Effects of gallium substitution on crystal growth and properties of gehlenite single crystal

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Highlights

  • High quality CAGS single crystal was successfully grown by Czochralski method.

  • Gallium substitution maked CAS crystal growth easier and improved crystal quality.

  • Dielectric and piezoelectric properties of CAS and CAGS were measured and compared.

Abstract

Gehlenite crystal, Ca2Al2SiO7 (CAS) crystal is found to be appropriate material to high temperature piezoelectric sensors, for its high electrical resistivity and high temperature stability of piezoelectric coefficients. But the challenges of growing high quality single crystal and the fragile characteristics limited its broad application and mass production in future. In this paper, a new member of the melilite family, bulk Ca2Al2-xGaxSiO7 (CAGS) single crystal, was successfully grown by the Czochralski (Cz) method, without any bubble, inclusion and cracking. By a small fractional of gallium substitution, we obtained a new potential high temperature piezoelectric single crystal CAGS with larger pulling rate (more than twice that of CAS), and better quality of single crystal. Furthermore, the dielectric and piezoelectric properties of CAS and CAGS single crystals were measured and compared. The measured piezoelectric coefficient d14 of CAGS single crystal is 10.52 pC/N which is much higher than that of CAS (6.68 pC/N). The electrical resistivity of CAGS was found to be remarkably high, reaching a value of ∼2.3 × 109 Ω cm at 600 °C.

Introduction

In the past few years, high-temperature (HT) piezoelectric sensors have attracted great of interest in industrial and scientific field owing to their extensive application, such as gas sensors, gas injectors, combustion pressure sensors directly placed in engine cylinders and so on. Ca2Al2SiO7 (gehlenite; CAS) single crystal is attractive for HT piezoelectric applications, because of its excellent HT performance. The CAS belongs to tetragonal system with space group P4¯21m [1,2]. Cations are found on three types of sites: large eightfold coordinated sites (Thomson cubes) occupied by the large cation Ca2+ and two types of tetrahedral sites: a regular one, T1, where half of Al3+ ions are located and a very distorted one, T2, smaller than T1, where Si4+ and half of Al3+ ions are statistically distributed [3]. This structure can also be described as stacking, along c-axes, of alternate layers of (T1 +T2) tetrahedral and Thomson cubes. CAS single crystal (melting point 1590 °C) was first grown in 1979 by the floating zone method and then grown in 1981 by the Czochralski method [4,5]. The resistivity of CAS was found to be two orders of magnitude higher than that of LTG and LGS crystals, which are notable candidate materials for combustion sensors [6]. Furthermore, the high temperature stability of resistivity and piezoelectric properties make CAS crystals appropriate material to high temperature piezoelectric sensors. Although there was no growth instability happened during the crystal growing procedure, such as spiral symptom during the growth of CAS single crystals, some inclusions and bubbles were found inside the boule frequently [7], which required low pulling rate to get high-quality single crystals during crystal growth and decreased the utilization of the crystals. Furthermore, the distinct cleavage of CAS crystals, along c-axes, would cause some inconvenience for sample preparation during the cutting process.

According to our knowledge, gallium substitution may decrease the viscosity and increase the melt fluidity greatly. In addition, gallium substitution usually lowers the melting point. Those may make crystal growth much easier, shorten the growth cycle and improve crystal quality. For example, the viscosity of Gd3Ga5O12 (GGG) is much lower than that of Y3Al5O12 (YAG) and it is easier to observe the flow of liquid. More importantly, unlike the YAG single crystal, there was no core strain found inside the GGG single crystal [8], which was greatly beneficial for large slab processing in the future. In consequence, we hypothesize that it may have the same effects on crystal growth of CAS. In this paper, we report the growth of high quality Ca2Al2-xGaxSiO7 (CAGS) single crystal and discuss the effects of gallium substitution on the structure, quality of single crystal, resistivity, and piezoelectric properties of CAS.

Section snippets

Experimental section

Polycrystalline Synthesis and Crystal Growth. CAS and CAGS single crystals have been grown by the Cz method, under highly pure nitrogen atmosphere and a little oxygen-containing (∼0.36%) nitrogen atmosphere respectively. The oxygen-containing nitrogen atmosphere is to prevent the oxidization of the crucible as well as decomposition and evaporation of the initial oxide. Ca2Al2SiO7 and Ca2Ga2SiO7 (CGS) were synthesized separately. The starting materials, 99.99% pure CaCO3, Ga2O3, SiO2 and α-Al2O3

Results and discussion

Polycrystalline Synthesis. The PXRD patterns of the sintered raw materials of CAS and CGS are shown in Fig. 1 separately, which indicates only the CAS and CGS phase throughout the entire sample.

Crystal Growth and Morphology of the CAS and CAGS Crystals. In the first trial of CAS crystal growth, we found bubbles accumulated on the surface of the melt, attaching to the crucible. The CAS melt had a high viscosity and low fluidity, causing the seeding process difficult to control. For example, it

Conclusions

Bulk Ca2Al2-xGaxSiO7 (CAGS) single crystal, a new member of the melilite family, was successfully grown by the Czochralski method, without any bubble, inclusion and cracking. The measured piezoelectric coefficient d14 of CAGS single crystal is 10.52 pC/N, and the electrical resistivity of CAGS was found to be remarkably high, reaching a value of ∼2.3 × 109 Ω cm at 600 °C. By a small fractional of gallium substitution, we obtained a new potential high temperature piezoelectric single crystal

Funding sources

This work is supported by the financial support from the key Research and Development Program of Shandong province (Grant No. 2018CXGC0410, 2018CXGC0411), the Natural Science Foundation of Shandong Province (ZR2018PEM007, ZR2018BEM024), the Young Scholars Program of Shandong University (Grant No. 2015WLJH36), and the 111 Project 2.0 (Grant No. BP201813).

CRediT authorship contribution statement

Yanru Yin: Conceptualization, Methodology, Software, Investigation, Writing - original draft, Writing - review & editing. Guiji Wang: Validation, Formal analysis, Software. Zhitai Jia: Validation, Formal analysis. Jian Zhang: Writing - review & editing. Qian Wu: Resources, Writing - review & editing, Supervision, Data curation. Wenxiang Mu: Writing - review & editing. Qiangqiang Hu: Writing - review & editing. Xutang Tao: Resources, Writing - review & editing, Supervision, Data curation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We gratefully acknowledge the financial support from the key Research and Development Program of Shandong province (Grant No. 2018CXGC0410, 2018CXGC0411), the Natural Science Foundation of Shandong Province (ZR2018PEM007, ZR2018BEM024), the Young Scholars Program of Shandong University (Grant No. 2015WLJH36), and the 111 Project 2.0 (Grant No. BP201813).

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