribute little to the anisotropy. The largest E' and 
most positive T.sub.f are observed when the TE.sub.01.delta. field during 
measurement is normal to the pressing direction. Almost within measurement 
error (.+-.5% maximum), Q seems to remain constant but systematic trends 
in the data suggest that a slightly lower Q is associated with the more 
positive T.sub.f orientation. The ceramic, whose measurement results are 
summarized in Table 2 contains 0.2 weight percent less TiO.sub.2 than the 
materials illustrated above. Measurements were made on samples extracted 
from the puck at different depths and at different orientations with 
respect to the pressure axis; see FIG. 3. Uniform but directional 
properties are clearly evident; note sample 8 which was extracted inclined 
to the pressing direction. 
TABLE 2 
______________________________________ 
Electrical Properties of Puck Samples 
Extracted According to FIG. 3. 
Sample Q (2.9 GHz) T.sub.f E' Density (g/cc) 
______________________________________ 
1 3480 .about.0.7 
80.1 5.72 
2 3525 .about.0.5 
79.6 5.71 
3 3410 .about.0.5 
79.7 5.72 
4 3710 .about.6.5 
79.2 5.71 
5 3710 .about.6.5 
79.2 5.71 
6 3710 .about.6.3 
79.4 5.72 
7 3670 .about.6.1 
79.4 5.72 
8 3700 .about.2.0 
79.5 5.72 
______________________________________ 
It also is known that many other oxides, such as lead and bismuth oxides, 
can be added to the barium oxide-rare earth oxide-titanium oxide ceramics 
of this invention in order to "tune" their properties. It therefore became 
not merely of interest but also important to determine whether these 
ceramics also exhibited similar anisotropy. 
EXAMPLE 2 
Several samples of dense, commercial 1:1:4.(Nd):PbO pucks were made 
available and these were tested as above. The pucks must have been pressed 
uniaxially because anisotropy in T.sub.f of 14 ppm/.degree.C. and in E' of 
-0.7 units are evident (Q.apprxeq.2300, 3 GHz). Bi.sub.2 O.sub.3 
-containing materials of the 1:1:4(Nd)-type (D. Kolar, Z. Stadler, S. 
Gaberscek, and D. Suvorov, Ber. Dt. Keram. Gas. 55, 346 (1978)) were 
fabricated by isostatic and by uniaxial pressing, then densified. The 
latter show anisotropy of 22 ppm/.degree.C. and .about.3 units in T.sub.f 
and E', respectively. Isopressed parts are relatively uniform, T.sub.f 
.apprxeq.10 ppm/.degree.C. and E'.apprxeq.91 (Q.apprxeq.1800, 3 GHz). 
Although we do not wish to be bound by any theory, and have not obtained 
the requisite experimental evidence to unambiguously support any 
particular theory, we have developed a working hypothesis and associated 
model which appears to account for much of our observations. First it is 
assumed that powder which has been heated at 1200.degree. C. 
(pre-sintering or proreaction temperature) and milled to an average 
particle size near two microns contains nuclei of appropriate composition 
that have acicular habit, that is, are pencilshaped. We then speculate 
that some fraction of the nuclei is aligned during uniaxial forming with 
their long axis (or axes) perpendicular to the pressing direction. Most of 
this alignment is accomplished at the lowest pressures. On the other hand, 
isostatic pressing does not cause any preferred orientation. But when 
isostatic pressing is applied to uniaxially pressed parts of low green 
density, the alignment of nuclei is preserved with only minor disruption. 
During densification, prooriented nuclei grow in the plain normal to the 
initial pressing direction at the expense of smaller particles and of 
adjacent secondary phases as chemical equilibrium is attained. The 
remainder of the ceramic also undergoes similar changes, but they are 
directionally random. The resultant product consists of a matrix of random 
crystallites plus a small volume fraction of oriented grains giving rise 
to the observed anisotropy in various electrical properties. 
This model assumes that nuclei of, e.g., 1:1:4 oxides exist in the 
processed powder. Therefore, their availability and concentration should 
be dictated by the magnitude of the initial calcination temperature. To 
test this, barium titanate, samarium oxide, and titanium dioxide (rutile) 
of a composition appropriate to give the 1:1:4 composition of the prior 
example were blended in alcohol, dried, and reblended. Samples of this 
powder, which of necessity contained no nuclei of the ceramic material. 
were pressed uniaxially and isostatically (90 MPa) and densified at 
1330.degree. C. Fired densities were 5.60 g/cc (&lt;4% porosity), sufficient 
to yield E'.apprxeq.77 and Q.apprxeq.3.2K (3 GHz). Both ceramics gave the 
same T.sub.f =-3.1 ppm/.degree.C. and the cube test showed no frequency 
variation indicating a complete lack of anisotropy. 
Where anisotropy in the ceramics of the barium oxide-rare earth 
oxide-titanium oxide class is intolerable, or even merely inconvenient, it 
can be avoided by using ceramics which are isotropic. These ceramics, of 
the same barium oxide-rare earth oxide-titanium oxide system, can be 
prepared merely by avoiding the formation of crystal nuclei prior to 
compaction. This can be done by pressing powders which have not been 
prosintered or proreacted, or which at the very least have been calcined 
at a temperature insufficient to cause nucleation. Compaction of the 
non-nucleated powders can be effected either by uniaxial or isostatic 
pressing at pressures which are typically in the region from 30 MPa to 
about 90 MPa, or by extrusion. The compacted pieces then can be sintered 
under usual conditions, which means temperatures in the range of about 
1250.degree.-1450.degree. C., but most usually at around 
1300.degree.-1380.degree. C., for a time which is typically on the order 
of 8 hours. The resulting ceramics will show densities indicating less 
than 5% porosity. Other electrical properties will be essentially 
unchanged from those found in the same ceramics exhibiting anisotropy, 
with the major functional difference being that all electrical properties 
will have the same value in every direction. 
On the other hand, the fact of anisotropy in the subject class of barium 
oxide-rare earth oxide-titanium oxide ceramics can be exploited, or at 
least accommodated, in several distinct ways, all of which have as their 
common theme that if a value, &lt;.alpha.&gt;, of an electrical property .alpha. 
is dependent on the direction in which the property is measured, .alpha. 
then can be considered a vector which is the resultant of its components 
along the three principal axes of the workpiece. For simplicity of 
discussion we will choose cartesian coordinates as our three principal 
axes, but it will be readily appreciated that another set of axes, such as 
those for a spherical coordinate system, also can be used with equal 
facility. 
If maintaining the reproducibility and constancy of the value, &lt;.alpha.&gt;, 
in any arbitrary but predefined direction is a prime consideration, then 
knowledge of the components of .alpha. along the three principal axes 
permits machining the workplace in such a way as to obtain ceramic 
particles exhibiting the same values of .alpha. along the predefined 
direction. This is more readily understood with reference to a specific 
example. FIG. 4 shows a cylindrical workpiece, or puck, manifesting 
anisotropy along its z axis. Let us exemplify .alpha. by T.sub.f, and 
assume T.sub.f =10 ppm/.degree.C. along the z axis and 0 ppm/.degree.C. 
along the x and y axes. Let us further assume for illustration that the 
desired value, &lt;.alpha.&gt;, in this case is T.sub.f =5 ppm/.degree.C. and 
that the ceramic article is a wafer where T.sub.f typically is measured 
along the direction given by the thickness of the wafer. Since in the 
workpiece the components of T.sub.f along the x and y axes are zero then 
&lt;T.sub.f &gt;=10 ppm/.degree.C. cos.theta. where, .theta. is the angle 
between the z axis and the plane of the x-y axis. Since it is desired that 
&lt;T.sub.f &gt;=5 ppm/.degree.C., cos.theta.=0.5. or .theta.=60.degree. . That 
is, the workpiece is cut so as to give wafers where the direction of wafer 
thickness is 60.degree. from the z axis. 
The example above can be generalized in order to obtain a preselected value 
or set of values of one or more electrical properties along a predefined 
direction in a final ceramic article which is fabricated from a bulk 
ceramic workpiece exhibiting anisotropy in the electrical properties of 
interest. The first stage in this method is to determine the principal 
axis of maximum anisotropy and to establish the value of the electrical 
properties in question along this principal axis. The value of the same 
electrical property needs to be established along the other two principal 
axes. Each of the measured values of the electrical property in question 
corresponds to the component of that property along the relevant principal 
axis. Knowing the direction along which the electrical property is to be 
measured in the final ceramic article, and recognizing that this is merely 
the vectorial resultant of the values found along the first, second, and 
third principal axes, one then can readily calculate the angles as defined 
by the three principal axes along which the anisotropic workpiece needs to 
be machined to afford the ceramic article with the predetermined 
electrical value or set of electrical values. By machining the workpiece 
we include such processes as cutting, grinding, polishing, and other 
analogous processes which will afford smaller ceramic articles of 
appropriate shape and dimension from the larger bulk workpiece. 
This same method is applicable not only to reproducibly obtaining a 
preselected value or set of values of some electrical property, but also 
may be used either to maximize or to minimize the value of at least 1 
electrical property along the direction in question. Among the electrical 
properties of interest are included the temperature coefficient of 
frequency, the temperature coefficient of capacitance, the dielectric 
constant, and the loss factor, Q. 
It should be clear that the methodology which is described above can be 
used not only to give articles with reproducibly precise values of a 
particular electrical property or set of properties, but also can be used 
to minimize loss (maximize utilization) in fabricating small ceramic 
articles from a bulk workpiece. Clearly, both are desirable outcomes and 
merely add to the advantages afforded by our methods.