Laboratory of preparation of optical materials and thermal analysis (J. Pejchal, R. Král)

Research team: Jan Pejchal, Robert Král, Karel Nitsch, Antonín Cihlář, Petra Zemenová, Aleš Bystřický

Scope

• Search and preparation of materials - crystals, nanostructures, and glasses - of high quality and purity with interesting optical properties and with their possible application as laser hosts operating in mid-IR region, as radiation detectors of neutrons, gama, and X-rays in industrial and medical application.
• Preparation of model crystalline systems for luminescence mechanism studies.
• Thermal analysis study of properties of solids and glasses prepared by rapid quenching.

Research and development in the field of materials preparation

Materials studied:

• Halides crystals - chlorides, bromides and iodides (pure and doped by transient metals and metals of rare earth) alkaline - Na, K, Rb, Cs, lead PbX₂ (X - Cl, Br, I), ternary alkali lead - KPb₂Cl₅, CsPbCl₃, CsPbBr₃, RbPb₂Cl₅, RbPb₂Br₅.
• Single crystals of complex oxides – materials based on aluminum garnets Lu₃Al₅O₁₂ (LuAG) and perovskites YAlO₃ (YAP) doped with rare-earth ions, lithium-containing crystals.
• Phosphate glasses - double phosphates (PO₃)^- of alkali metals (Li, Na, K, Cs) and metals of rare earth (Y, Ce, Gd, Pr. La) doped with (Ce, Er, Pr, Yb).

Experimental method:
Preparation:

• starting materials of high purity by means of chemical methods and zone refining
• halide single crystals from their melt by the vertical Bridgman method (Fig. 1d,e,f,h)
• complex oxide single crystals from their melt by micro-pulling-down method (Fig. 1b)
• phosphate glasses (Fig. 1a)
• single crystals from the high temperature solutions

Study and optimization of crystal growth conditions:

• Study of molten state influence (characterized by the thermal treatment and thermal history of the melt)
Study of influence of growth conditions, i.e. temperature gradient in the furnace and the pulling rate of the crystallization container on the final crystal quality
• Study of influence of the growth conditions on the position and the shape of the crystal-melt interface by direct observation (Fig. 1g) and inducing striations (Fig. 1c)
• Study of influence of the growth conditions on the position and the shape of the crystal-melt interface by direct observation (Fig. 1g) and inducing striations (Fig. 1c)
• Numerical modeling of crystal growth by vertical Bridgman method (cooperation with Dr. J. Hron, Faculty of Mathematics and Physics, Charles University in Prague, Prague, Czech Republic)

Characterization methods:

• Characterization of state of the melt using electric resistance measurements
• In-situ temperature field measurements in special quartz ampoule during simulated vertical Bridgman crystal growth

Fig. 1. Prepared phosphate glassy ingots of Er:LiY(PO₃)₄ (pink) a Ce:LiGd(PO₃)₄ (yellow) (a) and (b), longitudinal cut of 0.5%Ag:PbCl₂ captured in optical stereomicroscope with induced striations (c), single crystals of undoped PbCl₂ (d) and (h), Yb³⁺:RbPb₂Cl₅ (e) and Cu²⁺:PbCl₂ (f). Captured crystal-melt interface of undoped RbPb₂Br₅ (g) during vertical Bridgman growth.

Micro-pulling-down method

Micro-pulling-down method is a unique technique for crystal growth from the melt, which was newly established in the Czech Republic for the first time and installed in our laboratory in 2015. This method allows preparation of single crystals in matter of hours with diameter of several millimeters (3-5 mm) and length up to several centimeters. When compared to the other conventional methods (Bridgman, Czochralski, etc.), the starting material and energy consumption is significantly lower and considering the high growth rate, cost-effective material composition screening can be easily performed. With our setup configuration, single crystals of oxide materials with melting point up to 2100°C can be prepared.

Fig. 2. Overview of the micro-pulling-down apparatus T-MPD-OX Akita Seiko (a), schematic of the growth chamber (b), and snapshot of crystal growth of ruby (Al2O3 + 0.1 mol% Cr) captured by the CCD camera (c).

In Fig. 2a, the overview of the micro-pulling-down apparatus is displayed. In the top right, there is the control unit and the high-frequency power source is below. In the left, there is the growth chamber and an induction coil placed in a protection cage with the pulling mechanism underneath. The scheme of the growth chamber containing a system of concentric quartz glass tubes, alumina or zirconia ceramics, and a crucible of noble metal (Ir, Pt, Mo, etc.) is shown in Fig. 2b. High frequency electromagnetic field is generated by the induction coil, which heats up the crucible in the growth chamber. During the crystal growth, the melt flows through the capillary in the bottom of the crucible out, where by setting suitable growth conditions (temperature gradient and pulling rate) the solidification, formation of crystal-melt interface, and growth of new crystalline phase takes place. The growing crystal is pulled down at a speed of 0.01 – 0.1 mm/min. Temperature gradient and temperature field around the crucible is modified with ceramic shielding and an afterheater (noble metal). In the growth chamber the atmosphere of desired composition can be controlled with a flow of selected gases. Nitrogen or argon are the most common ones, while for the oxidizing conditions N₂ + O₂ (max 2% to prevent Ir crucible oxidization) can be used, and for the reducing ones Ar+H₂ is the most common gaseous mixture. The growth process is continuously monitored by a CCD camera, see Fig. 2c and video, where a crystal growth of a ruby (Al₂O₃ + 0,1 mol% Cr₂O₃) is captured. The as-grown single crystal of a ruby prepared with our apparatus is shown in Fig. 1b.

Snapshot video of ruby (Al₂O₃ + 0,1 mol% Cr₂O₃) crystal growth by micro-pulling-down method.

Research in the field of thermal analysis

• Thermal analysis – materials purity control, determinations of phase transitions, construction of phase diagrams, thermal properties of the glasses, crystallization study of glasses, measurement of glassy melts viscosity using penetration method.
• Thermomicroscopy – study of glassy melts nucleation and crystallization.

Laboratory equipment:

• Zone refining apparatus
• Vertical Bridgman crystal growth apparatuses
• Micro-pulling-down apparatus T-MPD-OX Akita Seiko Co., Ltd. (Fig. 2a)
• Simultaneous thermal analyzer Setaram Setsys Evolution 16 for DTA, DSC, and TG analysis, temperature range 25 – 1500 ^oC
• Thermo-mechanical analyzer Setaram Labsys TMA, temperature range 25 – 1400 ^oC
• Apparatus for thermo-microscopy, magnifications: 6,3 - 144 x, temperature range: -186 - +600 ^oC.