Materials Sciences and Applicatio n, 2011, 2, 739-748
doi:10.4236/msa.2011.27102 Published Online July 2011 (http://www.SciRP.org/journal/msa)
Structural and Electrical Characterization of
Sintered Silicon Nitride Ceramic
Imran Khan, M. Zulfequar*
Department of Physics, Jamia Millia Islamia, New Delhi, India.
Received December 28th, 2010; revised March 28th, 2011; accepted May 5th, 2011.
ABSTRACT
The electrical conduction phenomena, dielectric response and microstructure have been discussed in sintered silicon
nitride ceramics at different temperature and frequencies. Microstructure and phase of the sintered samples was inves-
tigated by Scanning Electron Microscope (SEM) and X-ray diffractometer (XRD). The electrical conductivity, dielectric
constant and dielectric loss increases exponentially with temperature greater than 600 K. The dielectric constant and
loss have been measured in the frequency range 100 Hz to 1 MHz. The a.c. conduction studies in the audio frequency
range 500 Hz to 1 MHz indicates that the conduction may be due to the electronic hopping mechanism. Silicon Nitride
ceramics became dense after sintering. The effect of grain size and role of phase on electrical and dielectric properties
have been discussed. These types of samples can be used as a high temperature semi conducting materials for device
packaging.
Keywords: Silicon Nitride, D.C. and A.C. Conductivity, Dielectric and Structural Properties
1. Introduction
Silicon nitride-based ceramics have many excellent
properties, high strength and relatively high fracture
toughness, good wear resistance, good oxidation resis-
tance and good corrosion resistance. For some time, they
have been under consideration as high-performance
structural materials because of their superior thermal
shock resistance relative to oxide ceramics [1]. Silicon
Nitride has been densified with sintering additives be-
cause of the highly covalent Si-N bonding. After sintering,
these additives remain as the amorphous grain boundary
phase, which severely deteriorates the high-temperature
mechanical behavior of Silicon Nitride ceramics [2]. Di-
electric films of silicon nitride ceramics play an integral
role in nearly every semiconductor device and integrated
circuit. Among this Silicon Nitride ceramic is used as a
gate dielectric layer, diffusion barrier and optoelec-
tronic-integrated circuits [3-5]. Low-density porous sili-
con nitride [6-12] ceramic is an important material with
properties including low dielectric constant (

 = 2.5 to
8, tan δ ≤ 3 × 10–3), good mechanical, high resistance to
rain erosion and sand erosion. Hence, low-density porous
silicon nitride ceramic is also a candidate for application
in radome materials [13]. Microstructural control of the
interface and interlayer requires
a complex interplay between initial composition and post-
heating transformations. The controlled crystallization of
refractory secondary phases from the remaining oxyni-
tride liquid generally results in an improvement of the
high-temperature resistance of sintered Si3N4 ceramics
[14-16]. Nowadays most silicon nitride ceramics are pre-
pared using α-Si3N4 powders. In silicon nitride ceramics,
the microstructure is similar to whisker-reinforced ce-
ramic composites, with large rod like β-Si3N4 grains as
the reinforcing agents [17]. The effect of microstructure
or grain size on dielectric properties are also already dis-
cussed [18]. In earlier studies on the micro structural cha-
racterization of silicon nitrides ceramics, it is noted that
all of them are conducted at relatively low TEM/SEM
magnifications and therefore are limited to the examina-
tion of the general microstructure. J. D Walten, P. Popper
and J. S. Throp [19,20] reveal the kinetics of densification,
mechanical, electrical and thermal properties of Si3N4 has
been studied. However no such systematic effort had been
made on the electrical conductivity and dielectric proper-
ties of silicon nitride ceramic.
The electrical conductivity and dielectrical measure-
ment are very important to check the insulating ability
and dielectric losses particularly at high temperatures.
The emphasis of the research, in particular, is on the
characterization of the grain-boundary microstructure by

Structural and Electrical Characterization of Sintered Silicon Nitride Ceramic 740
electron imaging and to determine the electrical conduc-
tivity, dielectric constant and loss of sintered silicon Ni-
tride ceramics.
2. Experimental Procedure
The samples prepared by grinding α-phase Silicon Ni-
tride (pure 99.999%), purchased from Alfa Aesar (A
Johnson Matthey company USA) and then the powders
made into pallet using Hydraulic press at a pressure of
100 MPa. The material used in this investigation was
sintered Silicon-nitride. Sintering was carried out in an
alumina crucible heated by a graphite heating element in
digital programmable furnace at the rate of 15˚C per min.
as an initial constant heating rate from room temperature
to 900˚C for 8 hrs, then to 950˚C at the same constant
heating rate for 2 h under normal pressure. For high
temperature sintering up to 1450˚C for 12 h at the rate of
15˚C per min. we use muffle furnace. The samples then
cooled from 1450˚C to room temperature at a same rate.
For dc conductivity measurement, the samples were
mounted in a specially designed metallic sample holder
where a vacuum of about 10–3 Torr could be maintained
throughout the measurement. The thicknesses of samples
were 1.02 mm and diameter ~10 mm. A dc voltage of 30
volts was applied across the samples and resulting cur-
rent was measured by a pico-ammeter (Keithley, model
6485). The temperature is measured by mounting a
chromel-alumiel thermocouple near the sample. Before
the I-V characteristics measurement, the samples were
annealed at 100˚C in vacuum to avoid the effect of
moisture. Dielectric measurements were performed by a
Wayne Kerr LCR Meter (model-4300) in audio frequent-
cies range of 100 Hz to 1 MHz as well as in the tem-
perature range 300 K to 1000 K. In order to observe the
morphology of Si3N4 samples, scanning electron mi-
croscopy (SEM) was used. SEM analyses were carried
out on surfaces of samples using a Scanning Electron
Microscope (JEOL model JSM 6380). The specimens
were silver paste coated in order to avoid charging ef-
fects. The crystalline phases present in the sintered ce-
ramics materials were identified by X-ray powder dif-
fraction PXRD (Powder X-Ray diffractometer) (PANa-
lytical X-Ray Diffractometer with PW1830 Generator)
(Ni-flltered Cu Kα radiation; λ = 1.5406 Å) [21].
3. Result and Discussion
3.1. DC Conductivity
To measure the DC conductivity (σdc) of sintered Silicon
Nitride ceramic, the temperature dependent current has
been measured. The dc conductivity is plotted as a func-
tion of temperature as shown in Figure 1. In the tem-
peratures region (T < 625 K), the dc conductivity is
nearly temperature independent. In the higher tempera-
tures region (T > 625 K), the dc conductivity increases
exponentially with temperature.
The Arrhenius behavior of the dc conductivity can be
expressed by the usual relation
0exp dc
dc
E
K
T








(1)
where 0

is the pre-exponential factor, Edc is the ac-
tivation energy for dc conductivity and k is Boltzmann
constant. The (I-V) curve is linear at room temperature
which shows the ohmic behaviour of the contact. The
Figure 1. Temperature dependent dc c onduc tivity of silicon nitride ceramics.

Structural and Electrical Characterization of Sintered Silicon Nitride Ceramic 741
lower temperature (<625 K) the behaviour of dc conduc-
tivity suggested by variable-range hopping conduction.
At high temperature (>625 K), in the thermal activated
conduction region, formation of large number of charge
carriers within the energy gap; start to participate in the
conduction mechanism. The pre-exponential factor (0

)
depends on the mobility and concentration of charge car-
riers which shows that the low value of pre- exponential
factor (0

) indicates the presence of contribution of lo-
calized states. Here the improvement in conductivity can
be attributed to an increase in the mobility of the charge
carriers. The electrical parameters calculated from the
“least-square straight-line fits” using Equation (1) is
given in Table 1. The high covalence of the principal
Si-N bonds, leads to the formation of substantial addi-
tional charge density re-distribution and finally increase
the conductivity with temperature [22].
The aspect of this charge hopping mechanism is that
the electron or hole tends to associate with local defects.
The activation energy for charge transport may also in-
clude the energy of free hole from its position next to the
defects [23,24], and conduction occurs by phonon-as-
sisted hopping between the localized states. The electri-
cal conduction of the sample follows a mechanism in
which the electron or hole may from one localized site to
the next, and the surrounding molecules respond to this
perturbation with structural changes and the electron or
hole is temporarily trapped in the potential well. The
electron resides at this site until it is thermally activated
migrate to another site [25,26]. The activation energy ΔE
in the high-temperature region calculated from the slope,
required for the movement of the electron or hole. The
electron trapped in such a potential well requires activa-
tion energy to overcome a barrier of a height equal to the
binding energy of the polaron in order to move to the
neighboring site. The electrical conductivity of the sin-
tered samples may form the basis for designing a new
class of solid electrolytes and fuel cells applications with
elevated operating characteristics.
3.2. Dielectric Properties
The alternative representations of the AC response of
silicon nitride material are: dielectric permittivity

*

 and electrical conductivity

*

 . In the pre-
sented work the measured quantities are the capacitance
Table 1. Electrical parameters at T = 800 K and F = 500 Hz.
Function At 1000 K At 625 K
σdc 6.45 × 10–9 (Ω–1cm–1) 1.61 × 10–13 (Ω–1cm–1)
ΔΕ 1.48 eV 0.16 eV
σ0 2.23 × 10–16 (Ω–1cm–1) 2.40 × 10–14 (Ω–1cm–1)
C and dissipation factor D. They have the transformation
relationships described as below. The real part dielectric
constant (


) and imaginary part dielectric loss ("

) of
the dielectric permittivity are extracted using the estab-
lished relationships:
0CAd




 and D


 
 (2)
The AC conductivity

has been calculated by the
relation
0
acT dc





 (3)
where d is the thickness, A is the cross-sectional area of
the sample, 0

is the permittivity of the vacuum and

is the angular frequency. Frequency and temperature
dependence dielectric parameters (

 and "

) of the
sintered silicon nitride ceramics are investigated in the
temperature range (300 K - 1000 K) at frequencies (100
Hz to 1 MHz). Figure 2 and Figure 3, show the tem-
perature dependent dielectric constant (

) and dielectric
loss ("

) at different frequencies (100 Hz to 1 MHz).
The values of


and "

from room temperature to
(>625K) remain almost independent of temperature. As
the temperature are increases,

 and "

increase
quite appreciably with temperature.
The increase in the dielectric constant and loss are de-
termined by the bulk conductivity of the grains, which
increases as the frequency decreases as shown in Figure
4 and Figure 5. It was established that a higher dielectric
constant is typical for materials based on Si3N4 from fur-
nace synthesis. We have observed that this is connected
with the state of the grain boundaries in the sintered ma-
terials. It was established that recrystallization processes
practically do not occur during sintering of these materi-
als; so as a result of interphase interaction between the
Si3N4 crystallites with the other phases, their surface
layer varies substantially to a depth determined by the
activity of the diffusion processes. The major crystalline
phases present in the products were α-Si3N4, β-Si3N4, and
Si2ON2 in addition to starting phase [27]. This leads to
formation of energy levels at the grain boundaries and
the electronic polarization increases.
During grain growth, more perfect crystallites are
formed without a defective outer shell, which reduces
their conductivity and thus the polarizability. The dielec-
tric loss ("

) arise due to the dielectric polarization and
dc conduction. To study the origin of the dielectric loss
in the operating temperature range, the dc contribution
was calculated using the relation;
0DC DC





(4)
The calculated and observed dielectric loss ("

) are
plotted in Figure 6 with temperature. It is observed that

Structural and Electrical Characterization of Sintered Silicon Nitride Ceramic 742
Figure 2. The dielectric constant () vs. temperature.
Figure 3.Tthe dielectric loss (') vs. temperature.
the dc conduction loss is less than the observed loss.
The variation of

 and "

with temperature and
frequency can not, therefore, be attributed to the dc con-
duction losses. The formation of defects in the structure at
high temperature is expected because of recrystallization.
Therefore, the samples should possess vacancies due to
recrystallization and generating charge carriers. These
results show insignificant conductive loss (
D
C

 ) over the
entire temperature range. However, the temperature de-
pendence of the dielectric loss ("

) for high tempera-
tures region (T > 800 K) shows the apparent increase
with increasing temperature, due to the recrystallization
and phase transition. Hence the dielectric loss cannot be
attributed to dc conduction in the entire temperature
range. It is attributed due to space charge polarization.
Figure 7, shows initially (in lower temperature region)
weak temperature and frequency dependent of AC con-
ductivity. But by increasing the temperature, the de-
pendence of temperature and frequency becomes more
significant.
The frequencies dependent ac conductivity shows
Figure 8, which follows the power law as given below
S
ac
A

  (5)
Here, A is a constant and s is the frequency dependent
exponent parameter calculated from the slop of the
straight line. The value of s is found 0.75 which is in
between 0.5 - 1.0 in the temperature range (300 K - 1000

Structural and Electrical Characterization of Sintered Silicon Nitride Ceramic 743
Figure 4. Frequenc y de pendent dielectric constant ().
Figure 5. Frequenc y de pendent dielectric loss ().
Figure 6. Temperature depende nc e conduc tion loss.

Structural and Electrical Characterization of Sintered Silicon Nitride Ceramic 744
Figure 7. Temperature depende nt a.c. conductivity at various freque nc ie s.
Figure 8. Frequenc y de pendent a.c. conductivity.
K). The all calculated dielectric parameters are given in
Table 2. We show that the conduction in Si3N4 ceramic
is hopping conduction which is due to the space charge
polarization.
The variation in

 and

ac with temperature & fre-
quencies may therefore, be attributed to space charge
polarization whose possibilities is quite large in the pre-
sent case. As we are dealing with polycrystalline samples
the grain boundaries and various other structural defects
may have different condition and a heterogeneous system
is formed. Due to different region of conductivity the
charge may be accumulated which may result into large
polarization at high temperature. The dielectric disper-
sion observed in the present case may, therefore, be un-
derstood in terms of space charge polarization. Based on
the simplest Maxwell-Wagner two-layer capacitor model,
the behaviour of such a capacitor under AC field has
been analyzed. It was found that both the overall dielec-
tric constant and the conductivity depend on the fre-
quency, the relative difference in layer thickness, the
dielectric constant, and the conductivity between the two
layers [28]. It is well known [29,30] that dielectric prop-
erties are strongly dependent on the dielectric polariza-
bility, porosity and the grain sizes. The dielectric con-
stant decreases with increasing porosity, and the geomet-
rical nature of the pores is also a factor that affects the
dielectric constant. Thus by measurement of the real part
of relative permittivity it is not possible to distinguish
between the effects of interfacial and orientational po-
larizability.
From equation
22
'1
s









 (6)
We also have


22
121 2
22
01 2
1
1
".
1
CR R

 

 
 (7)

Structural and Electrical Characterization of Sintered Silicon Nitride Ceramic 745
Table 2. Dielectric parameters at T = 800 K and F = 500 Hz.
Function At 1000 K At 625 K
F = 160 Hz F = 1 MHz F = 160 Hz F = 1 MHz
ε' 56.94 2.00 27.1 1.74
ε'' 673.03 1.53 10.13 0.16
σac(Ω–1cm–1) 2.5 × 10–8 8.2 × 10–8 3.5 × 10–10 3.8 × 10–10
ε''ac 592.06 2.19 5.56 2.16
Hence


22
01 2
1
"1
s
CR R






 (8)
where

12
01 2
21 12
12
sCR R
RR
RR













And R1 and R2 are the resistance of grain boundary
layers and the grain. The second term of Equation (8) is
exactly the Debye relaxation equation, but there is an
additional term inversely proportional to frequency. This
means that the losses, represented by "

, tend to infinity
as ω tends to zero. Thus the case of interfacial polariza-
bility may be distinguished from Debye relaxation by
observing the variation of "

below the relaxation fre-
quency. In the Debye case "

drops towards zero as the
frequency is lowered.
In general, the resistivity of boundary layers (1

) is
larger than the resistivity of the grain (2

), Consider
&
1x12

  then
'12
02
12
x




 (9)
It is clear from equation that the relaxation time (τ) for
the interfacial polarization is proportional to the product
of the resistivity of the two layers. The frequency at which
these types of polarization become effective which there-
fore depends upon the resistivity ρ2 of the grain. As the
resistivity decreases at higher temperature, this type of
polarization becomes quite effectives at these frequencies.
Due to the decrease in resistivity, the dielectric constant
(

) increase and dielectric loss ("

) also increases.
From the above discussion, one can interpreted that the
space charge polarisation may to predominate the polari-
sation mechanism in Silicon Nitride Ceramic.
3.3. Structural Characterization
In Figu re 9, the microstructures of the sintered samples are
shown in the SEM images of fractured surface of the sam-
ples. The volume of pores and grain size can be seen clear-
ly. During sintering, silica on the surface of Si3N4 and some
of the nitride forms an oxynitride at a high temperature,
which promotes the densification of the material. The sur-
face of this particular elongated grain shows distinct evi-
dence of crack growth along the surface of the grain. Such
a growth mode helps to produce a complex crack path,
which in turn contributes to crack deflection and bridging,
thereby improving the toughness of these ceramics. The
large grains surrounding some fine grains can form open as
well as closed pores [16,17]. This effect results in the rear-
rangement stage and rapid initial densification. So these
dense Silicon Nitride ceramics samples confirm certain
electrical and dielectrical properties at high temperature.
The X-ray diffraction of Si3N4 ceramics is shown in
Figure 10. There are ten major peaks indicate the
-phase of silicon nitride five peaks indicate -phase of
silicon nitride and four peaks of another phase of silicon
oxy nitride (SiON). X-ray diffraction Pattern indicates
that the sintered samples basically preserve the characters
of the starting powders. The sintering mechanism up to
some extent, confirms certain electrical properties of the
sintered samples [31,32].
4. Conclusions
We have discussed the electrical conductivity of Silicon
Nitride Ceramic (Si3N4) ceramics in the temperature
range (300 K to 1000 K). We have studied the dc con-
ductivity and dielectric parameters (

and "

) of the
sintered silicon nitride ceramics with structural charac-
terization. The electrical properties are significantly af-
fected with temperature as well as frequency. The elec-
trical conduction of the Si3N4 sample follows a mecha-
nism in which the electron or hole may from one local-
ized site to the next. The a.c. conduction studies in the
frequency range 500 Hz to 1 MHz indicates that the con-
duction may be due to the large polarization at high tem-
perature. As it is shown from experimental results that
sintered silicon nitride ceramics material shows very
good results by high polarizability over entire frequency
range. The electrical and dielectric properties are also
depended on the grain size. The qualitative dielectric
dispersion observed in the present case confirms space
charge polarization. During sintering the diffusion proc-
esses leads to formation of surface energy levels at the
grain boundaries and enhance the polarization. The grain
growth formed more perfect crystallites without a

Structural and Electrical Characterization of Sintered Silicon Nitride Ceramic 746
Figure 9. Grain boundaries in sintered silicon nitride ceramics at 1450˚C.
Figure 10. X-RD pattern of sintered silicon nitride ceramics.

Structural and Electrical Characterization of Sintered Silicon Nitride Ceramic 747
defective outer shell. SEM images confirm the rear-
rangement stage and rapid initial densification and X-ray
diffraction pattern indicates that the sintered samples
basically preserve the characters of the starting powders.
These types of samples can be used as a high temperature
semi conducting materials.
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