Optical properties of traditional ceramic with different sintering
temperatures in terahertz range
Xin Y. Miaoab, Qing N. Yangc, Cheng J. Fengb, Ri M. Baob, Kun Zhao*b, Li Z. Xiaoa
a
State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum,
Beijing 102249, China
b
Beijing Key Laboratory of Optical Detection Technology for Oil and Gas, China University of
Petroleum, Beijing 102249, China
c
College of Materials Science and Engineering, Sichuan University, Chengdu 610065, China
ABSTRACT
Terahertz spectroscopy was used to study the sintering process of traditional ceramic by scanning sample with different
final temperatures (from 100-1450C). Absorption coefficient (α) and refractive index (n) were obtained with different
final temperatures. The sintering process was divided into four stages on the basis of α and n, which characterized the
ceramic sintering process well. The results coincide with the actual situation. Therefore, THz-TDS represents a
promising technique to monitor the synthesis process of materials.
Keywords: Ceramic, Sintering, Temperature, Terahertz spectroscopy
1. INTRODUCTION
Ceramics are widely used due to excellent physical and mechanical properties. Traditional ceramics have been produced
for thousands of years in history and nowadays China is still the largest producer in the world. Sintering is the key step in
the processes of the ceramic production since firing conditions have crucial impact on the performance of ceramics[1-4].
Manufacturers and researchers have been focusing on the sintering behavior and changes in properties of ceramic caused
by high temperature. In general, the sintering process of ceramic was investigated with increasing calcination
temperature through transmission electron microscope, scanning electron microscopy, thermal analysis and X-ray
diffraction at present[5-7].
Terahertz wave fills the gap between microwave and infrared spectroscopy[8]. They can be transmitted through a wide
variety of substances such as plastics, rocks, semiconductors, paper and ceramics just like radio waves and be propagated
through space, reflected, focused and refracted like light waves[9-13]. In addition, as a radiation with low energy, terahertz
wave has been proposed as a useful tool in non-destructive testing, such as defect detection and heritage conservation[1416]
. Terahertz time domain spectroscopy (THz-TDS), based on direct measurements of the amplitude of ultrashort
electromagnetic pulses, has been proved to be a multifunctional tool for determination of the optical parameters such as
absorption coefficient, extinction coefficient and refractive index of a sample. Moreover, THz-TDS is an appropriate
method for process monitoring because of its online properties and simple measurement conditions, which are necessary
for the rapid identification of components and structure of materials in the industry[17-20].
Generally, the traditional ceramic sintering process is divided into four stages[21-23]: the exclusion of the remaining
moisture (below 300C), oxidative decomposition and transformation of crystal (from 300 to 900 C), porcelain-forming
(from 900  C to the final temperature) and cooling stage (from the final temperature to normal temperature). In this
research, we mainly focused on the nondestructive evaluation of heat treated ceramics using THz-TDS since terahertz
wave is sensitive to structure changes (such as water content, density and grain size)[24-26]. Two kinds of traditional
ceramics heated from low temperature to high temperature (100-1450C) were analyzed using THz-TDS. We propose a
new method to characterize several stages during the sintering process and determine the range of optimal final
temperature.
Selected Papers of the Photoelectronic Technology Committee Conferences held June–July 2015, edited by
Shenggang Liu, Songlin Zhuang, Daren Lv, Lijun Wang, Guangjun Zhang, Michael I. Petelin, Libin Xiang,
Proc. of SPIE Vol. 9795, 979532 · © 2015 SPIE · CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2209580
Proc. of SPIE Vol. 9795 979532-1
2. EXPERIMENTAL METHODS
Purple Sand (Zisha Tao) clay and White (Bai Tao) clay were studied and the main mineral of both kinds of clay is Kaolin.
Table 1 presents typical chemical analysis of the tested materials. It can be seen from the table that the main difference
between them is the content of silicon dioxide, aluminum oxide and ferric oxide. The aforementioned materials
investigated here are typical commercial products from the ceramic industry. They were made into mud by adding water
and shaped in molds.
The experimental setup employed a conventional THz-TDS system using transmission geometry from Zomega Terahertz
Corporation. An femtosecond laser beam (800 nm) was split into two beams, the pump beam was focused onto the
surface of a biased GaAs photo conductive antenna for THz generation and the probe beam for electro-optic detection.
THz pulses were focused onto a sample by optical lens and the THz beam carrying sample’s information met the probe
laser beam at the ZnTe crystal in THz detector. To avoid moisture absorption in the air and enhance the signal noise ratio
(SNR), the setup was covered with dry nitrogen.
Solid state diffusion, which is dependent on sintering temperature, has a significant effect on microstructure and
mechanical properties. The optimal sintering temperatures of Purple Sand and White pottery are 1180-1210 C and 13001330 C, respectively. Therefore, the samples were heated to different temperatures repeatedly. The heating experiment
and THz experiment were conducted alternately while we changed final temperature of sintering each time. The green
bodies were heated up to temperatures in the range of 100-1450C in a box furnace. The holding time was 30 min and
the heating rate and the cooling rate were both set at 10 C/min. The long exposure to the high temperature was intended
to make sure that the heat penetrates into the core and uniformly increases the temperature of the specimen. All THzTDS measurements were performed for four times at four mutually perpendicular positions of each sample and the
average value was used to calculate the optical parameters.
Purple Sand
White pottery
SiO2
Table 1 Chemical analysis of the tested materials
Al2O3
K2O
Na2O
MgO
CaO
Fe2O3
TiO2
LOI
66.81
52.11
15.80
31.31
5.70
1.59
0.86
-
5.37
11.08
MS
3.08
2.30
0.52
0.17
THz Detector
1.20
0.69
0.58
0.40
L
g
O
Computer
A
THz Emitter ITOI
GaAs
M`
M1
Controller
>
Femto- Second LASER
Figure 1. Schematic diagram of transmission THz-TDS setup
3. RESULTS AND DISCUSSION
We could obtain the amplitude and phase information of samples simultaneously by THz-TDS. Figure 2 shows the timedomain waveforms of the specimens at several final temperatures. They were variant from each other at the peak
amplitude (Amp) and the delay time (Δt). Amp was higher at lower treating temperatures compared to higher ones. Δt
reduced first, then increased with the temperature rising.
Proc. of SPIE Vol. 9795 979532-2
f1as°
(b)
\
00-
1350
1300
1250
1200
110
1050
^
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.
A.,
16
600
400
700
600
400
300
200
100
300
200
100
dt
Ama
Reference
24
22
20
18
1000
900
800
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700
At
:
00
1250
1100
1050
1000
900
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1350
15
17
1'6
20
19
1'8
Reference
21
22
23
24
25
Time (ps)
Time (ps)
Figure 2. THz time-domain spectra of (a) White pottery and (b) Purple Sand samples at different final temperatures.
0.13
(b) 6.0 _r-0- Kaolin
(a) 0.12
-- Purple Sand
0.11
5.5
/
,-
0.10
0.09
S 0.08
Q
E 0.07
Q 0.06
-\ / J
0-0-0-41.....
0.05
0.04
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--\,j
0.03
0
200
400
600
800
1000
1200
1400
0
200
400
600
800
1000
1200
1400
Temperature ( °C)
Temperature ( °C)
-- White pottery
-- Purple Sand
E 1.8
E
1.7
---
----e
i
E,
--_.-.
1.6
1.5
1.40
1/
200
400
600
800
1000
1200
1400
Temperature ( °C)
Figure 3. Variations of (a) Amp, (b) Δt and (c) d with final temperature
In order to express more clearly, Amp and Δt were extracted from the time-domain spectra and the average value was
calculated. Variation of the samples thickness (d) with each final temperature was measured. The temperature
dependence of average Amp, Δt and d were displayed in Figure 3., where the changing tendency of both samples was
substantially the same for all the three parameters. Amp first increased in the range of 100-400C, then basically remained
unchanged till a sharp decrease at 900C. The fall stopped in the 1200-1300C and then dropped again till the maximum
temperature. Δt declined slightly in the 100-1000C, then rose steeply and leveled off at the turning points of 1200C and
Proc. of SPIE Vol. 9795 979532-3
1350  C for Purple Sand and White pottery, respectively. According to the Beer-Lambert law the absorbance of THz
waves was proportional to the path through which the light travels while the concentration remains constant. However,
the attenuation of Amp increased in the 1000-1200C and 1300-1450C range, while d shortened first and then expanded.
A similar situation was also reflected in the delay time. When a THz pulse waveform transmitted through the sample and
met the detector, there was a time delay in the signal compared to the reference signal caused by the increase of optical
path. Δt nearly extended monotonically in the range of 1000-1450C while d changed nonmonotonically, corresponding
to the variation in density of the sample. Thus, we calculated the absorption coefficient (α) and refractive index (n). The
dependence of average α and n on the heating temperature was displayed in Figure 4, where α was calculated from the
spectrum functions of the reference and sample respectively, and then divided by d. n was obtained by Δt and d. 0.8 THz
was selected due to acceptable SNR.
(a)16 -
(b)
-(;)- Purple Sand
`-0- White pottery
2 .1
* -White pottery
- *- Purple Sand
2.0
14
Overburn
it*i
Porcelain- forming
12
Porcelain- forming
10
t£t
8
*--*
Removal of free water
_
6
1 Removal of bound water
:»*
4
2
0
200
400
600
*
\
*- *
Porcelain- forming
Porcelain- forming
800
1000
1200
1400
1600
*i*
0
200
Temperature ( °C)
400
600
800
1000
1200
1400
1600
Temperature ( °C)
Figure 4. Sintering temperature dependent (a) α and (b) n at 0.8 THz.
The observed trends of α were very noticeable. Generally, the whole sintering process of Purple Sand pottery could be
divided into four stages of 100-300  C, 400-900  C, 1000-1200  C and 1250-1450  C in accordance with the heating
temperature, while that of White pottery with four stages of 100-300C, 400-900C, 1000-1300C and 1350-1450C.
Free water existed in pores of the green bodies and evaporated slowly in the first stage (100-300 C). It is known that the
hydrogen bond collective network formed by water molecules changes on a picosecond timescale and THz regime is
sensitive to its picosecond-scale inter molecular dynamics. Therefore, α dropped while the water content reduced with
increasing temperature. In the second stage, loss of bound water in Kaolin reacted in 400-600  C, causing the trifling
decrease of α. Meanwhile, the quartz crystalline was slightly expanded at 573 C and the carbonate decomposed which
led to a decline in density. Thus the refractive index n decreased. The third stage was a critical stage in sintering while
liquid phase appeared and the mullite was generated by feldspar, aluminum oxide and quartz, which improved the
ceramic strength. As the temperature rose, small particles dissolved and recrystallization occurred, which favored the
growth of the grain. Elevating temperature during this phase increased the density progressively and caused the steep
linear rise of α and n. As the temperature continued rising, a faster expansion of the grain occurred and the structure was
changed irregularly (the overburnt stage). The drop of n and α at 1200C or 1350C indicated the decrease of density,
and the enhanced α at 1300-1450C or 1400-1450C was caused by scattering due to surface irregularities. According to
the analysis above, it can be concluded that the optimal final temperature was nearby the demarcation point of the third
stage to the overburnt stage in the range of 1200-1250  C (Purple Sand) and 1350  C (White pottery), which was
consistent with the data provided by the manufacturer.
4. CONCLUSIONS
In this study, we investigated the impacts of final temperature to the sintering of two kinds of traditional ceramic by
THz-TDS. Variations of THz-parameters with different temperatures were obtained and analyzed. The sintering process
was divided into four stages by α and n, which well characterized the ceramic sintering process. Therefore, THz
technique can be used as a promising tool to monitor the synthesis process of materials and the future applications will
be expected.
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ACKNOWLEDGMENTS
This work was supported by the Specially Funded Program on National Key Scientific Instruments and Equipment
Development (Grant No. 2012YQ140005), the National Key Basic Research Program of China (Grant No.
2014CB744302) and the NSFC (Grant No. 61405259).
REFERENCES
[1] Chao, S., Petrovsky, V., Dogan, F., “Effects of sintering temperature on the microstructure and dielectric properties
of titanium dioxide ceramics,” J. Mater. Sci. 45(24), 6685–6693 (2010).
[2] Hsiang, H. I., Hsi, C. S., Huang, C. C., Fu, S. L., “Sintering behavior and dielectric properties of BaTiO3 ceramics
with glass addition for internal capacitor of LTCC,” J. Alloys Compd. 459(1-2), 307–310 (2008).
[3] Xu, Q., Wu, S., Chen, S., Chen, W., Lee, J., Zhou, J., Sun, H., Li, Y., “Influences of poling condition and sintering
temperature on piezoelectric properties of (Na0.5Bi0.5)1−xBaxTiO3 ceramics,” Mater. Res. Bull. 40(2), 373–382 (2005).
[4] Nahm, C. W., “The effect of sintering temperature on varistor properties of (Pr, Co, Cr, Y, Al)-doped ZnO ceramics,”
Mater. Lett. 62(29), 4440–4442 (2008).
[5] Huang, C. L., Wang, J. J., Huang, C. Y., “Sintering behavior and microwave dielectric properties of nano alphaalumina,” Mater. Lett. 59(28), 3746–3749 (2005).
[6] Huger, M., Fargeot, D., Gault, C., “High-temperature measurement of ultrasonic wave velocity in refractory
materials,” High Temp. High Press. 34(2), 193–201 (2002).
[7] Baimukhamedov, E. K., Gladum, G. G.., Sharinkhanov, A. A., “Self-propagating high-temperature synthesis of hightemperature lightweight refractories,” Inzh.-Fiz. Zh. 65(4), 490–491 (1993).
[8] Dai, J., Xie, X., Zhang, X. C., “Detection of broadband terahertz waves with a laser-induced plasma in gases,” Phys.
Rev. Lett. 97(10) (2006).
[9] Wilk, R., Pupeza, I., Cernat, R., Koch, M., “Highly accurate THz time-domain spectroscopy of multilayer structures,”
IEEE J. Sel. Top. Quantum Electron. 14(2), 392–398 (2008).
[10] Bao, R., Wu, S., Zhao, K., Zheng, L., Xu, C., “Applying terahertz time-domain spectroscopy to probe the evolution
of kerogen in close pyrolysis systems,” Sci. China Physics, Mech. Astron. 56(8), 1603–1605 (2013).
[11] Naftaly, M., Miles, R. E., “Terahertz time-domain spectroscopy for material characterization,” Proc. IEEE 95(8),
1658–1665 (2007).
[12] Siegel, P. H., “Terahertz technology in biology and medicine,” IEEE Trans. Microw. Theory Tech. 52(10), 2438–
2447 (2004).
[13] Hoffmann, M. C., Monozon, B. S., Livshits, D., Rafailov, E. U., Turchinovich, D., “Terahertz electro-absorption
effect enabling femtosecond all-optical switching in semiconductor quantum dots,” Appl. Phys. Lett. 97(23) (2010).
[14] Kamba, S., Nuzhnyy, D., Savinov, M., Šebek, J., Petzelt, J., Prokleška, J., Haumont, R., Kreisel, J., “Infrared and
terahertz studies of polar phonons and magnetodielectric effect in multiferroic BiFe O3 ceramics,” Phys. Rev. B Condens. Matter Mater. Phys. 75(2) (2007).
[15] Rihani, S., Faulks, R., Beere, H., Page, H., Gregory, I., Evans, M., Ritchie, D. A., Pepper, M., “Effect of defect
saturation on terahertz emission and detection properties of low temperature GaAs photoconductive switches,” Appl.
Phys. Lett. 95(5) (2009).
[16] Schwerdtfeger, M., Castro-Camus, E., Krügener, K., Viöl, W., Koch, M., “Beating the wavelength limit: threedimensional imaging of buried subwavelength fractures in sculpture and construction materials by terahertz time-domain
reflection spectroscopy,” Appl. Opt. 52(3), 375–380 (2013).
[17] Chan, W. L., Deibel, J., Mittleman, D. M., “Imaging with terahertz radiation,” Reports Prog. Phys. 70(8), 1325–
1379 (2007).
[18] Mittleman, D. M., Jacobsen, R. H., Neelamani, R., Baraniuk, R. G., Nuss, M. C., “Gas sensing using terahertz timedomain spectroscopy,” Appl. Phys. B-Lasers Opt. 67(3), 379–390 (1998).
[19] Jackson, J. B., Mourou, M., Whitaker, J. F., Duling III, I. N., Williamson, S. L., Menu, M., Mourou, G. A.,
“Terahertz imaging for non-destructive evaluation of mural paintings,” Opt. Commun. 281(4), 527–532 (2008).
Proc. of SPIE Vol. 9795 979532-5
[20] Zimdars, D., White, J. S., Stuk, G., Chernovsky, A., Fichter, G., Williamson, S., “Security and non destructive
evaluation application of high speed time domain terahertz imaging,” 2006 Conf. Lasers Electro-Optics 2006 Quantum
Electron. Laser Sci. Conf. (2006).
[21] Al-Hilli, M. F., Al-Rasoul, K. T., “Influence of glass addition and sintering temperature on the structure, mechanical
properties and dielectric strength of high-voltage insulators,” Mater. Des. 31(8), 3885–3890 (2010).
[22] Cheng, C. W., Shih, C. F., Behera, R. K., Hsu, W. D., “Investigation of initial stages of nano-ceramic particle
sintering using atomistic simulations,” Surf. Coatings Technol. 231, 316–322 (2013).
[23] Tielrooij, K. J., Timmer, R. L. A., Bakker, H. J., Bonn, M., “Structure dynamics of the proton in liquid water probed
with terahertz time-domain spectroscopy,” Phys. Rev. Lett. 102(19) (2009).
[24] Li, S., Jadidi, M. M., Murphy, T. E., Kumar, G., “Terahertz surface plasmon polaritons on a semiconductor surface
structured with periodic V-grooves,” Opt. Express 21(6), 7041–7049 (2013).
[25] Taday, P. F., Bradley, I. V., Arnone, D. D., Pepper, M., “Using Terahertz pulse spectroscopy to study the crystalline
structure of a drug: A case study of the polymorphs of ranitidine hydrochloride,” J. Pharm. Sci. 92(4), 831–838 (2003).
[26] Kuzel, P., Nemec, H., Kadlec, F., Kadlec, C., “Gouy shift correction for highly accurate refractive index retrieval in
time-domain terahertz spectroscopy,” Opt. Express 18(15), 15338–15348 (2010).
Proc. of SPIE Vol. 9795 979532-6