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o
INORGANIC DIELECTRICS RESEARCH
PROGRESS REPORT NO. 1
Contract No. DA-36-039~3c-39lUi
November I, 1961 to February 1, 1962
Investigators (Full Time)
E. J. Smoke C. J. Phillips E. L, Kastenbein D- R. Ulrich R. C. Pitetti J. R. Smyth C. Breen
Signal Corps Research Program File No. 01007-PM-62-93-93
N. J. CERAMIC RESEARCH STATION Rutgers, The State University John H. Koenig, Director
Assistants (Part Time)
J. Thomson W. Hoagland J. Rubin J. Wasylyk J. Buckmelter
JISIA
«•*
Your comments, suggestions and criticisms on
the work reported would be appreciated. Longhand
notations on this sheet will suffice. Please
det^Sh and send to Dr. J. H. Kocnig, 6chool of
Ceramics, Rutgers, The State University, New
BrunswicP?, N.J.
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TABLE OF CONTENTS
ABSTRACTS.....................................
Part I - Low Loss Microwave Ceramic Dielectrics, Part II - Transparent Polycrystalline Ceramics.. Part III - Hot Extrusion........................
PART I - LOW LOSS MICROWAVE CERAMIC DIELECTRICS...
Page
i
I i
ii
1
Introduction. 1 I. Hot-Pressing. ........!!!..'!! I! 2 II. Investigation of Crystalline Phases Present...., 9
A. Introduction. 9 B. Literature Research. 10 C. Experimental Work.......................... ll D. Results... .................. . . 16
III. Summary. l8 Future Work , iQ
PART Ix - TRANSPARENT POLYCRYSTALL INE CERAMICS,,. 20
Introduct ion 20 Experimental Work s................ , '.„'. 21 Results». .,,,.,, 23 Future Work ?A
PART III HOT EXTRUSION 28
Introduct ion , Equ i pment . , Experimental Work Discussion of Results. Conclusions Future Work
23 29 31 3? 33 33
Part I
LOW LOSS MICROWAVE CERAMIC DIELECTRICS
Abstract
Three compositions in the La203-Al203-Si02 system were hot-
pressed and the electrical properties measured at several
frequencies and temperatures. A study was made of the crystalline
phases appearing in a number of compositions within the system.
Two lanthanum silicate compounds, lanthanum aluminate and mull He
were found in the various compositions. Attempts to form each
lanthanum silicate compound by itself are being made in order to
evaluate their electrical properties. This information will be
used in attempting to tailor bodies to meet the specific require-
ments of this project.
Part II
TRANSPARENT POLYCRYSTALLINE CERAMICS
Abstract
Specimens of pure alumina were pressed and sintered using a
two-fire procedure. For the initial firing, one set of specimens
was fired in air and one set was fired in argon. The second
firing of both sets was performed in argon. Bulk densities of
the two sets of specimens were measured and compared. Specimens
which were initially fired in air were less dense than those
fired wholly in argon.
Part III
HOT EXTRUSION
Abstract
The hot extrusion apparatus is described and its operation
explained. The results of three extrusion attempts are
described.
-1-
Progress Report - Part. I
LOW LOSS MICROWAVE CERAMIC DIELECTRICS
Introduct ion
The object of this problem is to develop low loss insulation
with a dielectric constant of 15 and a power factor not to exceed
O.OOOlj. at microwave frequencies and at temperatures between -SO
and +300CF. The dielectric constant is above that of the
recognized range for insulation.
In earlier work, two lanthanum alumino-si 1icate compositions
exhibited these desired values as determined at 1 megacycle and
room temperature. The compositions were LAS-2 (1.96 weight
percent of AI2O3) and LAS-5 (4-76 weight percent of AI2O3)1. Both
of these compositions were fired conventionaL/.after an involved
preprocessing procedure. The processing of these compositions
consisted of drying the LagOz for one day prior to batching, wet
ball milling the compositions for one day, calcining the
compositions three times, screening the material between each
calcining operation, pressing the desired pieces, and then slow
firing the pieces to vitrification. This processing of the
compositions took about one week to complete.
1. Inorganic Dielectrics Researfch, Progress Report No. 1, March 1, 1960
o Rutgers, The State University Signal Corps Contract No. DA-36-039-sc-89111.1
Efforts were made to reduce the processing time by two
methods: by a high temperature prefiring, and by a single cal-
cination in place of the three calcinations used previously. Both
of these attempts resulted in materials exhibiting critically
short firing ranges, porous bodies and, consequently, unrepro-
ducible and unreliable results. For this reason, hot-pressing
the compositions was tried and resulted in the production of
dense bodies of approximately the desired properties within at
least a portion of the frequency and temperature range.
This report deals with the results obtained by hot-pressing
and with analysis of the crystalline phases formed in fired
pieces of a number of compositions in the ternary system.
I. Hot-Pressinq
Since firing of these bodies by conventional means was not
successful without calcining three times prior to the final
firing, it was decided to try fabrication by hot-pressing. This
technique produced bodies that had 0.002 moisture absorption and
electrical properties that were near the desired values.
Three bodies were hot-pressed: K15, K15+1 and KI5+3. The
compositions of these bodies in weight and mol percentage are
given in Table I.
Table I
K15
K15+1
K15+3
LaaOa wt. % MoJ_. %
61.56 26.lt
61.32 26.3
60.67 25.i|
SiOz Wt. % Mol. _#
33«1U 65-0
32.93 6U.5
32.68 61).. 1
Al203 wt. % Mol. %
5.30
5-75
6.65
8.6
9.2
10.5
The LazOs is extremely hygroscopic and was treated for 20
hours at l[|.00oF to drive out the water. The materials were then
weighed, dry-mixed for two hours, and wet ball-milled for twenty
hours. This material was then dried and passed through a 100
mesh screen.
Hot-pressing of the screened material was performed in
1" ID x 3" OD x 3" high graphite dies which had been lined with
0,005" sheet molybdenum to minimize carbon migration. The
dielectric constant of composition K15 decreased the greatest
amount with an increase in frequency between 8.52 and 9.6^ KMC.
Graphic presentations of the change of the dielectric constant
with frequency and temperature are given in Figures 1 and 2 on
pages [(. and 5 respectively. The graphs indicate that the diel-
ectric constant is not very temperature sensitive at these fre-
quencies, but that there are large changes in the dielectric con-
stant with changes in the frequency.
Table II
Body
K15
K15 + 1
K15+3
Bulk Density (g/cm3) Frequency (KMC) _°F
U.13
U.20
i+.U
8.52 9.6^ 8.52 9.61|
8.52 9.62+ 8.52 9.6i+
8.52 9.6U 8.52 9.6lj.
Temp. Diel sctric oF Constant
72 13 9 72 10 .15
300 li+ 0 300 10 35
7P- 12 2 72 11 1
300 12 US 300 11 3
72 10 8 72 9.U8
300 10 8^ 300 9 8)4
•»H
u.
5
H
8 u 1—1
o w u
i
u
u c
cr CD I-,
o o OC\J
o o OOJ OI>-
vr\ t*>
U, tt,
o r-
U
(-1 iS\
00
-r-
^UB^SUOQ otj-^oatata
CO
u CO I
'T- r^ O
i vO r^
-< a t- • CD O > 3
C *-> D o CD (-■
-!-> -kJ cd a +-> o
(D en
H (- o
■>U <n
(D (0 O) c
oc; to
•5-
U u y y y
+ + lAUMA
Vof
V
u as
M a,
5
H l/i
O a h 3 cn
OS H cj U
o It] u
u\
CO
0
J- CM ^t -^ vD UA ^D O
0s OD O ('■
0 • x a
o o
1 1 ( ^ ^1 -H
a-
bk o ""-' 0)
o u o p r\j
(0 t- ^H
(U -ct CL ^-H
B O- CJ CD (- I
U
I
m o
>, 1 -►-> < •tH a to (a • cv a > 2
■»H
g ■i-> s o OJ h O -tJ *J ^-^ CO C
*J o CO o
c^ H
0 D. u O « O
U1 u r~*
W 03 OTC
4-J cn 3 •r*
OS lO
• After hot-pressing the specimens were machined to fit the
waveguide in the dielectrometer (0.250" x O.396" x 0.890"). When
this size specimen was placed in the waveguide of the dielectro-
meter, it was found that the length dimension (.250") was too
small to determine tan accurately. Tan Is given by:
Tan S = f TCK^A)
where F(K, d. A) depends on dielectric constant of the specimen,
specimen length, and wavelength in the waveguide. However, the
effect of d, the sample length, on F(K, d.^ ) is minor compared
to its effect on £* . The 4 X, then, produced by the specimen,
is approximately proportional to d and tan $ . Therefore, for
low loss specimens, d must be made large enough to produce a X
which is much larger than the 4 X caused by waveguide losses.
The dielectrometer specimens were also used to obtain
approximate measurements of the dielectric constant and power
factor at 1 megacycle and room temperature using the Q-meter. The
size and shape of the samples did not lend themselves to direct
measurement by the Q-meter so the data obtained had to be
corrected for edge effects. There were no calculated corrections
for the size sample used so that the values given in Table III
are only close approximations. The plus sign given next to the
dielectric constant indicates the given value is somewhat smalle'r
than the actual value would be if the sample was a size where
-7-
there would be no "edge effects." The minus sign, next to the
power factor indicates that the actual value is smaller than the
value given.
Table III
P. F. Body
K15
K15+.1
K15+3
K
+16.0
+23.8
+15.1|
-.00108
-.00058
-. 00081+
BHiA-Den s i_ty (g/cma)
k'18
k-20
k- 11
If the values for the dielectric constant calculated at 1
megacycle and room temperature are considered with the higher
frequency values at 720F, then much more Information can be ob-
tained. With body K15, there is a sharp decrease of the
dielectric constant in the higher frequency regions. With body
K15+3, there is also a greater decrease at the higher frequency
but not quite as great as body K15. Composition K15+1 appears
to have a more linear decrease in the dielectric constant over
the entire frequency range investigated. A graph of this infor-
mation appears in Figure 3 on page 8. New specimens are being
hot-pressed for more precise electrical measurements.
It was apparent that a small amount of carbon or molybdenum
had minratpH intn -ill +1-,^ v,„i. _, . »taveu »ntg nn tne not-pressed specimens. in all probabil.
ity, this migration increased the power factor of these bodies.
An attempt will be made to hot-press these bodies in ceramic dies
to eliminate this contamination.
Figure 3
VARIANCE OF DIELECTRIC CONSTANT WITH FREQUENCY AT ROOM TEMPERATURE
Frequency (KMC)
X KIS o K15+1 • K15+3
3 8.52 9.61| 10
Rutgers, The State University- Signal Corps Contract No, DA-36-039-sc-391,'1
The application of the hot-pressing technique has produced
results which approach the requirements of the project. Further-
more, if the results of the dielectric properties at 1 megacycle
can be held reliable, then we can see that there is an increase
in the dielectric constant with increasing alumina percentage up
to 9.2 mol percent and then there is a decrease In the dielectric
constant with greater percentages. With the power factor the
reverse is true. The power factor is high at 8.6 mol percent of
alumina, low at 9.2 mol percent and then a slight increase at
10.5 mol percent. These results can be made reliable only through
further experimentation with these compositions.
Ii. Investigation of Crystalline Phases Present
A. Introduction
Since compositions within the LaaOs-AlzC^-SIO2 system
exhibit promise of producing bodies satisfying the technical
requirements desired in this phase of work, a study of the
crystalline phases occurring was deemed desirable. If these
crystalline phases can be identified, attempts can be made to
produce bodies containing a maximum quantity of each given phase.
Evaluation of the properties of these phases should be extremely
fui »r, tallorina HOHIRS to meet the technical requirements of
the contract.
-10-
B= Literature Research
The preliminary research done for this investigation was the
accumulation of information on the various binary compounds. The
binary compound of alumina and silica is mullite (3Al203°2Si02).
This is a well explored binary and thus there is sufficient in-
formation dealing with the identification of the compound.
Two compounds exist in the lanthanum oxide-alumina system:
LazOs-AlzOs and LaaOa-12AI2O3. The 1:1 compound has a perovskite-
type structure and is stable from room temperature to its melting
point at 2130oC. The X-ray powder diffraction pattern has been
given in earlier reports. The 1:12 compound is similar in
structure to CaO-6Al203 and is stable to 1920oC, where it melts
p congruently .
A recent thesis by Warshaw-^ has shown that there are two
compounds in the lanthana silicate binary: La203-Si02 and
La203°2Si02• The compound La203=2Si02 is typical of the pyro-
silicates of the larger rare earths, comprising La, Ce, Pr, Nd,
Sm and Eu. There is a high and low temperature form of La203o2Si02
{ß and oc. respectively). The orthos i 1 icatc (La203'Si02) melts
congruently at 1980oC while the 1:2 compounds melts incongruently
at 1760oC forming the 1:1 compound and a liquid. As in the case
~.p T_ r\ .OCJA- i^^+K^^^im ^r^ + Vi^ir-ili/~'3+o oloo Vipc ti-fn no! ■vrmn r nh ^
2. Warshaw, Israel, Phase Equilibrium and Crystal Chemical Rela- tions in Rare Earth Systems, Department of Chemistry, Penn State U.
3« ibid.
n
■ 11.
with the transition temperature being 12750C.
The phase equilibrium diagram given by Warshaw, Figure I4. on
page 12, bears little resemblence to that published by Toropov
and Bondar^ which was given in earlier reports. Instead of
finding two compounds, they found only a 2:3 compound. This
difference would completely alter the phase equilibrium relation-
ships in the remainder of the system. Only the compounds reported
by Warshaw have been identified in the present study.
C. Experimental Work
A number of compositions were prepared to investigate the
compound formation occurring in the ternary system. The lanthanum
oxide was dried at 1^00oF for 20 hours before the oatches were
weighed. The compositions were then dry bail-milled for seven
hours. One inch discs were pressed and fired with no soak.
Vitrification and compound formation was obtained using draw
trials method. The fired specimens were crushed to pass through
a 200 mesh screen and then X-rayed.
The compositions used in this study are given in Table IV on
page 13 and their locations are shown in the two ternary diagrams.
Figures 5 and 6 on pages li^ and 15. Indicated also in these
i^.. Warshaw, Israel.
Q
0) la
Y \
\
\ s.
v O P
(0 "H
\ V
\ S,
X \
O CO
>
/
/
/
/ ^ Of
Cr
C.i
m
—i
+
\ r.
0J
,-J
O
C\J
—p- o o CO
-I—' o o
CM
I
-t-J I
Si o
ir
CM
TJ
U H
rj
•T-l
H
o N td
S
. o OO
o
- o ■^3
o
to o
o . o -^
CM
«0
(0 O HJ
o CM
o o o
o
o U0
{0o) aan^BJadmai
CO c
u 01 I
O-
O I
vO CO
>»Ji *» 3 -■ Q (/) u • Q> O > s C *->
^^ co t- -P *J cd C *-> O (/) O
JC ft
O •vU
a) to en c +J O) 3 -<
DC LO
-13-
Tabl e IV
Compound wt. La203 % Mol. % Wt.
Si 02 % Mol. % Wt.
Al. %
'.03 Mol. %
1 70 35.6 2k 56.0 6 8.U 2 y. 2i|.2 28 55.6 17 20.2
3 hb 17.8 30 5k-9 2 3 27.3
h 33. 3 11.3 33- 3 55-U 33. 3 33-3
5 75 U6.2 10 28.0 13 25.8
6 60 29.8 15 35.1 25 35.1
7 k5 18.5 23 k7.k 30 314.2
3 50 18.U U5 76.5 5 5.1
8A 50 16.1 ho 69.9 10 ll+.O
9 U5 16.5 ko 68.1 13 15.U
10 40 11|.6 35 59.5 23 25.8
.Ik-
Figure 5
LOCATIONS OF COMPOSITIONS IN WEIGHT PERCENTAGES
SiOz
IB.ZO3 AI2O3
W.eight Percent
Rutgers, The State University Signal Corps Contract No, DA-36-039-sc-89lUl
n
-15-
Figure 6
LOCATIONS OF COMPOSITIONS IN MOL PERCENTAGES
SiOz
/ \
Laz03°2Si02
LazOs-SiOz 50
LazOs LazOs'AlzOs
Mol Percent
LazOa«12AI2O3 AI2O3
Rutgers, The State University Signal Corps Contract No. DA-36-039-sc-891l4.1
n
ü
-16-
diagrams are the locations of the compositions that had been hot-
pressed. Comparing the two diagrams (one in weight percent and
the other in mol percent), it can be seen that the compositions
fall in the high lanthana area when using a weight percent diagram
but they fall in the low lanthana part of the ternary when using
mol percent» It has been postulated that if a ternary compound
would exist it would be in the low mole percent lanthana region .
The results of the X-ray analysis of this work indicated that
there were no ternary compounds in the area evaluated. Two
lanthanum silicate compounds, lanthanum aluminate, mullite and
the three starting oxides in various forms were found in different
compositions. The compounds found in the different compositions
are shown in Table V on page 17-
D. Results
As can be seen from Table V, there appears to be no ternary
compound. Furthermore, La203»2Si02 appears to have formed in
most of the compositions, but LazO^'SiOz appeared in only a small
number of compositions.
It will be noted that all three of the compositions (Kl5,
Kl5+lj and K15+3) on which electrical properties have been
obtained contain mullite, La203°2Si02 and l^Oa'A^Os. The
electrical properties of all of these phases except La2U3"2Si>-'2
are known. Therefore, it is important to attempt to produce
5. Warshaw, Israel, ibid.
-17-
Table V
Crystalline Phases
LazOs" ILazOa- ILazOa" ILazOa- LazOs' Body MuH i_te AlzO* 12AI2O3 2Si02 2SIO2
1 X
2 X
3 X
k X
5
6
7 X
8 X
8A X
9
10
K15 X
K15 + 1 X
K15+3 X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Si02 La2 Ol AlaOj
X X
X X
X X
x(?) X
X
X
X X
Crist.
X
X
X
s
X
K
X = indicates crystalline phase present
-18-
the latter compound and determine its electrical properties. If
the properties of both the 1:1 and 1:2 lanthana-si1ica compounds
can be determined, this information should be most helpful in
preparing bodies to meet the desired technical requirements.
HI. Summary
The dielectric properties of Kl5> K15+l> and K15+3 prepared
by hot-pressing at least approach the desired values. It appears
that these compositions are only slightly temperature sensitive
at the higher frequencies but are quite frequency sensitive. The
highest dielectric constant recorded was found using 9.2 mol
percent of AI2O3 and the lowest tan was also recorded at this
percentage. Any increase in the alumina content decreased the
dielectric constant and increased the tan
The compositional study of the ternary system revealed that
La203°2Si02 was the dominant compound formed in the areas studied.
Also formed within the ternary were Laz^s'SiOz, mullite, LaAlOs.
All of the compositions evaluated exhibit short firing ranges
which indicated that vitrified bodies will be extremely difficult
to form using the conventional firing techniques.
Future Work
1. Hot-press new specimens K15, K15+1 and K15+3 to evaluate
tan at microwave frequency.
2. Hot-press LazOs-SiOa and LazOs'ZSiOz to determine the
dielectric properties.
-19-
3- Hot-press ternary compositions and evaluate their
dielectric properties.
k- Use information obtained in 2" above to prepare compo-
sitions believed to have the most promise of fulfilling the
technical requirements of this project.
o
"•
-20-
Progress Report - Part II
TRANSPARENT POLYCRYSTALLINE CERAMICS
Introduction
The objective of this program is to produce a transparent
or highly translucent, polycrystal1ine5 single-oxide dielectric
of very high density and essentially void free. In addition to
transparency, the material should also possess the following
properties: a low dielectric constant, low power factor,
dielectric strength of 1+00 volts/mil for l/k" specimens, and high
mechanical strength.
As has been stated in previous reports, the preliminary work
in this investigation has been on alumina. Alumina was chosen for
this work since transparent, polycrystal1ine specimens of it have
been produced by other investigators1. Furthermore, a considerable
amount of information on the sintering of alumina is found in
the literature. Therefore, in starting with a study of alumina,
a material is being used which is not only known to be capable of
producing transparent, polycrystal1ine specimens, but about which
a maximum number of the conditions required for the production of
. . „„^ vr^.m rin^e examination of the transparent specimens ai^ nnuv—
I. Aus tralian Patent, "Improvements in or Relating to Transparent Alumina."
Rutgers, The State University Signal Corps Contract No. DA-36-039-sc-891^1
(>
-21-
parameters involved in production of transparent, polycrystal1ine
alumina should be extremely valuable in future work on other
oxides. The use of Isotropie crystalline materials should allow
production of specimens having a higher degree of transparency
than alumina since alumina is anisotropic.
Two methods of maturation arc planned: sintering and hot-
pressing. However, as of this date, only the former technique
has been studied. The transparent alumina previously mentioned
was fabricated by sintering. The aluminum oxide used was of high
purity, having only 0.3Ä of magnesia. Alpha alumina was the only
crystalline phase present. The crystal size after sintering ranged
from 15 to 65 microns.
In the present investigation, the procedure outlined in the
Australian patent was followed closely, with certain modifications.
One of these was that a different gaseous environment was used.
Experimental Work
The apparatus used for sintering has been described in the
previous report. A diagram of the sintering furnace appeared in
Figure 1 on page 38 of that report. It is essentially a water-
cooled, induction furnace, containing an induction coil, and a
molybdenum susceptor, surrounded by a quartz tube and Insulated
with zirconia sand as loose fill. The sample rests on a small
piece of molybdenum on a zirconia setter.
■ 22-
r The procedure as described in the above-mentioned patent
calls for two separate firings: First, a firing at a temperature
from 1650° to 1750oC for a period of from 50 to 300 minutes, to
eliminate most of the voids and gas containing pores; secondly
a firing at a temperature of from 1850° to 1950oC, for a period
of not less than fifteen minutes. The first firing may be done
in any atmosphere, while the body is always subjected to an
atmosphere of hydrogen in the second firing.
In the present program, argon was substituted for the hydro-
gen. There are several reasons why this was done. First, it was
felt that hydrogen could not be safely used with the present
set-up. Further, a literature review failed to reveal any
previous work utilizing an argon atmosphere. Hence, much new
information could be obtained in this area. Finally, the argon
serves to protect the molybdenum susceptor from oxidation.
Samples of pure alumina (99.9+%) in a colloidal form having
a maximum particle size of 0.3 microns, were pressed into compacts
l/2 inch in diameter and l/8 inch thick. Half were fired in a
gas-fired kiln in a slightly oxidizing atmosphere for one hour at
1700oC. The other half were fired in argon at the same temperature
and for the same length of time. Ail samples were then subjected
to a second firing to 19000C in an atmosphere of argon and held
for periods of 15, 60, and 300 minutes respectively.
o
-23-
Results
Densities, obtained with the samples of fired alumina are
listed in Table I on page 2^. The results are shown graphically
in Figure 1= As can be seen, the densities obtained were quite
low, and the specimens were not transparent.
The samples were all coated with a thin silvery metallic
layer, probably from condensation of vaporized molybdenum. How-
ever, the films were very thin and were easily removed by
grinding. The coatings did not appear to have diffused into the
specimens. Hence, the lack of transparency was undoubtedly due,
not to the contamination of the metal, but to the large percentage
of voids present in the fired piece. An examination of the table
reveals that the most dense sample obtained still contained voids
to the extent of nearly $%, In order to obtain transparency, less
than 0,$% of voids is believed to be necessary.
It is postulated that the presence of these vodds is due to
uninhibited grain growth. If grain growth is too rapid, gaseous
inclusions become entrapped within the crystals rather than at
crystal boundaries. Pores within individual crystals have been
found to very difficult to remove. Grain growth must be slowed
down enough to permit diffusion of interstitial pores from the
body. In order to inhibit this growth and to allow it to proceed
at a rate slow enough to permit elimination of the voids, a small
amount of some other material must be added. The next series of
samples is therefore being made up with 0,7% of magnesia added
to the alumina.
A.
-2h-
Tablc I
DENSITIES AND MOISTURE ABSORPTIONS OF SINTERED ALUMINA
Samples initially fired in air (1700oC)
Add itional Fire, 1900oC
Bulk Densi Lz
% True Densi ty
Moi sture Absorption
0 min. 3-67 92.0 0.88 15 " 3-71 93.0 0.11 60 " 3.75 9U.0 0,02
300 " 3-76 9U.5 0,02
B. Samples initially fired In argon (1700oC)
Additional Fire, 1900oC
Bulk Densi ty
% True Density
Moisture Absorption
0 min. 3.77 94-5 0,39 15 " 3.82 95.5 0,06 60 " 3.82 95.5 0,02
300 " 3.82 95.5 0.02
(Theoretical density of oC-alumina = 3.987)
n
-25-
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-26-
It is also planned to fire in an atmosphere containing
aluminum vapor. The purpose of the aluminum is to create
structural defects. The increase of vacancies in the lattice
should increase the sintering rate by enhancing the diffusion
of pores through the structure. It is hoped to thus be able to
eliminate the voids and produce theoretically dense bodies.
Table I reveals that those samples fired initially in argon,
have higher densities than those which were given an initial
fire in air. The significance of this is open to question at this
time since only a small number of specimens have been tested.
Polished sections of the specimens are now being made for micro-
scopic examination. This will permit determination of grain size
and amount, size, and location of pores which will be necessary
in evaluating the effects of different sintering conditions.
{
Future Work
1. Polished sections will be made of sintered specimens.
2. Specimens will be pressed with alumina containing a
small amount of MgO to inhibit grain growth, and will be fired in
the manner described above.
3- The furnace set-up will be altered slightly to permit
firing in an atmosphere containing aluminum metal vapor.
1|. A furnace is currently being built which will permit
firing in a vacuum or in various atmospheres. As soon as it is
finished, alumina will be subjected to firing in an atmosphere
of hydrogen.
o
-27-
5- Samples have previously been prepared by hot-pressing.
However, they were contaminated from the graphite die. Future
samples will be prepared by hot-pressing in ceramic dies.
6. When work is completed on alumina, investigation of
cubic phases, particularly magnesia, will begin. The literature
reveals that MgO can be fabricated to theoretical density by
hot pressing. However, no mention is made of transparency.
-28-
Progrcss Report - Part III
HOT EXTRUSION
Introduction
Hot extrusion of certain ceramic materials is foreseen as a
method of producing materials having the desirable properties of
hot pressed materials but being a more economical production
method. For instance, earlier work has shovm that barium
titanate can be hot extruded and that the extruded material
possesses qualities similar to that produced by hot pressing. The
previous work was only preliminary and, although it did provide
encouragement that hot extrusion of ceramic materials is feasible,
it also demonstrated that there are a number of problems which
will have to be solved before truly successful hot extrusion of
these materials can be accomplished. Briefly the problems are:
1. Lubrication of the material being extruded so that it
will flow readily through the die.
2. Maintaining the material being extruded at the proper
temperature during the time necessary for extrusion.
3. Controlling the cooling rate of the extruded material
to eliminate failure due to thermal shock.
The extrusion of materials such as glasses is relatively
simple since these materials exhibit a gradual softening over
Rutgers, The State University Signal Corps Contract No. DA-36-039-sc-891l4.1
-29-
a reasonably large temperature range and essentially true viscous
flow. However, crystalline ceramic materials have distinct
melting points, going from a solid material exhibiting only a
small degree of flow to rather fluid melts, over a very narrow
temperature range. Although it is known to be possible to slowly
extrude crystalline ceramics at some temperature below their
melting point, rapid extrusion at such temperatures is not
believed to be possible without lubrication. One means of pro-
viding lubrication without dilution of the desired crystalline
phase Is by coextrusion of a metal or glass cladding around the
crystalline material. The coextrusion of a metal was used in
the previous work on barium titanate and has also been success-
fully used by Nuclear Metals, Inc. for the hot extrusion of UO2 .
The coextrusion of metal cladding around the ceramic is the
method of lubrication being used in the present study.
Equipment
An exploded view of the extrusion assembly being used pre-
sently is shown in Figure 1. The press used is an electrically
operated, hydraulic, 150 ton, constant pressur« Dake Press, Model
No. 21-150. The ram speed of this press Is approximately 1?
Inches per minute.
J, G. Hunt and P. Loewenstein, "The Fabrication of Clad Massive UO2 Fuel Elements by Coextrusion," Fourth Quarterly Report, Contract No, AT(30-1)-1565, Nuclear Metals, Inc., Concord, Mass., June 6, I960.
-30-
Figure 1 Exploded Cross Section of Hot Extrusion Assembly
m p —h— i ^ ̂
////. ^ ̂ ^ 2 ̂
V///, '///A ^\\ s\\\< 1 1
Extrusion Ram Heat treated l+l^O steel , Rockwell Cl+O-li^
Coextrusion Can O.D. approximately 1.70". depends on expansion of metal and extrusion temperature
Extrusion Die
1-3/11" O.D, Heat treated 'l-lijO steel 5 Rockwell Chfl-hS
Extrusion Barrel 6-1/2" O.D., 1-3A" I.D. Heat treated Ij-lq-O steel , Rockwell Cl).0-I|.5
Base Plate 2k" x 18"
Rutgers, The State UAiversity Q„II i Siqnal Corps Contract No. DA-36-039-sc-89 Ul 1
-31-
In use, the extrusion barrel with the die in place rests
upon the base plate. It is positioned on the base plate directly
under the ram and over a hole to allow the extruded rod to pass
through the plate. The positioning of the barrel is accomplished
simply by pushing the barrel firmly against three pins set
tangentially along the circumference of a circle having the same
diameter as the barrel. The angular distance between each pin
being approximately 50°'
The extrusion ram is placed on top of the extrusion barrel
and off to the side enough so that it is clear of the inside
diameter of the barrel. The press ram is then run down until it
is only a fraction of an inch above the top of the extrusion ram.
The coextrusion can and the material to be extruded are
heated, either separately or together depending upon temperature
requirements, and arc then quickly placed in the extrusion barrel.
Experimental Work
The work performed during this period was concerned mainly
with testing the operation of the hot extrusion apparatus. The
proper operation is somewhat complex and depends largely upon
timing and teamwork of a group of three or four men. This is
necessary to accomplish the extrusion as rapidly as possible
before too much heat is lost to the cold extrusion barrel.
For this preliminary work, it was decided to use a material
that had relatively well known flow properties and which would
extrude at a lower temperature than barium titanate.
■32-
During this period three extrusion attempts were made. The
ceramic material used was a proprietary, glass containing compo-
sition used in producing transfer molded parts. This material
was chosen since the temperatures at which it can be transfer
molded are known.
The mix was hand tamped into a coextrusion can of 1018 steel
and placed in an electric furnace to heat. The die and barrel of
the extruder were lubricated with a mix of graphite and stearic
acid in CCI4. After soaking for about 10 minutes at the desired
temperature^ the coextrusion can was removed from the furnace and
quickly placed in the extrusion barrel. The extrusion ram was
placed in the barrel and the full extrusion pressure applied as
rapidly as possible. The time between removal of the can from the
furnace and application of full pressure was approximately 5-7
seconds. Neither the die nor the extrusion barrel were preheated.
The temperatures to which the material and coextrusion can were
heated were ii+OO, 1^00 and 1700oF.
Discussion of Resu1ts
Extrusion did not occur on any of the three extrusion
attempts. In fact there was very little difference in the appear-
ance of the three slugs after the attempted extrusion. In each
case, the coextrusion can did doform sufficiently to take the
shape of the die and barrel and each had a slight "nose" where
the can had been foreed a short distance into the straight portion
-33-
of the die. The ceramic itself did not appear to have been
heated sufficiently prior to the attempted extrusion since all
pieces were porous although the porosity of the piece preheated
to 1700oF was quite low» It appears that a ten minute soak at
peak temperature is not sufficient for thoroughly heating the
ceramic powder. This is probably due to its low thermal
conduct ivi ty.
Conclusions
The preliminary hot extrusion attempts have shown, at least
tentatively, that:
1. Using the present design of equipment, assembly of the
necessary parts and application of full extrusion pressure can
be obtained within 5 to 7 seconds after removal of the pieces
from the furnace.
2. A soaking time of ten minutes is apparently not long
enough to allow the ceramic extrusion blank to reach equilibrium.
3» The use of a coextrttslon can having a sof^ning
temperature than 1018 steel is desirable for use in extruding
the composition employed in the present study.
Future Work
A number of extrusion experiments will be performed using
1100-F aluminum. Trial runs will be made to determine the
temperature and pressures needed for extrusion of solid slugs of
■3h-
this aluminum. This will help in determining the parameters
necessary for coextrusion of the aluminum with the ceramic
material. According to the literature the aluminum should ex-
trude between 750 and 900oF..2 It Is felt that the present
extrusion apparatus will probably work most favorably if the co-
extrusion can is made of a metal which will extrude at a lower
temperature than the ceramic since the coextrusion can is in
direct contact with the cold barrel and die of the extruder dur-
ing the extrusion period. If the ceramic is heated to a higher
temperature than the coextrusion can it will act as a source of
supply of heat to the can during the extrusion and, it is hoped,
that this will enhance extrusion. If this method produces
successful extrusions, a similar approach will be tried in the
hot extrusion of barium titanate, choosing a coextrusion metal
which has a higher extrusion temperature than aluminum. If it
becomes apparent that the cooling of the coextrusion can by the
room temperature extrusion barrel and die is greatly hampering
successful extrusion, preheating of the barrel and die will be
tried. In this case, care must be taken not to get the barrel and
die hot enough to cause them to lose their temper.
2. Herbert R. Simonds, Archie J. Weith and William Schack, "Extrusion of Plastics, Rubber and Metals," Reinhold Publish- ing Corp., New York City, 1952.
•35-
Another method which may be used if the previous methods are
not satisfactory will require a slight modification of the present
assembly. The method consist essentially of having a liner made
which can be preheated and placed Inside the extrusion barrel
just prior to the extrusion attempt. The coextrusion can will fit
inside of this liner. The liner should be made of a material
which will not flow at the extrusion temperature. It will serve
as a buffer between the cold die and the coextrusion can thus
keeping the temperature of the can from dropping so rapidly dur-
ing the time required for extrusion.
I
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