Phase transformation and percolation effect in PbO2-1/12Ag2O-xC system

Article abstract of DOI:10.1016/S0038-1098(02)00884-0

The structure and electronic transport properties have been investigated for composite PbO₂-1/12Ag₂O-xC (0 ≤ x ≤ 2) materials prepared by mechanical milling in an O₂ atmosphere. The solid-state reactions result in the occurrence of abundant phase transformations, when the graphite content increases. With the phase transformations occurring, the percolation effect is observed for the electronic transport of PbO₂-1/12Ag₂O-xC system, while its conductivity undergoes an insulator-metal transition. The percolation threshold is determined by DC conductivity measurement and analysis of X-ray diffraction patterns.

Full text of DOI:10.1016/S0038-1098(02)00884-0

Solid State Communications 125 (2003) 493–497
www.elsevier.com/locate/ssc
Phase transformation and percolation effect
1
in PbO – Ag O–xC system
2
2
12
a,b
D. Li , Z.D. Zhang
a,b,*
a,b
a,b
, D.Y. Geng , W.F. Li ,
a,b a,b
X.P. Song , G.W. Qiao , Y.Z. Wang
a,b
a
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences,
Wenhua Road 72, Shenyang 110016, People’s Republic of China
b
International Centre for Materials Physics, Chinese Academy of Sciences, Wenhua Road 72, Shenyang 110016, People’s Republic of China
Received 9 July 2002; accepted 9 December 2002 by H. von L o¨ hneysen
Abstract
The structure and electronic transport properties have been investigated for composite PbO – Ag O–xC ð0 # x # 2Þ
1
12
2
2
materials prepared by mechanical milling in an O
phase transformations, when the graphite content increases. With the phase transformations occurring, the percolation effect is
2
atmosphere. The solid-state reactions result in the occurrence of abundant
1
12
observed for the electronic transport of PbO – Ag O–xC system, while its conductivity undergoes an insulator–metal
2
2
transition. The percolation threshold is determined by DC conductivity measurement and analysis of X-ray diffraction patterns.
q 2003 Elsevier Science Ltd. All rights reserved.
PACS: 81.20.Ev; 61.43.Gt; 71.30. þ h; 72.15.Rn
1
12
Keywords: A. The PbO – Ag O–xC system; D. Electronic transport; D. Percolation effect
2
2
1
. Introduction
The percolation threshold effect is a well-known
forms of carbon or finely dispersed metals. Recently, the
2
systems of PbCO ·2PbO–Ag O and PbCO ·PbO–Ag O
3
2
3
were reported to exhibit possible superconductivity [7,8].
We investigated the electronic transport properties of
PbO –Ag O–xC system, which shows metal–nonmetal
phenomenon widely observed in physical systems [1],
like, filler-matrix systems as the extreme change of certain
physical properties within a rather narrow range of filler
concentration. The conductive composites [2,3], consisting
of an insulting polymeric matrix and conductive filler, and
the nanocomposite magnets [4], consisting of hard and soft
magnetic phases, frequently demonstrate such a behavior.
Several models have been proposed to describe the
percolation behavior of electroconductive composites [5,
2
2
transition and percolation effect [9]. In this work, we find
that abundant phase transformations and percolation effect
exist in PbO – 1 Ag O–xC system with increasing the
2
12
2
graphite content. The percolation effect is analyzed for
electronic transport properties of the (2PbCO ·PbO þ
3
PbCO )/Pb nanocomposite in which Pb serves as conductive
3
filler. We also succeed in synthesizing Ag Pb O compound
2 6
5
6]. The most frequently used electroconductive particles
served as fillers for conductive composites are either various
by annealing the PbCO ·PbO/Ag nanocomposite in an O
3
2
atmosphere.
*
Corresponding author. Address: Shenyang National Laboratory
for Materials Science, Institute of Metal Research, Chinese
Academy of Sciences, Wenhua Road 72, Shenyang 110016,
People’s Republic of China. Tel.: þ86-24-23971859; fax: þ86-
2
. Experimental
24-23891320.
The composite materials were prepared by analytical
reagents of PbO (rutile-type), Ag O and 99.7%-pure
E-mail address: zdzhang@imr.ac.cn (Z.D. Zhang).
2
2
0038-1098/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.
PII: S0038-1098(02)00884-0

494
D. Li et al. / Solid State Communications 125 (2003) 493–497
graphite with the mol ratio of 1: 11 :x ð0 # x # 2Þ: The
progress, depending on the change of the graphite content.
The broad peaks in the XRD patterns show that the crystal
structures of the phases are not very perfect with the
existence of defects that induced by mechanical milling as
usual. From the phase constituents shown by XRD patterns,
it is clear that some solid-state reactions occur indeed during
2
2 2
powder mixture of PbO , Ag O and graphite was sealed in a
hardened steel can in an oxygen atmosphere and mechani-
cally milled using a high-energy ball miller at a voltage of
1
2
10 V for 5 h. The mass ratio of balls to powders was kept at
0:1. The as-milled powders were pressed into pellets by
using a 760 MPa axial-pressure with a steel die. The pellets
were ground to a rectangular parallelepiped with dimensions
milling. Without any graphite (x ¼ 0), PbO can be
2
decomposed to Pb O and O by the violent hurtling of
2
3
2
3
of 2 £ 10 £ 0.6 mm to fit the test holder. X-ray diffraction
the steel balls. The possible reaction is: 2PbO !
2
1
2
(
XRD) patterns were recorded at room temperature using
Pb O þ O : When there exists a small amount of graphite
2
3
2
Cu Ka radiation with a Rigaku D/Max-gA rotation target
diffractrometer. The temperature dependence of electrical
resistance was measured between 77 K and room tempera-
ture by the DC four-probe method using 15 mm silver wires
and silver paint with contact resistance ranging from 3 to
ð0 , x # 0:25Þ; lead oxide PbO
.55
appears and its amount
1
increases gradually because of the partial reduction by
2 3
graphite. For 0:25 # x # 0:4; Pb O and PbO1.55 phases
disappear gradually, while lead oxycarbonate PbCo ·PbO
3
emerges, with possible reactions: 2PbO þ C ! PbCO ·
2
3
2
0 V.
PbO and C þ O ! CO : In this process for x less than 0.4,
2
2
none of elemental Ag peaks can be found in XRD patterns.
According to our recent work [9], it is evaluated that a very
small amount of Ag Pb O compound may exist, doping the
3
. Results and discussion
5
2 6
other main phases, because the content of Ag O is far less
2
2
than that of PbO in the starting materials. The possible
The brown, dark or silver gray powders were produced
1
reaction is: 5Ag O þ 4PbO ! 2Ag Pb O þ O : When x
after milling for 5 h. When the cans were opened, some
gases were ejected from the cans. When the gases were
collected, a part of the gases could be absorbed by a solution
2
2
5
2
6
2
2
is larger than 0.4, the elemental Ag appears according to
XRD patterns, which can be ascribed by the reduction of
graphite. When x ¼ 0:5 and 0.6, we can easily obtain the
2
of ,20% NaOH, which was considered as CO . The gases
composite of PbCO
3
·PbO–Ag (with Ag=Pb ¼ 1=6 in a
produced during milling may be either oxygen or carbon
dioxide, depending on the content of graphite in the
precursors. XRD patterns (Fig. 1) of the as-milled powders
show that different phases are produced during the milling
simple process with one step, instead of the complicated
preparation processes with experimental conditions of high
press/high temperatures as described in literature [6,7]. With
increasing the graphite content x up to 0.70, another lead
3
oxycarbonate 2PbCO ·PbO can be synthesized and it is
difficult to estimate the possible reactions. When 0:73 ,
x # 0:80; the amount of 2PbCO ·PbO reduces gradually and
3
it disappears at x ¼ 0:80; while PbCO and Pb appear. When
3
0:90 # x # 2:0; lead oxides PbO also appear due to the
oxidation of the very fine Pb powders. When x is larger than
.20, the main phase is Pb, which is mixed by PbCO and
1
3
PbO. In this process, the role of graphite is not only to react
to synthesize products, but also to create a CO atmosphere
during the milling synthesis. All the compounds have been
produced in the mixture of O and CO atmospheres and the
CO partial pressure can be controlled by the amount of
2
2
2
2
graphite in the starting compositions (except the sample
x ¼ 0).
The as-milled samples of x # 0:74 are insulators, whilst
other samples with more graphite in starting materials show
metallic conductivity and the pellets pressed show metallic
luster. With increasing x from 0.75 to 1.4, the resistivities of
these samples at 290 K decrease from 0.29 to
2
5
6
tivity of these metallic pellets are represented in Fig. 2,
.12 £ 10 V cm. The temperature dependences of resis-
which are characterized by a linear function of r ¼ r þ AT
0
(
where r is resistivity of the composite, r residual
0
resistivity due to impurity scattering and A the temperature
coefficient of resistivity) in the measurement temperature
range. It can be illustrated by the scattering of conduction
Fig. 1. X-ray diffraction patterns of as-milled PbO – 1 12 Ag O–xC
samples.
2
2

D. Li et al. / Solid State Communications 125 (2003) 493–497
495
literature [5]. Here, s and s are the electrical conduc-
c
0
tivities of the composite and the conductive filler particles
Pb), V, V the volume fraction and the volume percolation
(
c
concentration of the conductive filler. In the present case,
the percolation phenomenon is slightly more complex
because the conductive filler Pb is produced by reducing
reaction with graphite and the relative amount of Pb depends
on the graphite content in the starting materials. Hence, a is
an adjustable parameter through which one can transfer
graphite content x to the volume fraction of the conductive
filler, and s a quantity determining the power of the
c
conductivity increase above V . Kirkpatrick [5] gave the
value for the exponent s ¼ 1:5 ^ 0:1 as point percolation
model. Our value of s ¼ 1:5 ^ 0:1 support the mechanism
of point percolation in the present system. With the graphite
content increasing ðx . 1:2Þ; the normalized conductivities
deviate from the fitting curve because of the excess graphite
added and the increase of the amount of PbO in the samples.
From Figs. 1 and 3, we can infer that the value of
Fig. 2. Temperature dependence of resistivity of the as-milled
1
12
PbO – Ag O–xC ð0:75 # x # 1:4Þ samples.
2
2
percolation threshold for the graphite content x should be
c
0
.75. In the PbO – 1 Ag O–xC system, the sample of x ¼
2
12
2
electron by thermal vibration of atoms, same as the curve of
x ¼ 1 and 1.25 in our recent work [9].
0:75 first shows a metallic conductivity. Compared to the
XRD patterns of samples with x ¼ 0:73; 0.74 and 0.75, it is
evident that there is no trace of Pb in the sample of x ¼
0:73; whilst the conductive phase Pb has been produced in
the samples of x ¼ 0:74 and 0.75. However, the x ¼ 0:74
sample is still an insulator because it is below the
percolation threshold. The x ¼ 0:75 sample becomes a
conductor, due to the conductive particles Pb forming a
conductive path through the sample. This transition is
actually an insulator-metal transition, originating from the
percolation effect of the conductive composite, which
changes the electronic transport properties of the
The graphite content dependence of the conductivity of
the as-milled samples ð0:75 # x # 1:4Þ is shown in Fig. 3.
The normalized conductivities s =s at 290 K are plotted as
c
0
a function of x, which represent the graphite content in the
starting materials. The relationship between the normalized
conductivity s =s and the graphite content x in the region
c
0
of 0:75 , x # 1:2 can be fitted by the equation s =s ¼
c
0
s
s
ðV 2 V Þ , aðx 2 x Þ ; same as the equation (1) in the
c
c
(
serves as conductive filler.
2PbCO ·PbO þ PbCO )/Pb nanocomposite in which Pb
3
3
The sample of x ¼ 0:5; consisting of PbCO ·PbO and
3
Ag, is interesting for comparing with the literature [7,8].
DSC curve of this sample, shown in Fig. 4, reveals that four
transitions occur between 100 and 400 8C with the onset
temperatures at 235.5, 327, 342 and 365 8C, respectively.
Except for the very weak endothermal peak onset at 342 8C,
the other three are exothermal peaks, among which the peak
at 327 8C is the weakest, and that at 365 8C is much broader
than that at 235.5 8C. We tried to identify these transitions
by annealing the sample at temperatures just above the onset
temperatures. The x ¼ 0:5 sample was annealed at 250, 335,
3
2
50 and 380 8C, respectively, in 0.1 MPa O flow for 1 h.
XRD patterns of the as-milled and annealed samples are
shown in Fig. 5. It is identified that some reactions occur
during annealing. The first peak, in Fig. 4, with the onset
temperature of 235.5 8C is consistent with the decompo-
sition of some unknown phases. As annealed at 335 8C,
PbCO ·PbO starts to decompose to be PbCO ·2PbO and
Fig. 3. Normalized conductivity s =s0 of the as-milled PbO – 1
12
c
2
3
3
Ag O–xC ð0:75 # x # 1:4Þ samples as a function of the graphite
2
CO
PbCO
3 4
increases with the appearance of Pb O
2
. With increasing temperature to 350 8C, the amount of
·PbO gradually reduces and that of PbCO ·2PbO
. When annealed at
starting content (x). The solid curve is the fitting result s =s ,
c
0
s
aðx 2 x Þ with fitting parameters of x ¼ 0:75 ^ 0:02; s ¼ 1:5 ^
3
3
c
c
0
:1; a ¼ 0:72 ^ 0:02:

496
D. Li et al. / Solid State Communications 125 (2003) 493–497
mal method [11], and high pressure synthesis [12].
However, all these annealed samples are insulators, which
is due to the small amount of Ag Pb . X-ray data
5
2 6
O
recorded from samples in Refs. [7,8] revealed the presence
of diffraction line at d ¼ 321 pm which were asserted to a
novel unknown material (cited as phase 321 in Refs. [7,8]).
Djurek et al. [7,8] assumed on the structure of the phase 321
as follows [7,8]. Pb layers could span hexanogal basal
2
þ
planes separated 321 pm along c-axis [7,8]. Pb cations
could be octahedrally co-ordinated by oxygen atoms with
octahedra face and vertex shared giving rise to formation of
22
(
2
Pb O
3
)
complexes which form three-dimensional cage.
They supposed the formation of PbyPb double bonds which
enable to Pb cattons, in the environment of ligands [7,8].
Unlike the literature [7,8], the phase 321 cannot be obtained
in the present work, which may be caused by the deficient
2 2
pressures of O and CO . We would like not to discuss in
detail this phase in this paper.
tF ei mg .p e4 r.at Du r Se C. graph of the sample of x ¼ 0:5 with increasing
4
. Summary
3
80 8C, both PbCO ·PbO and PbCO ·2PbO disappear and
3 3
In summary, the effects of oxygen atmosphere and
graphite content on synthesizing the composite of PbO – 1
Ag O–xC system by mechanical milling have been
Ag
5
Pb
2
O
6
forms. Before annealing at 380 8C, the elemental
2
12
Ag is mechanically mixed in other major phases. The
conductive Ag Pb phase [10] is produced directly by
reaction from Ag and PbCO ·2PbO at ambient pressure,
2
5
2
O
6
investigated. Under an oxygen and carbon dioxide atmos-
phere, there exist abundant phase transformations as the
graphite content varies in this system. With the phase
transformation occurring, the percolation threshold effect is
observed for the conductivity of PbO – 1 Ag O–xC
3
which is different from mechanical milling [9], hydrother-
2
12
2
system, changing from insulator to metallic conductivity.
The percolation threshold ðx ¼ 0:75Þ of the (2PbCO ·PbO þ
3
3
PbCO )/Pb nanocomposite is found by DC resistivity and
XRD measurements.
Acknowledgements
This work has been supported by the National Nature
Science Foundation of China (Grant Number 59725103) and
the Science and Technology Commissions of Shenyang.
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