Crystal structure and microwave dielectric properties of (1 - x)NdAlO3-xCaTiO3 ceramics

Article abstract of DOI:10.1016/S0025-5408(01)00793-0

Crystal structure and microwave dielectric properties of (1-x)NdAlO₃-xCaTiO₃ ceramics have been investigated. Crystal structure of the specimens changed with the composition. Rhombohedral structure was found for the specimens with x ≤ 0.1. When 0.3 ≤ x ≤ 0.7, the specimens had the tetragonal structure and it changed to the orthorhombic structure as x exceeded 0.7. Two types of the second phases were observed in (1-x)NdAlO₃-xCaTiO₃ ceramics. For the specimens with x ≤ 0.5, Nd₄Al₂O₉ phase was observed and Al-rich phase was found in the specimens with x ≥ 0.7. The dielectric constant (ε_r) and the temperature coefficient of the resonant frequency (τ_f) increased with the increase of x. The Q × f value of the specimen increased with x and exhibited the maximum value when x = 0.5. The microwave dielectric properties of Q × f = 45,000 GHz, ε_r = 45 and τ_f = -1.5 ppm/°C were obtained for 0.3NdAlO₃-0.7CaTiO₃ ceramics.

Full text of DOI:10.1016/S0025-5408(01)00793-0

Materials Research Bulletin 37 ꢀ2002) 605±615
Crystal structure and microwave dielectric properties
of ꢀ1 À x)NdAlO3±xCaTiO3 ceramics
Min-Han Kima, Chang-Soo Wooa, Sahn Nahma,*, Chang-Hack Choib,
Hwack-Joo Leec, Hyun-Min Parkc
aDepartment of Materials Science and Engineering, Korea University, 1-5 Ka, Anam-Dong,
Sungbuk-Ku, Seoul 136-701, South Korea
bAdvanced Materials Technology, 17-2 Jamwon-Dong, Seocho-Ku, Seoul 137-030, South Korea
cNew Materials Evaluation Center, Korea Research Institute of Standards and Science,
TaedukScience Town, Taejon 305-600, South Korea
ꢀRefereed)
Received 20 November 2000; received in revised form 8 May 2001; accepted 17 August 2001
Abstract
Crystal structure and microwave dielectric properties of ꢀ1 À x)NdAlO3±xCaTiO3 ceramics
have been investigated. Crystal structure of the specimens changed with the composition.
Rhombohedral structure was found for the specimens with x ꢀ 0:1. When 0:3 ꢀ x ꢀ 0:7, the
specimens had the tetragonal structure and it changed to the orthorhombic structure as x
exceeded 0.7. Two types of the second phases were observed in ꢀ1 À x)NdAlO3±xCaTiO3
ceramics. For the specimens with x ꢀ 0:5, Nd4Al2O9 phase was observed and Al-rich phase
was found in the specimens with x ꢁ 0:7. The dielectric constant ꢀer) and the temperature
coef®cient of the resonant frequency ꢀtf) increased with the increase of x. The Q Â f value of
the specimen increased with x and exhibited the maximum value when x ˆ 0:5. The microwave
dielectric properties of Q  f ˆ 45,000 GHz, er ˆ 45 and tf ˆ À1:5 ppm/8C were obtained
for 0.3NdAlO3±0.7CaTiO3 ceramics. # 2002 Elsevier Science Ltd. All rights reserved.
Keywords: A. Ceramics; A. Electronic materials; C. X-ray diffraction; D. Dielectric properties
1. Introduction
Microwave dielectric materials of a high quality factor ꢀQ-value), a good stability
of temperature coef®cient of resonant frequency and a high dielectric constant have
* Corresponding author. Fax: ‡82-2-928-3584.
E-mail address: snahm@korea.ac.kr ꢀS. Nahm).
0025-5408/02/$ ± see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 2 5 - 5 4 0 8 ꢀ 0 1 ) 0 0 7 9 3 - 0

606
M.-H. Kim et al. / Materials Research Bulletin 37 52002) 605±615
been extensively studied because of their applications to microwave devices [1,2].
Many investigations were carried out on the materials with a high dielectric constant
such as BaO±Re2O3±TiO2 ꢀRe ˆ rare earth) and ꢀPb, Ca)ZrO3 systems [3±5]. The
complex perovskite materials with a low dielectric constant and a high Q-value were
also intensively studied [6±8]. Recently, the interests on the dielectric materials with a
intermediate dielectric constant ꢀer ꢁ 45) and a high Q Â f value ꢀ>43,000 GHz) have
been increased. ꢀZr, Sn)TiO4 ꢀQ  f ˆ 45,000 GHz, er ˆ 38) ceramics is known to
have a high quality factor but the dielectric constant is relatively low [9]. The
ꢀ1 À x)LaAlO3±xCaꢀor Sr)TiO3 system has been investigated and the microwave
dielectric properties of Q  f ˆ 47,000 GHz, er ˆ 38 and tf ˆ 5 ppm/8C were
obtained for 0.35LaAlO3±0.65CaTiO3 ceramics [10,11]. The Q-value of
0.35LaAlO3±0.65CaTiO3 ceramics is high but the dielectric constant is still low.
The microwave dielectric properties of ꢀ1 À x)LaꢀZn0.5Ti0.5)O3±xCaꢀor Sr)TiO3
system were also studied but the results were not satisfactory [12,13].
According to the previous work, NdAlO3 has a rhombohedral structure and the
dielectric properties of NdAlO3 are similar to those of LaAlO3 [14,15]. Therefore, if
CaTiO3 were incorporated to NdAlO3, it would be possible to develop a dielectric
material which has an intermediate value of dielectric constant and a high Q-value.
The objective of this work is to investigate the crystal structure and the microwave
dielectric properties of ꢀ1 À x)NdAlO3±xCaTiO3 ceramics and to ®nd a new materials
with er ꢁ 45, Q Â f > 43,000 GHz and tf of zero.
2. Experimental
The starting materials were Nd2O3, Al2O3, CaCO3, and TiO2. They were mixed for
24 h in nylon jar with ZrO2 balls, and then dried and calcined at 1200±14008C for
10 h. After remilling, the powder was dried and pressed into disk and sintered at
1420±16008C for 6 h in air. Phase identi®cation of the specimen was carried out using
a Rigaku D/max-B diffractometer with a conventional Cu Ka radiation. Lattice
parameters were calculated by least-square re®nement of data collected with an
internal Si standard. TREOR was used for least-square re®nement. The microstruc-
ture of the specimens was studied by using scanning electron microscopy ꢀSEM)
ꢀHitach S-4300). The density of the sintered specimens was measured by water-
immersion technique. The dielectric properties in the microwave frequency range
were measured by a dielectric post resonator technique suggested by Hakki and
Coleman [16] and Courtney [17]. The temperature coef®cient of the resonant
frequency was measured at 5 GHz in the temperature range of 25±858C.
3. Results and discussion
Fig. 1 shows the X-ray diffraction patterns of ꢀ1 À x)NdAlO3±xCaTiO3 ceramics
with 0:0 ꢀ x ꢀ 0:9 sintered at 14508C. All the peaks were indexed in terms of the

M.-H. Kim et al. / Materials Research Bulletin 37 52002) 605±615
607
Fig. 1. X-ray diffraction patterns of ꢀ1 À x)NdAlO3±xCaTiO3 ceramics with 0:0 ꢀ x ꢀ 0:9 sintered at
14508C.
simple perovskite unit cell. Two types of the second phases were observed in
ꢀ1 À x)NdAlO3±xCaTiO3 ceramics. For the specimens with x ꢀ 0:5, the peak for
the Nd4Al2O9 second phase indicated by full circle was found. The intensity of the
peak for Nd4Al2O9 phase decreased with the increase of x and disappeared for
specimens with x > 0:5. When x exceeded 0.5, the peak for the unknown phase
indicated by the asterisk was found and the intensity of the peak increased with the x.
In addition, the peak for the chemical ordering was not observed in these specimens.
The shape of the X-ray peaks changed with x [see the ꢀ220) peak] implying the
variation of the crystal structure with the composition. In order to ®nd the crystal
structure of the specimens, the lattice parameters of the specimens were calculated
from the X-ray diffraction patterns and the results are illustrated in Table 1. Silicon was
Table 1
Lattice parameters of ꢀ1 À x)NdAlO3±xCaTiO3 ceramics
Ê
Lattice parameters ꢀA)
Composition
a ˆ b ˆ g
Structure
a
b
c
x ˆ 0.0
x ˆ 0.1
x ˆ 0.3
x ˆ 0.5
x ˆ 0.7
x ˆ 0.9
x ˆ 1.0
3.7522
3.7642
5.3470
5.3770
5.4000
5.4320
5.4424
±
±
90.355
90.330
90
Rhombohedral
Rhombohedral
Tetragonal
±
±
±
7.5760
7.5930
7.6250
7.6370
7.6417
±
90
90
Tetragonal
Tetragonal
±
5.3810
5.3807
90
90
Orthorhombic
Orthorhombic

608
M.-H. Kim et al. / Materials Research Bulletin 37 52002) 605±615
used as the internal standard for the calculation of the lattice parameters of the speci-
mens. As shown in Table 1, the specimens with x ꢀ 0:1 could be indexed to rhombo-
hedral structure. When 0:3 ꢀ x ꢀ 0:7, the tetragonal structure was the more probable
than any other model structure and it changed to orthorhombic structure as x exceeded
0.7. It is worth to mention that according to the electron diffraction analysis, the cubic
phase was found in the 0.1NdAlO3±0.9CaTiO3 ceramics sintered at 16008C. Since
CaTiO3 is cubic above 13278C [18], a small amount of the high temperature cubic
phase is considered to coexist with the orthorhombic phase in 0.1NdAlO3±0.9CaTiO3
ceramics sintered at 16008C. More detailed results on the crystal structure developed in
ꢀ1 À x)NdAlO3±xCaTiO3 ceramics will be published in the near future.
Fig. 2ꢀa)±ꢀe) show the SEM images of ꢀ1 À x)NdAlO3±xCaTiO3 ceramics sintered
at 14508C for 6 h. The average grain size of specimens increased with the increase of
x. Two types of second phases were also found in SEM images. For the specimens
with x ꢀ 0:5, the second phase with needle shape was observed and it is considered
Fig. 2. SEM images of ꢀ1 À x)NdAlO3±xCaTiO3 specimens sintered at 14508C: ꢀa) x ˆ 0:1, ꢀb) x ˆ 0:3,
ꢀc) x ˆ 0:5, ꢀd) x ˆ 0:7 and ꢀe) x ˆ 0:9.

M.-H. Kim et al. / Materials Research Bulletin 37 52002) 605±615
609
Fig. 2. ꢀContinued ).

Fig. 3. EDS spectra of the second phases existed in ꢀa) 0.9NdAlO3±0.1CaTiO3 ceramics, and ꢀb) 0.1NdAlO3±0.9CaTiO3 ceramics.

M.-H. Kim et al. / Materials Research Bulletin 37 52002) 605±615
611
as Nd4Al2O9. For the specimens with x ꢁ 0:7, the second phasewith the different shape
was observed. In order to identify the composition of the second phases, EDS analysis
was carried out on 0.9NdAlO3±0.1CaTiO3 and 0.1NdAlO3±0.9CaTiO3 ceramics.
Fig. 3ꢀa) illustrates the EDS spectrum of the second phase existed in 0.9NdAlO3±
0.1CaTiO3 ceramics. According to the quantitative analysis, the ratio of Al to Nd was
about 0.5 and the amount of Ca and Ti was negligible. Therefore, this result con®rms
that the second phase formed in the specimens with x ꢀ 0:5 is Nd4Al2O9. Fig. 3ꢀb)
shows the EDS spectra taken at the second phase found in the 0.1NdAlO3±0.9CaTiO3
ceramics. Even though, the exact composition of the second phase was not identi®ed,
the high concentration of Al ions was found at the second phase. Therefore, the
second phase formed in the specimens with x ꢁ 0:7 is the Al-rich phase.
Fig. 4 shows the variation of the relative density with x for ꢀ1 À x)NdAlO3±
xCaTiO3 ceramics sintered at different temperatures. The relative densities of the
specimens with 0:1 ꢀ x ꢀ 0:9 were high ranged between 95 and 98% of the
theoretical density. However, the relative density of NdAlO3 ceramics sintered at
1450 or 16008C was very low. Therefore, it is considered that NdAlO3 ceramics is not
completely sintered at 1450 or 16008C. Fig. 4 also shows that the relative density of
0.1NdAlO3±0.9CaTiO3 ceramics sintered at 16008C is lower than that sintered at
14508C. The density of the high temperature cubic CaTiO3 phase is lower than that of
orthorhombic phase [18,19]. Therefore, the low density of 0.1NdAlO3±0.9CaTiO3
ceramics sintered at 16008C could be explained by the presence of cubic phase. In
addition, as shown in Fig. 5, the average grain size of the specimens with x < 0:9
sintered at 16008C is slightly larger than that sintered at 14508C. However, it suddenly
Fig. 4. Variation of the relative density with x for ꢀ1 À x)NdAlO3±xCaTiO3 ceramics.

612
M.-H. Kim et al. / Materials Research Bulletin 37 52002) 605±615
Fig. 5. Variation of the average grain size with x for ꢀ1 À x)NdAlO3±xCaTiO3 ceramics.
increased for 0.1NdAlO3±0.9CaTiO3 ceramics sintered at 16008C. Therefore, the
sudden grain growth could be also responsible for the low density of 0.1NdAlO3±
0.9CaTiO3 ceramics.
The dielectric constant of ꢀ1 À x)NdAlO3±xCaTiO3 ceramics with 0:0 ꢀ x ꢀ 0:9
was illustrated in Fig. 6. It increased with x but the rate of the increase varies with the
composition and the structure. The change of the er was negligible for the specimens
with x ꢀ 0:1 which have the rhombohedral structure. For the specimens with
0:3 ꢀ x ꢀ 0:7 which have the tetragonal structure, er increased with x but the rate
of the increase was small. As x exceeded 0.7, the tetragonal structure changed to the
orthorhombic structure and the er increased signi®cantly. The er of 0.3NdAlO3±
0.7CaTiO3 ceramics was about 45. In addition, the er of 0.1NdAlO3±0.9CaTiO3
ceramics sintered at 16008C is relatively low compared with that sintered at 1420 and
14508C. It is probably due to the presence of the cubic phase, which decreased the
density of the specimen. However, since the dielectric properties of the cubic phase
have not been known, the effects of the cubic phase on the dielectric properties of the
specimen are not completely understood at this moment.
Fig. 7 illustrates tf of the ꢀ1 À x)NdAlO3±xCaTiO3 ceramics. The tf of NdAlO3
ceramics was about À50 ppm/8C. The behavior of tf with x was similar to that of the
er. The variation of tf was negligible when the specimen has the rhombohedral
structure. For the specimens with 0:3 ꢀ x ꢀ 0:7, tf of the specimen slowly increased
with x. However, it greatly increased for the specimens with x > 0:7. The tf of
0.3NdAlO3±0.7CaTiO3 was about À1.5 ppm/8C.
The variation of the Q Â f value with x for ꢀ1 À x)NdAlO3±xCaTiO3 ceramics is
shown in Fig. 8. According to the previous work, the Q Â f value of NdAlO3 ceramics

M.-H. Kim et al. / Materials Research Bulletin 37 52002) 605±615
613
Fig. 6. Variation of the dielectric constant with x for ꢀ1 À x)NdAlO3±xCaTiO3 ceramics.
Fig. 7. Variation of the temperature coef®cient of resonant frequency with x for ꢀ1 À x)NdAlO3±
xCaTiO3 ceramics.

614
M.-H. Kim et al. / Materials Research Bulletin 37 52002) 605±615
Fig. 8. Variation of the Q Â f value with x for ꢀ1 À x)NdAlO3±xCaTiO3 ceramics.
is about 60,000 GHz [15]. Therefore, it was expected that NdAlO3 has the highest
Q Â f value and it decreases with the increase of x as found in ꢀ1 À x)LaAlO3±xCaTiO3
system. However, the Q-value of NdAlO3 was very low and it increased with x and
exhibited the maximum value as x ˆ 0:5. This anomalous result could be explained by
thepresenceofNd4Al2O9 secondphase.AsshowninFig.1,theNd4Al2O9 phaseexistsin
the NdAlO3 and the amount of Nd4Al2O9 phase decreased with x. Therefore, the low
Q Â f value of the specimens with x < 0:5 is attributed to the presence of Nd4Al2O9
phase. In addition, for NdAlO3 ceramics, the low relative density is also responsible
for the deterioration of Q-value. Fig. 8 also shows the variation of the Q Â f value with
the sintering temperature. The Q Â f value of the specimens sintered at 14208C
was relatively low. However, for the specimens sintered above 14208C, the variation
of Q Â f value with the sintering temperature was not signi®cant. The Q Â f value
of 0.3NdAlO3±0.7CaTiO3 ceramics sintered at 14508C was about 45,000 GHz.
Therefore, 0.3NdAlO3±0.7CaTiO3 ceramics exhibited excellent dielectric properties
and it can be a good candidate for the microwave dielectric materials.
References
[1] K. Wakino, K. Minai, H. Tamura, J. Am. Ceram. Soc. 67 ꢀ4) ꢀ1984) 278.
[2] W. Wersing, in: B.C.H. Steele ꢀEd.) Electronic Ceramics, Elsevier, New York, 1991, p. 69
ꢀChapter 4).
[3] D. Kolar, Z. Stadler, S. Gaberscek, D. Suvorov, Ber. Dr. Keram. Ges. 55 ꢀ1978) 7.

M.-H. Kim et al. / Materials Research Bulletin 37 52002) 605±615
615
[4] H. Oshato, J. Sugino, A. Komura, S. Nishigaki, T. Okuda, Jpn. J. Appl. Phys. 38 ꢀ1999) 5625.
[5] J. Kato, H. Kagata, K. Nishimoto, Jpn. J. Appl. Phys. 30 ꢀ1991) 2343.
[6] S. Nomura, K. Toyama, K. Kaneta, Jpn. J. Appl. Phys. 21 ꢀ1982) L624.
[7] S. Kawashima, M. Nishida, I. Ueda, H. Ouchi, Am. Ceram. Soc. 66 ꢀ1983) 421.
[8] S.B. Desu, H.M. O'Bryan, J. Am. Ceram. Soc. 68 ꢀ10) ꢀ1985) 546.
[9] G. Wolfram, H.E. Gobel, Mater. Res. Bull. 16 ꢀ1981) 1455.
[10] J.H. Moon, H.M. Jang, H.S. Park, J.Y. Shin, H.S. Kim, Jpn. J. Appl. Phys. 38 ꢀ1999) 6821.
[11] P. Sun, T. Nakamura, Y.J. Shan, Y. Inaguma, M. Itoh, T. Kitamura, Jpn. J. Appl. Phys. 37 ꢀ1998)
5625.
[12] S.Y. Cho, I.T. Kim, K.S. Hong, Jpn. J. Appl. Phys. 37 ꢀ1998) 593.
[13] D.H. Yeo, J.B. Kim, J.H. Moon, S.J. Yoon, H.J. Kim, Jpn. J. Appl. Phys. 35 ꢀ1996) 663.
[14] F.S. Gallaso, Structure, Properties and Preparation of Perovskite-type Compounds, Pergamon
Press, Oxford, 1994.
[15] D.G. Lim, Ph.D. thesis, Korea University, South Korea, 1999.
[16] B.W. Hakki, P.D. Coleman, in: Proceedings of the IEEE Transactions on Microwave Theory.
Tech. MTT-8, 1960, p. 402.
[17] W.E. Courtney, in: Proceedings of the IEEE Transactions on Microwave Theory. Tech. MTT-18,
1970, p. 476.
[18] JCPDS Diffraction Data Card No. 43-0226.
[19] JCPDS Diffraction Data Card No. 42-0423.