Transcript Diapositiva 1 - Materials Knowledge

Knowledge Based Materials, Marie Curie Summer Schools , Älvdalen 2006
Observations of the Electrical Properties near to the Percolation
Threshold in Si3N4-ZrC composites
I.Gallici°*, V. Petrovsky**, F. Spizzo***and S.Meriani*
° IKTS - Fraunhofer Institute Keramische Technologien und Systeme, Winterbergerstraße 28, 01277 Dresden Germany
* D.M.R.N – University of Trieste, via Valerio 2, 34127 Trieste, Italy
** Materials Science Institute NASU, Krhzyhzanovsky str. 3, 03124 Kiev, Ukraine
*** Electrolux Home Products Italy S.p.A. – Core Technology & Innovation, Corso Lino Zanussi 30, 33080 Porcia (PN), Italy
Corresponding author : [email protected], [email protected]
Introduction
General Abstract
Studies of the electrical behaviour of multi-component Ceramics
Matrix Materials are not numerous and their results are not yet
sufficient to design and engineer technical components with
properties better than those of the individual constituents. Most of
the work on CMM composites was done to improve materials
mechanical properties, but less to improve their electrical behaviour
as in the case of studying the electronic conductivity as a function
of composition, shape and dimensions of the particles.
The main question concerning the development of advanced
resistive CMMs is the prediction of their effective electrical
behaviour, starting from composition and size to focus on their
microstructure evolution during the technological cycle of
production.
Technical Abstract
The Research deals with microstructure vs. properties relations
in electronically conducting ceramic matrix composite; in
particular the project aims at the characterization and
development of new electro-conductive ceramic composites
based on nitrides and transition metals carbides as
conducting secondary phase. Ceramic nitrides can be used for
a variety of thermo-mechanical devices, in this case the
composites should find application as heating elements. In this
context, it is also important to study and develop analytical
methods based on electro-conductivity measurements, that
should allow the design of more efficient heaters and provide a
key for the interpretation of the phenomena that link
temperature, microstructure and electrical conduction
behaviours.
Materials
Results
Method
T °C
1,E+02
1,E+03
1,E+04
1,E+05
1,E+06
20
50
100
150
200
250
300
350
400
500
550
600
650
700
750
800
850
900
450
Re(σ)[Ωcm-1]
1,50
T °C
1,00
0,50
1,E+02
1,E+03
1,E+04
1,E+05
Intensity / cps
20
50
100
150
200
250
300
350
400
550
600
650
700
750
800
850
900
950
450
500
2
Graf 1 : XRD spectra of sample with15% vol ZrC and 5%vol MgO; (
peaks of SiC,
peaks of ZrC,
peaks of Si3N4)
1,E+05
1,E+06
The samples studied show, a flat response in the
impedance plot as a function of frequency confirming
primarily a conducting behaviour;
The XRD spectra reveal the presence of SiC that can
contribute to increase the electrical conduction;
Sample with fine powder, according to the percolation
models, show low value of resistance at lower volume
fraction of conducting filler;
The addition of the MgO seems to bring to higher
value of density, due to the formation of more liquid
phase,
according
to
the
following
reaction:
6MgO+Si3N4=6Mg(g)+3SiO2+2N2;
The composites studied exhibit a positive temperature
coefficient over the whole temperature range, which
supports metallic-like electron conduction.
 Only in one case the conductivity increase with
temperature, the matrix is supposed to exhibit a strong
influence on the electrical Resistivity of the composite,
that may produce a semi-conducting behaviour as a
function of temperature, yet not fully understood.
1,E+06
500
70
0
1,E+04
2,00E-04
1000
60
1,E+03
3,00E-04
Freq(Hz)
50
1,E+02
Graf 4: Plot of Real part of Impedance versus Frequency at room temperature,
for four samples of different composition.
4,00E-04
0,00
1,E+01
40
20%volZrC-5%MgO
5,00E-04
2,00
2 Theta / °
20%volZrC-5%Al2O3
Freq (Hz)
20%vol ZrC-5%MgO
30
1,E+00
1,E-01
6,00E-04
Graph 2 : The Real part of conductivity depending on the
measuring temperature and frequency for a sample containing
13% vol of ZrC and 5%MgO, Red arrow indicates the
increasing temperature .
1
1,E+01
1,E-02
1,E+01
Freq(Hz)
0
1,E+02
10%volZrC-5%MgO
13%vol ZrC-5%MgO
0,00E+00
1,E+01
1500
1,E+03
13%volZrC-5%MgO
1,00E-04
Fig 1 : Polished and plasma etched surface of sample containing
30% vol of ZrC and 5% vol of MgO (white area Si3N4-black
particle ZrC) .
1,E+04
Z' (Ohm)
Experiments were performed using Hewlett-Packard
4192 impedance analyzer. The data were collected
between 5Hz-13MHz and between room temperature to
about 1000°C under inert atmosphere.
Each sample to be investigated for complex impedance
was coated with a Pt-paste before being fixed between
Pt electrodes.
Re(σ)[Ωcm-1]
Commercial powders of Si3N4 and ZrC were used.
Al2O3 and MgO in the amount of 5 vol.% were used as
activators.
ZrC powders were subjected to air sizing to prepare
mixtures with both fine (3 m) or coarse (28-42m) size.
Samples with 5-30% vol of ZrC have been studied since
the percolation threshold has been found around ~20%
vol ZrC (9).
Sintering was obtained by reactive hot-pressing in
CO
atmospheres.
Samples
were
sintered
in
HP
.
furnace with graphite moulds at 1680-1720°C for 2545 minutes under about 20 MPa pressure.
20
In this contribution impedance spectroscopy has been used to
characterize composites based on silicon nitrides and transition
metal carbides near to the percolation threshold. In particular, a set
of dense Si3N4-ZrC ceramic systems, with Al2O3 and MgO as
sintering aids, and various amounts of ZrC, have been realized and
investigated. Measurements in a wide range of temperatures and
frequencies have been carried out to study the conduction
mechanisms. Bulk and surface modifications have been examined
using ESEM, XRD and EDS techniques.
The electrical response of sintered bodies has been correlated with
their micro-structural and crystalline phases evolution, in order to
substantiate the ability of impedance spectroscopy to disclose the
role of processing parameters on the conductivity behaviour .
Graph 3 : The Real part of conductivity depending on the
measuring temperature and frequency for a sample containing
20% vol of ZrC and 5%MgO. Red arrow indicates the increasing
temperature .
The aim of the future activity will be:
 Evaluating the Impedance spectra using
Effective Media Theory;
 Compare the electrical properties of the sample
produced with different process of production HP
and SPS;
 Use of impedance spectroscopy as a nondestructive quality control.
Glossary
HP, Hot Pressing: the pressing process occurs at high
temperature. Basically, the process combines pressing with
firing; SPS, Spark Plasma Sintering : novel sintering technique
characterized by pulsed DC current that directly passes
through a graphite die, as well as the powder compact, in
case of conductive samples.Transition Metal Carbide or
Interstitial Carbides : compounds between Carbon and
elements of the IV,V,VI group of the period table. Large
difference in electro-negativity, the carbon atom has much
smaller size than the other atoms allowing it to nest in the
interstices of the lattice. The bonding is mostly metallic, their
composition is often indeterminate, electrical and thermal
conductivities are high, they have high melting point, high
hardness and chemical inertness.
Percolation threshold :Volume fraction around which the
second phase of a composite becomes three-dimensionally
connected throughout the medium; Percolation theory: This
theory gives a phenomenological equation for the
conductivity of a system (or composite medium) near to a
metal-insulator -transition; Effective Media Theory: States that
in a binary (or higher) mixture each ellipsoidal grain is , on
average, surrounded by a mixture which has the effective
conductivity of the composite medium; MIT, metal–insulatortransition: Within a very small composition range , a material
can go from being an insulator to a conductor or vice versa
(see Percolation Threshold ).
References
Resistivity : Also known as specific electrical resistance, is a
measure of how strongly a material opposes the flow of
electric current. Impedance: Is a measure of opposition to a
sinusoidal electric current. The concept of electrical
impedance generalizes Ohm´s Law to AC circuit analysis. The
impedance of an electric circuit can be a complex number
Z(ω)= Z‘-j Z‘‘. Impedance Spectroscopy: This method uses an
AC signal to probe the material, measuring the input-output
voltage, current and phase angle at different frequencies. 4point Probe Measurement: Technique used for measuring
Resistivity. A current source forces a current through the
sample outer probes, a voltmeter measure the voltage V
through the inner two probes.
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