TABLE II Activation energies, E (with standard deviation from linear regression), for the decomposition of organic residues of PT—PZ gels
!
with steam treatment!
Sample
E from DTGA peak
(kJ mol\ꢀ)
E from DTA peak
(kJ mol\ꢀ)
E from DTGA peak
(kJ mol\ꢀ)
E from DTA peak
(kJ mol\ꢀ)
!
!
!
!
Acetone formation
Oxidation
Oxidation of carbon residues
PT
113$5(5)
126$5(4)
121$3(2)
125$8(7)
116$5(5)
144$4(3)
162$8(5)
134$5(4)
128$8(6)
137$7(5)
145$4(3)
151$5(3)
124$3(2)
130$3(3)
PZT(10)
PZT(25)
PZT(35)
PZT(45)
PZT(75)
PZ
399$15(4)
263$16(6)
499$37(7)
404$28(7)
!Values in parentheses are percentages.
formation of carbon residues. PT shows a small
3.4. Reasons for the formation of carbon
residues
°
exothermic crystallization peak at 480 C. The crystal-
lization of PZT(45) is less exothermic and occurs at
higher temperatures.
Comparison with pyrolysis behaviour of other sam-
ples prepared by the sol-gel process gives insight into
the reasons for the formation of carbon residues.
Without preceding steam treatment, samples of ZT(x)
form carbon residues from residual methoxyethoxide
groups during pyrolysis. Steam treatment removes all
the methoxyethoxide groups, which can be recognized
by disappearance of corresponding bands in Fig. 1,
and prevents the formation of carbon residues. This
leads to the conclusion that in zirconium-rich PZT
gels after steam treatment the acetate groups must be
the starting material for the formation of carbon resi-
dues.
In Table II the activation energies of the pyrolysis
steps of various samples, PT—PZ, after steam treatment
are collected. The activation energy of the oxidation
step can be evaluated directly from the exothermic
DTA peak. The DTGA peak of acetone formation
overlaps with the weight loss due to oxidation at
somewhat higher temperatures. The DTA oxidation
peak can be scaled in such a way that it is in good
coincidence with the DTGA peak at about
°
260—300 C. Subtraction of this curve from the DTGA
peak leaves only that part of the DTGA peak caused
by acetone formation at lower temperatures, From the
Lead acetate and titanyl acetate are pyrolysed with-
out carbon residues. In the PZT series, a problem
arises with growing zirconium content. This indicates
that acetate groups bonded to zirconium are the rea-
son for the carbon residues. Although most of the
acetate groups are bonded to lead, a small amount
bonded to zirconium can be sufficient to cause black
coloration of the samples. Pyrolysis of zirconium acet-
ate at high heating rates also shows the formation of
residues, which are decomposed only at temperatures
¹
shift of these curves with heating rate the activa-
ꢀ
tion energy of acetone formation was evaluated. For
PZT(75) and PZ oxidation occurs at higher tempera-
tures, causing only a shoulder on the DTGA acetone
formation peak. These DTGA peaks can be evaluated
directly for calculation of the activation energy of
acetone formation.
The temperature range, the activation energy and
the yield of acetone from PZT gels are very similar to
those of lead acetate. This is a clear indication that
most of the acetate groups in the gel are bonded to
°
above 400 C.
An examination of the water in the vessel used for
steam treatment shows a decrease of pH from 5.5
before to 3.5—3.8 after treatment. I.r. spectra of the
water exhibit weak bands of acetic acid, formed in the
reaction
ꢂ>
Pb . The activation energy of the oxidation step of
the PZT gels is also similar to that of the first oxida-
tion step of lead acetate, but in PZT gels pyrolysis is
°
completed with this oxidation up to 300 C, and for
lead acetate a second oxidation step follows at higher
temperatures. This shows that (Zr/Ti) atoms have
some influence on pyrolysis at higher temperatures.
PZT gels with more than 52% zirconium contents
are pyrolysed with the intermediate formation of car-
(Zr/Ti)!Ac#H OP(Zr/Ti)!OH#HAc
ꢂ
In zirconium-poor PZT gels, the few acetate groups
bonded to zirconium are removed by steam treatment,
and the samples can be pyrolysed without carbon
residues. In zirconium-rich samples, too many zirco-
nium bonded acetate groups are present and cannot
be removed even with repeated steam treatment. The
newly formed (Zr/Ti)-OH groups are acidic and can
bon residues and small amounts of PbCO , leading to
ꢁ
°
an exothermic weight loss at 450—500 C, even after
steam treatment. The results of DTA—TGA—i.r.
measurements resemble those of PZT(45) without
steam treatment. The activation energies of acetone
formation and oxidation are similar to the activation
energies for the titanium-rich samples. The activation
prevent the formation of PbCO in steam treated PZT
ꢁ
gels. The Raman spectra of PZT(45) and ZT(45) con-
tained in Fig. 1 show a slight modification in the low
wave-number region due to steam treatment, which
could be caused by rearrangements in the amorphous
°
energies of the decomposition step at 450—500 C are
different for the DTA and DTGA peaks and vary with
sample composition.
(Zr/Ti) O octahedra network.
ꢇ
4347