Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to the preparation of, and to, asolid ceramic that is a precursor for a hydroniu~ ion conduc-
tor.
While it is known that single crystals of hydronium ~
and ~" aluminas can be formed, the creation of a solid poly-
crystalline hydronium ~-" alumina has escaped the creative abi-
lity of those skilled in the art.
Such said hydronium polycrystalline ~" aluminas are pos-
sible host material for hydrogen fuel cells or for use as a
membrane to decompose water - electrolysis cells, since the
polycrystalline material could be formed into any desired
shape as might be required by the geometric or design para-
meters of the cell.
It is known that the ~" alumina phase is the preferred
polymorph of ~ alumina as the former has three conductive
planes (sometimes referred to in the prior art as three spinel
blocks), while the ~ phase possesses only but two.
It is known in the prior art how to create single crys-
tals of sodium ~"-A12O3 (sodium beta double prime alumina),
and the prior art alleges knowledge of how to replace the so-
dium ion with hydronium so as to create a hydronium ion con-
ductor. In the prior art, when the ion re~lacement of sodiurn
with hydronium takes place, within a single crystal, the crys-
tal expands in size (to accommodate the hydronium ion as will
be hereafter explained), while within solid polycrystals of
sodium ~"-Al~O3, the polycrystal shatters.
We have determined that the reasons for the shattering
of the said polycrystals, as will become more apparent in this
application, is because the hydronium ion is so much larger
than the sodium ion which it replaces, or is begged to rep-
lace, by appropriate processes, that the crystal lattice stru-
cture of the sodium ~" aluminas will not accommodate its size;
the fracture strength of the polycrystal is not large enough
so as to overcome the undue stress placed upon it when the
hydronium ion replaces the sodium ion that is, the pressure
put on the polycrystal lattice structure by the hydronium ion
exceeds its molecular bonding threshold whereby it shatters
2 --
The prior art is replete with allegations that the ~"- A12O3
ion would be a good conductor of the hydrogen ion, but the
ability to yield defacto such polycrystalline conductors, pre-
ferably ~olids, has yet to be achieved.
It is an object of this invention to disclose the opti-
mum conductivity of a ~ " alumina polycrystalline solid which
is a precursor for such hydronium analogues and to fabricate
this solid into any desired shape. The resultant is a high
density mechanically strong solid ceramic conductor with high
resistance to acidic and alkaline corrosion.
Some of the inventors herein have earlier disclosed in a
co-pending application [1], filed in Canada, 28 April, 1981 as
Serial No. 376,561-0 now entitled THE PREPARATION OF A PRECUR-
SOR POWDER FOR THE MANUFACTURE OF A CERAMIC HYDROGE~ ION CON-
DUCTOR, a potassium ~ alumina compound and method of senera-
ting the same. The resultant, thereof, is a fine crystalline
powder with a high weight density of ~" alumina.
For the convenience and for understanding, it is appro-
priate to define the following function:
~+ ~
It is an object of the invention, therefore, to achieve
a solid (alkali) compound (a high density mechanically strong
ceramic) possessing alumina and ~" alumina phases where f(~)
= 0.01 - > 1, which preferably is in the range of 0.25 --~
0.48 and specifically 0.37 while preferably using mixes of
oxides of sodium and potassium (Na2O/K~O). In an alternative
embodiment a preferred value for f(~) is in the range of 0.5
- > 0.55. At or about the value of f(~) = 0O37 (within the
range 0.25 --~ 0.48) conductivity of the compound is greater
than 10-2 (ohm)~1 (cm)~l at 300C. ( 10~3 (ohm)~l (cm)~l at
room temperature ). The preferred compound with f(~ = 0.37
is of a chemical formula;
(NaO-6K0-4)2O (3 W/o MgO) ~ A12O3
In order to explain the properties of such compound, a
theory has been evolved known as the MAP theory and the same
is partially disclosed herein. The theory accurately predicts
-- 3 --
the experimental results of conductivity for mixes of alkali
(Na/K) ~/~" aluminas.
A method of making such polycrystalline solid is also
disclosed and consists generally of the steps:
(a) selecting compounds of a least two alkalies, and an
aluminum compound soluble in water;
(b) dissolving the same in water;
(c) mixing the components at the atomic level;
(d) removing the water therefrom so as to derive white
polycrystalline powder, wherein the aforesaid compounds
are now mixed at the atomic level and wherein f(~) is
predetermined;
(e) compressing the powder into a predetermined shape;
(f3 sintering the compressed powder so as to relieve
. the polycrystals of their high surface energy, but for a
short interval of time so as to not alter significantly
the aforesaid predetermined value f(~);
The water removal step (d) includes the step of calcina-
20 tion.
Specifically the method comprises the steps of:
(a) selecting soluble sodium and potassium compounds,
and an aluminum compound;
(b) dissolving said compounds in water to mix the same
at the atomic level;
(c) removing the water therefrom so as to derive a plu-
rality of crystallites forming a white crystalline pow-
der wherein the sodium, potassium and aluminum compounds
are mixed in the crystallites at the atomic level;
(d) calcining the powder to obtain a predetermine value
of f(~);
(e) compressing the powder into a predetermined shape;
and,
(f) sintering the compressed powder, into a solid cer-
amic, so as to relieve the polycrystallites of th~ir
high surface energy, but for a short interval of time so
as to not alter significantly the value f(~);
,".
l~g~18
Specifically the alkalies are in the preferred weight
percent ratios 60 Na2O/40 K2O.
In the preferred method, the selecting step (a~ comp-
rises selecting feed alkalies in ratios of 60 wt% Na2CO3; 40
wt% K2CO3 (of the total alkali); 3.0 wt% MgO and the balance
A12O3 in the form of A12(SO4)3; while the preferred sintering
step (d) takes place for approximately 1 minute at a tempera-
ture of 1610C.~ 2C.
Figure 1 is a prior art phase diagram for Na2O-A12O3.
Figure 2 plots the density of the resultant material of
this invention as a function of Na2O content.
Figure 3 plots ~ f versus Na20 content.
Figure 4 plots density as a function of Na2O content~
Figure 5 plots surface area and particle size vs Na2O
content.
Figure 5 is a flow chart depicting two routes of obtai-
ning the preferred product, by two alternative method se-
quences.
Figure 7 are x-ray diffraction results of the spacing of
the ~" and ~' phases, along the C-axes of the crystal illust-
rating, along the dashed line, the calculated results and
plots from prior art reports, while the solid line, the exper-
imental results of the inventors, all as a function of f(~.
Figure 8 is a conductivity plot as a function of A f,
for mixes of two alkalis with ~ ~ " alumina phases (Na/K ~ ~ "
aluminas).
Figure 9 is a plot of the experimental results according
to this invention and according to the disclosed MAP Theory.
Figure 10 is a theoretical plot of figure 9, at two tem-
peratures, room t~mperature and 300C.Background to the Invention
It is now known that the proton conductivity of the ~"
phase is higher than the ~ phase, therefore, one desires to
achieve the value of f( ~) = 0. One of the co-inventors herein
has earlier noted [2] that the ~" alumina is less stable than
the ~ alumina phase because of the defect nature of the
crystal structure of ~"A12O3.
B
-- 5 --
It is also known that one may create sodium ~ " alumina
in a powder or solids where f(~) ~ O and these have been des-
cribed in the literature.
Further, one would ultimately wish to create H30+ super-
ionic conductors by replacing the sodium ion (Na+) with the
hydronium ion (H30+) but the success in doing so has been
limited because of the "crystal shattering" during the rep-
lacement of the Na+ by the H30~. This shattering is known as
structural damage.
From measurements, it is known that the dimensional size
of the sodium, potassium and hydronium ions are as set forth
in the following table:
K+ = 1.4 A
Na+ = 0.9 A
H30~ = 104 A
From the size of each of the aforesaid ions the crystal
shattering of the prior art can be explained; when the hydro-
nium ion ~H30+) replaces the sodium ion (Na+) in the polycry-
stal lattice, s:ince it is almost 50% larger, the polycrystal
lattice structure is stretched beyond its polycrystal fracture
strength; it thus shatters.
From the foregoing, it would be preferred, therefore, to
replace the Na+ partially with K+ and hence, to create a
sodium-potassium ~" alumina since the potassium ion is of the
exact magnitude as the hydronium ion. Two of the inventors
herein have, in fact, created a K-~"A1203 compound (no sodium)
where f(~) = O, or is approximately O and have disclosed the
same in the aforesaid Canadian Patent Application. Such
compound, however, is but a powder.
It is desired to create the aforesaid polycrystalline
powder into a solid. On sintering of the aforesaid powder K-
~"A1203, the f(~) increases dramatically and the high weight
percent of the preferred ~ "A1203 is lost. This can now be
explained with reference to figure 1, being a phase diagram
for Na20-A1203. (Note: one would prefer a K20-A1203 phase
diagram similar to that of figure 1, but none is available.
It is thus assumed that the phase diagram for K20-A1203 is
-.,~
-- 6 --
similar to that of Na2O A12O3). The dashed line 10 thereof is
the stability range for Na~"A12O3 and it becomes destabilized
at about 15~03C. Sintering above that temperature, therefore,
collapses the ~" crystal structure in a short time and the
same is lost.
It is known that Na~"A12O3 powder is more stable than
the potassium version thereof and it was conceived, therefore,
to mix both potassium and sodium versions thereof.
The Invention
Materials were prepared with soda contents of 40, 50 and
50 wt~ of total alkali by three methods, all utilising spray
freeze/freeze drying procedures for powder synthesis.
(a) mixing K ~ ~"Al~O3 and Na~"A12O3 powders (series 1).
(b) mixing K-Na~ Al2o3 (Rl-05M9-30A110-3317) with
Na~"A1203 powder (series 2).
(c) mixed alkali powder (series 3).
Each composition was made up with 40, 50 and 60 wt%
total alkali as Na2O the balance of the alkali K2O. The re-
sults of this series of experiments are summarized in figure
2. The lowest of f(~) values were obtained in series 3. The
best sample from the density and f(~) standpoint had a maximum
density of 95% theoretical and f(~) = 0.46.
A further decrease in f(~) was explored by inserting MgO
and varying its content. Compositions with 2 and 3 wt% MgO
and 40, 50 and 60 wt% (total alkali) as Na2O (balance K2O)
were sprayed, calcined, pressed and sintered. The change of
f(~) on sintering (defined as ~f) is shown in figure 3. The
3 wt~ MgO sinters are superior with respect to low /\f values,
and in the case of the 60 wt% Na2O samples, 3 wt% MgO also
gave the maximum densities. The possibility of further reduc-
tion in f(~) was investigated by reducing the sintering tempe-
ratures. /\ f versus the Na20 fraction is plotted in figure 3
for 2 and 3 wt~ MgO compositions for these lower tempera-
tures. The 3 wt~ MgO compositions are again superior with the
lowest /\f values while satisfactory densities are achieved at
1609C. ' 2C~, eg. 1610C. in the case of 60 wt~ Na2O samples
,~,,
(figure 4). The increased Na2O probably leads to the form~
ation of liquid phases at the sintering temperature which
catalyze sintering.
These results, therefore, show that a satisfactory Na/K
~ ~" alumina with respect to density and f(~) can be prepared
by method 3 having the composition of the total alkali 60 wt%
as Na2O, 40 wt% as K2O (of the alkali mix); 3.0 wt~ MgO, and
the balance A12O3. The optimum sintering-- temperature is
1610~C., ~ 2C. and optimum sintering time is 1 minute. The
product i5 (Na0 6 Ko-4)2 (3 W/o Mgo)~ "Al2o3 sub nom 6N3.
Having determined the increased sinterability of the 6N3
composition, the surface area, lattice parameters and sintered
densities of the powders and sinters were measured. The
change in surface area with wt% Na2O is shown in figure 5.
Close to the K~ " A12O3 composition, the surface area dec-
lines markedly (shown by the dotted line in figure 5~.
6N3 can thus be made by series 1 or series 2 procedures
as aforesaid, but the preferred method is that of series 3r
apd referring to figure 6, the following are the steps:
~ The feed chemicals contain Na+, K+, A13+, Mg2+ prefer-
ably as Na2CO3, X2CO3, A12(SO4)3-16H2O and MgCO3 and are
dissolved in water and then sprayed into liquid nitro~en which
freezes the constituent materials and they form a very fine
powder. In fact, what happens, a "mixing" takes place of the
feed materials at the atomic level. Thus, a white powder is
formed and it is placed in a freeze dryer and freeze dried for
approximately 4 days. Alternatively, the dissolved feed mat-
erials can be spray dried in a spray dryer and the same very
very fine powder achieved (see figure 6).
The freeze drying or alternatively the spray drying has
the effect of driving off the liquid water. The resultant is
then calcined, for about 2 hours and a fine crystalline white
powder is stabilized thereby and has the general chemical
structure Na(K)-3/~"-A12O3 and the preferred chemical composi-
tion (Nao.6Ko~4)2o (3 W/o MgO) ~ A12O3 sub nom 6N3 with an
f(~) = 0.0 The powder then is packed into a mold which con-
sists essentially of an inner metal mandrel and a flexible
La;18
outer sleeve which fits over the mandrel, and defines therebe-
tween a space whose shape is the shape of the desired solid
form, which in our preferred embodiment is a closed end tube
(of the approximate size of a test tube, but it should be
noted any size appears possible). The powder is loaded into
the space through an aperture defined in the sleeve and when
fully loaded, a plug is seated in the aperture to close the
same. The filled mold is then placed in an oil press and the
hydraulic press is driven to pressures of 50,000 lbs. whereup-
on the powder is compressed into a solid of the desiredshape. The solid then is removed and sintered quickly for 1
minute at 1610C. me solid precursor for the manufacture of
the ceramic hydrogen ion conductor is achieved as all that
needs to be done is to replace the sodium and the potassium
ions therein with hydronium ions.
me reason for sintering at 1 minute is to relieve the
crystals of their high surface energy and to produce a high
density mechanically strong ceramic; while, the short time in-
terval has the effect of not changing significantly, the value
of f( ~) which has already been achieved as a result of the
calcination.
It is preferred that the ratios of the feed chemicals,
in order to achieve sintered product 6N3, be with a total 60
wt% Na2O; 40 wt~ K2O of the total alkali; 3.0 wt% MgO and the
balance A12O3. The face analysis of this composition, 6N3, is
60% ~"-A12O3 and 40% ~-A12O3 (f(~) = 0.4). This suggests that
the K+ ion resides in the ~ -alumina phase [as its fraction
is equal to that of f(~)] and that the Na+ ion resides in the
~" phase. Proof of this hypothesis is shown in figure 7. In
this figure, the lattice parameters of ~/~ "-A12O3 of the
composition 6N3 are plotted as a function of f(~). When f(~)
is less than 0.4 and K+ ions are forced into the ~"-A12O3
phase, the lattice parameter of this phase increases, whereas
that of the ~-A12O3 phase remains constant. On the other
hand, for 6N3 with f(~) greater than 0.4, Na+ ions are forced
into the ~-A12O3 phase causing its lattice parameter to
shrink, while that of the ~"-A12O3 phase remains constant.
The experimental points on this graph support this hypothesis.
m is 6N3 product, f(~) = 0.4/ has a maximum conductivity
for the Na20 K20-3/~" alumina system (greater than 10-4
(ohm)~l (cm~~l at room temperature).
This conductivity has been demonstrated and may be cal-
culated from what will be known as the Mixed Alkali Percola-
tion Theory (MAP), a theory developed by several of the co-in-
ventors, yet unreported [33, which takes into-account two exi-
sting theories and combines the same in order explain the
aforesaid phenomenon. It takes into account the Effective
Media Percolation Theory (EMPT) in a polycrystalline system,
and the Ion Distribution Theory For Mixed Alkalies sometimes
called the Mixed Alkali Effect (MAE).
As a background to MAP and referring to EMPT, it is
known that in practical situations, conductivity measurements
are carried out on materials which contain a mixture of two
phases with different conductivities~ If the two phases are
intimately mixed, the measured conductivity will be a func-
tion, related to the ratio of the separate conductivities of
components of the mixture, and thus be an average of the two
constant conductivities al and ~2 where ~1 and a2 are the con-
ductivities of the first and second materials. The "average"
conductivity ~m is explained by EMPT by
am = k2-klX + [(k2-klX)2 + gala2]_1 (1)
where kl = 3tal _ a~)~ k2 = 25l a2 and
X = the volume fraction of phase 2 and a m is the conductivity
of the mix.
From equation (1), the solid curve plot in figure 8 is
derived; while the experimental result taken trace out the
dash curve. Thus the theoretical curve for EMPT must be shif-
ted to the left which is the anisotropic effect and the two-
dimensional nature of the ~-A1203 lattice effectively increa-
ses the proportion of the resistive phase, for it is recog-
nized that conduction is taking place in a polycrystalline
material in which the conductivity in each crystallite is
two-dimensional, that is, takes place along the conduction
planes between the spinel blocks. It is clear that when many
119~
-- 10 --
of these crystals are incorporated into a ceramic, the conduc-
tion planes of some neighbouring cystals may be oriented per-
pendicular to the other. Under these conditions, no current
flows across the boundary. The theory assumes that such crys-
tals form part of the non-conducting phase and an effective
volume fraction of the ion conducting phase may be defined as
Xeff = f(~) + b[l - f(~)]
where b is the fraction of the ~ n phase particles which are
misalligned. Use of this theory reveals that ~ = 0.3, that is
30% of the particles are misalligned, a finding which is rea-
sonable to those skilled in the art. However, theoretically
the predicted conductivity to (~ 1 is much lower than
what might be expected. This must be explained as the effect
of presence of both X+ and Na+ ions.
The so-called MEA is well known in glasses, and can be
observed for ~ alumina and its isomorph ~ gallate. Associated
with this effect is a large decrease in the conductivity and
an increase in activation energy at some intermediate composi-
tion. The MEA effect kas been explained in terms of preferen-
tial site occupation and ion pair formation. From site energycalculations, it has been suggested that the larger ions pre-
fer Beevers-Ross sites in the ~-A1203 and ~"-A1203 lattices
and displace the smaller ions to paired interstitial mid-oxy-
gen sites. In any event, it is assumed that the Mixed Alkali
Effect is responsible for the very low conductivity at the
percolation threshold, it should be possible, therefore f to
simulate the experimental data if the ion distribution between
the two phases is known. Suffice it to say, it can be shown
that Na-~" alumina and K-~" alumina mixes are equivalent to a
mechanical mixture of K- ~alumina in a matrix of Na ~" alumina
whereupon the conductivity according to MEA is readily calcu-
lated. Ion exchange experiments with NaNO2/KNO3 melts ha~e
shown that potassium ions prefer to reside in the ~ phase.
Considering this marked stability of K-~ alumina, one could
propose, as a first approximation, that, when the mole frac-
tion of potassium ions (N) in the total system (~+~") equals
the proportion of ~ phase [i.e. N - f(~)], the potassium ions
'~3
~ .~
will primarily reside in the ~ phase. Thus, as the proportion
of ~ phase decreases, i.e. N>f(~), more and more potassium
ions are forced to reside in the ~" phase and we can write for
the mole fraction of potassium ions in this phase:
X3" = N ~ fl~' (2)
X~ = N
These equations can be inserted into that of equation (1) and
plotted as in figure 9. See how close they come to the actual
conductivity measurements on 6N3, at room temperature. The
curve of figure 9 is at room temperature, while that of figure
10 illustrates the conductivity plots (according to MAP) at
room temperature (curve C~T~) and at 3Q0C.( curve o 300).
The conductivity peaks are observed at both temperatures when
f(~) = 0.37, the theoretical value of f(~) for 6N3, and this
value which is within the error measurement of the experimen~
tal results. ~eferring to figure 10, at room temperature,
notice the discontinuity of the curve where f(~ ) - 0.50 to
0.55; the absolute minimum conductivity at room temperature of
around when f(~) exceeds approximately 0.5. The discontinuous
properties of the conductivity at room temperature have inte-
resting applications.
Returning now to a review of figures 10, 3 and 4, spec-
fically as to figure 3, note that ~f is lowest when the sodium
content versus potassium content as at 0.6 to 0.4. Thus, the
preferred alkali mix is the ratio 0.6 sodium oxide/0.~ potas-
sium oxide . From figure 4, the maximum density, which is
well above 95~ of that of the ceramic solid, is achieved as
well when the alkali mix is 0.6 Na20 and 0.4 K20 - the point
indicated as 6N3. Also note that in these two figures, figur2
3 and 4, this value is achieved with the magnesium content at
3 wt%. Hence, the nomenclature 6N3 stands for 60 wt~ of
sodium in the (sodium oxide - potassium oxide~ alkali mix and
3 wt~ magnesium oxide for stabiliziny the ~ " alumina
~,
~19~ 8
- 12 -
phase~ It is with this ceramic solid that f(~) = 0.37 for
which the maximum conductivity is illustrated in figure lOo
Referring to figure 5, note the change in the surface area
which declines when the sodium oxide content of the alkali
exceeds 60% wt. Vi5 a vis potassium oxide.
Footnotes:
.
[1) Also correspondingly filed as European Patent Applica-
tion SN. 82103395.8 filed 22 April, 1982, Published 10
November, 1982, Bulletin 82/45, as Publication No.
Al-0,064,226; sub nom: Ceramic Hydrogen Ion Conductor
and its Preparation.
[2] Nicholson et al; The Relative Stability of Spray -
Frozen/Freeze-Dried ~ "-A12O3 Powder, Matts. Res. Bull.,
Vol~ 15, pp. 1517-1524, 1980.
[3] Bell et al; A Percolation Model for the Conductivity of
Mixed Phase, Mixed Ion Aluminas to be published in
Stockholm Sweden 9 July, 1983.
., ,, ~.
