A heating stage for the microscopic investigation of phase transitions A. Schranz a , M. Grüninger a , H. Tschrischnig a , N. Herres a , T. Schober b a Interstaatliche Hochschule für Technik Buchs NTB, Werdenbergstrasse 4, CH-9471 Buchs (SG) b Institut für Festkörperforschung (IFF), Forschungszentrum Jülich, D-52425 Jülich Specifications of the heating stage • to deliver - defined temperatures at the sample from RT to above 100°C - a defined temperature gradient at the sample • to fit: - the x-y-stage of a Zeiss Axioskop2MAT microscope - reflected and transmitted light mode • to allow: - use of different sample holders (modular design) - easy loading and unloading of samples Specifications of the control program and the user interface • to display - temperatures and temperature gradient clearly • to enable - easy setting of temperatures and temperature gradients - adjustments to the settings be made on the screen (virtual instrument) as well as with rotating knobs (potentiometers) - closed loop temperature control • to assist - in the configuration of the interface settings • to allow - storing and printing of the settings Realization • thermally decoupled arrangement of the heating stage and the microscope table • two heating cartridges each of 3 W electrical power • two thermal conductors from high-conductivity graphite • two Pt-100 temperature transducers • magnetic clamping of the sample holder • separate panel with two potentiometers for small temperature adjustments • DAQ card and SC–2345 module from National Instruments (NI) • LabVIEW software from NI for closed loop heating stage control • user interface written with LabVIEW System overview Motivation The microscopic observation of a phase boundary can reveal useful details about the structural changes occurring at or near the transition temperature. Commercial heating stages are usually designed to keep either a sample temperature as stable as possible to a targeted value or to move the whole sample as homogeneous and linear as possible from one temperature to another. In both cases the location at the sample where a phase transition occurs first will usually not be predictable. In particular with samples showing undercooling, the onset of a spontaneous nucleation is often difficult to visualize and the high growth velocity complicates the observation which may then be followed only by a high-speed camera. Our heating stage overcomes some of these problems using a temperature gradient at the sample. The heat is fed to the sample via two independent heating cartridges under closed-loop temperature control. Visualization and defined manipulation of the location of a phase boundary is easily achieved. Power Supply / Electronic Module heating stage with top cover two knobs (potentiometers) for small temperature adjustments PC with User Interface two Pt-100 transducers Observation of the domain structure of niobium hydride near to the phase transition at 88 °C Fast „growing“ domain structure at a shalllow temperature gradient and a slowly falling temperature Dissolution of the domain structure at a moderate temperature gradient and a slowly rising temperature Slowly „growing“ domain structure at a shallow temperature gradient and a quick lowering of the temperature stabilized transition region at T U = (88.0 ± 1.1) °C temperature gradient: 1.5 K/mm low temperature phases α and β high temperature phases α and α´ A A Sketch showing the formation of an orthorhombic domain pattern from a cubic lattice [3]: 1) Formation of a single-domain plate 2) Formation of twinned domains adjacent to the original domain in order to keep overall deformations low 3) Additional twinning leading to further domain orientations mounted sample with temperature transducers heating cartridge 20 V / 3 W Acknowledgements Technical support during various stages of the work has been received from: - Prof. F. Baumgartner, Prof. A. Dommann, Mrs. B. Schlecht, Mr. U. Künzler, Mr. H. Tobler, Mr. M. Wohlwend and the mechanical workshop of NTB Buchs, CH. - Dipl. Chem. FH S. Mattle (APM Technica AG, Heerbrugg, CH) - Mr. M. Grüninger (Fa. Berhalter AG, Widnau, CH) and Mr. H. Geller (Grabs, CH). - Mr. W. Dintheer (Phoenix Contact AG, Tagelswangen) - Mr. G. Caduff (Inficon, Balzers (FL) and Prof. P. Koidl (FhG-IAF Freiburg, D) T U high-conductivity graphite Lower part of the device showing how the heating stage has been thermally decoupled from the stage housing by use of springs Details The stage housing is flapped to load and unload the sample Applications of the new heating stage The device has been used to study phase transitions: The solid-state phase transformation of hydrogen loaded niobium crystals at 88°C has been investigated some time ago in great detail [1], [2], [3]. It served here to show the suitability of the device for the investigation of martensitic phase transformations (see the phase diagram and the microscopic pictures to the right). At temperatures above 88°C the NbH crystal investigated did not show any contrast due to the presence of the cubic α and α´ phases. On cooling down below this temperature narrow stripes appeared, which denote the presence of cubic α and orthorhombic β domains. The domains are visible here because of slight alternating inclinations at the surface contrasted by differential interference contrast in reflected light (C-DIC). The precise positioning of the transition zone on the sample made it possible to obtain detailed images of the NbH domain structures near the border between the high- and the low-temperature phases. In order to visualize the transition region, a temperature gradient between the (warmer) region in the upper left corner and the (cooler) region in the lower right corner has been established. The boundary between the high temperature phase region and the low temperature phase region is denoted by A-A. It was moved to the field of view by adjusting the base temperature of the device, thus rising and lowering the temperature of both heating cartridges by the same amount at the same time. The position is stabilized by means of the temperature gradient and by virtue of the cloosed loop temperature control. The steepness of the adjustable temperature gradient determines how well the transition region is defined: the shallower the gradient is, the more local mechanical distortions take over and determine the shape of the borderline and the onset of domain formation. The new heating stage is thus a suitable means to investigate the kinetics of phase transformations. The solid-liquid phase transition of the low melting temperature crystals Salol and Benzophenone have been displayed and manipulated in transmitted polarized light. For educational puposes the formation of crystals and their dissolution can be shown in real time. Here, the movement and incorporation of air bubbles or the varying shape of the borderline during crystallization and melting may be of interest. Further possible applications of the heating stage comprise the investigation of the curing process of adhesives, of polymerisation processes and even the growth of small single crystals and whiskers. Literature [1] T. Schober: Wasserstoff in Niob, Tantal und Vanadium: Strukturen, Phasen- diagramme und Morphologien. Kernforschungsanlage Jülich GmbH: Jülich, 1978 (Jül-Spez 17). [2] T.B. Massalski (ed.): Binary Alloy Phase Diagrams. 2 nd ed.; ASM International, Materials Park, Ohio (USA); 1990; S. 2044. [3] T. Schober: Tantalhydrid – Phasenübergänge, Publikation zu wissenschaft- lichen Filmen, Institut für wissenschaftlichen Film: Göttingen, 1981. Fast „growing“ domain structure at a shalllow temperature gradient and a slowly falling temperature View publication stats View publication stats