Structural and Electrical Characterization of Sintered Silicon Nitride Ceramic

doi:10.4236/msa.2011.27102

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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.

Email: [email protected]

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

















(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

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 (



 ) 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





 (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















 (6)

We also have



121 2

01 2

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



01 2

CR R













 (8)

where



01 2

21 12

sCR R



























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







 (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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