A versatile hightemperature furnace for neutron fourcircle diffractometers G. Heger, W.F. Kuhs, S. Massing To cite this version: G. Heger, W.F. Kuhs, S. Massing. A versatile hightemperature furnace for neutron fourcircle diffractometers. Revue de Physique Appliquee, 1984, 19 (9), pp.735738. <10.1051/rphysap:01984001909073500>. <jpa00245248> HAL Id: jpa00245248 https://hal.archivesouvertes.fr/jpa00245248 Submitted on 1 Jan 1984 HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
Ce The Revue Phys. Appl. 19 (1984) 735738 SEPTEMBRE 1984, 735 A versatile hightemperature furnace for neutron fourcircle diffractometers G. Heger, W. F. Kuhs (+) and S. Massing Kernforschungszentrum, Karlsruhe, F.R.G. Institut LaueLangevin, 156X, 38042 Grenoble Cedex, France 2014 Résumé. aux besoins d un diffractomètre de neutrons à quatre cercles. Il est adapté à des températures entre 330 et 1 200 K avec une stabilité supérieure à 0,05 K. Les mouvements des cercles sont très peu limités : seule la rotation 03A6 est limitée à 200. Le rapport décrit un four à haute température, facile d emploi, léger (0,9 kg) et adapté four fonctionne sous vide (typiquement de 105 mbar) et est refroidi à l eau. La température est contrôlée par trois thermocouples chromelalumel, l un flexible est en contact avec l échantillon. La puissance électrique est de 60 W à la température maximale. Seulement deux réflecteurs de vanadium à paroi mince (0,1 mm) et une enveloppe extérieure sphérique en aluminium (enveloppe cylindrique en Al dans une version précédente) se trouvent dans le faisceau de neutrons. Le four a été utilisé avec succès, pour collecter des données sur la phase incommensurable située entre les phases 03B1 et 03B2 du quartz. 2014 Abstract. design and performance of a versatile closed, light (0,9 kg), high temperature furnace adapted to the needs of a neutron fourcircle diffractometer is reported. It operates between 330 and 1 200 K with a long term stability of better than 0.05 K. There are few restrictions on the movement of the circles; only the 03A6range is limited to 200. The furnace operates under high vacuum (typically ~ 105 mbar) with a watercooled base. The temperature is controlled by 3 chromelalumel thermocouples, one of which is flexible to allow it to be fixed directly at the sample. The maximum electrical power requirement is 60 W. Only 2 thinwalled (0.1 mm) ~ vanadium reflectors and an outer spherical aluminium can (cylindrical Al can in a former version) are in the neutron beam. The furnace was used successfully e.g. to collect data of the incommensurate phase at the 03B103B2 transition of quartz. 1. Technical specifcations. During the last few years the demand for neutron diffraction measurements at high temperatures has been increased considerably. Especially the temperature range up to about 1 200 K is of interest for experiments on e.g. structural phase transitions, ionic conductors, disorder and anharmonicity. For sufficiently precise diffraction data in most cases single crystal measurements of high quality are required. We have adopted a watercooled vacuum furnace to the needs of a neutron fourcircle diffractometer with little restrictions of the angular ranges of crystal orientation. The versatility of the device results from its low weight (0.9 kg) combined with highly flexible supply tubes for vacuum and water cooling. In this way extended data sets of reflection intensities can be collected, comparable to the situation without furnace. In the neutron beam there are only two cylindrical vanadium reflectors (0.1 mm thick each) and a thinwalled (0.5 mm) aluminium can (spherical or cylindrical). Therefore, the background is rather low and can be corrected accurately in particular by using the wscan technique. A sectional view of the furnace is shown in figure 1 ; its characteristics are summerized in table I. The resistivity heating is realized with a thermocoaxial wire within a flexible steel tube. In this way there is no danger of a short circuit. The life time of the heater unit is not restricted by poor vacuum operation. The maximum temperature of the heater is limited to 14001500 K. In figure 2 there are plotted the temperatures of the two thermocouples fixed at the middle of the heater unit and at the sample position, respectively, as a function of electric power. These curves are taken at a vacuum of about 10 6 mbar and with clean, new Vreflectors. The temperature gradient increase with increasing temperature. It is recommended to operate the furnace only in the range up to 60 W corresponding to temperatures at the sample of about 1 200 K and at the heater of about 1 400 K. For the higher temperature region > 900 K a good reflectivity of the metal shields is essential. Nbfoils may be used instead of V to reduce the incoherent scattering background (if the much larger coherent scattering cross Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:01984001909073500
Characteristics Temperatures 736 Table 1. of the furnace for neutron fourcircle diffractometers. water cooling of the furnace base Fig. 1. Vertical section through the furnace. section of Nb is not troublesome). The fumace was operated successfully also at lower temperatures (330700 K) with Alshields. A sample crystal can be either directly glued on top of the Wsample holder or wrapped in a metallic foil for a better homogeneity of temperature. In case of high vapour pressure of the sample material a closed metal or quartz container can be inserted. Samples with dimensions up to 10 mm can be used For the preparation of an experiment the furnace with the mounted crystal visible (without reflectors and vacuum can) can be centred within the Eulerian cradle of a fourcircle diffractometer at room temperature (Fig. 3). Fig. 2. measured at the middle of the heater unit ( e) and at the top of the sample holder (in direct contact with the pin) ( x ) as a function of electric power. The difference between the two curves gives the temperature gradient between heater and sample. These data were obtained with clean, highly reflecting vanadium shields. The broken line indicates the upper limit for routine operation of the furnace. 2. Experimental verification. The performance of the furnace was tested by neutron diffraction experiments. The main points of interest
Temperature 737 Fig. 3. View of the open furnace mounted on the Eulerian cradle of the fourcircle diffractometer P32 at thé Kernforschungszentrum Karlsruhe. were the stability and homogeneity of temperature. Due to the small dimensions of the furnace important temperature gradients between the sample and the heater have to be taken into account (see Fig. 2). A test was performed on the fourcircle diffractometer D9 of the Institut LaueLangevin using a 3 x 3 x 6 mm3 quartz single crystal mounted with ceramic glue on top of the sample pin with its largest axis parallel to the pin. Apart from the two Vshields no further precautions were taken in order to homogenize the sample temperature. In a first run the intensity of the (113) Bragg reflection was recorded near the ceincflphase transition ; this reflection is reasonably strong in the aphase, while the intensity is almost zero in the pphase. The temperature was changed in steps of 1 K approximately every 15 min and controlled at the central heater thermocouple. The results of these measurements are shown in figure 4. From the observed xinc coexistence range a temperature gradient of about 10 K/cm across the sample is deduced. The différence between the sample and heater temperatures was measured as 60 K (probably with an important gradient in the ceramic glue). A measurement at fixed temperature in the (xinc coexistence range gave no significant intensity changes over a period of several hours. A second test run was performed controlling the temperature directly at the sample position. The long term stability (two days) was found to be better than 0.05 K, while controlling at the heater always gave slightly drifting temperatures (typically 1 K/day) probably due to environmental changes i.e. improving vacuum. In a following experiment [1] performed on the fourcircle diffractometer D 10 of the Institut Laue Langevin further information on the performance of Fig. 4. variation of integrated peak intensities of Bragg and satellite reflections at the 03B1INC03B2phase transition in quartz : a) (113) Bragg peak measured with a freely mounted 3 x 3 x 6 mm sample on heating and cooling, b) ( 0.03 2 2) satellite peak measured with a 7 x 7 x 7 MM3 sample wrapped in aluminium foil on cooling [1]. Ti denotes the INCp phase transition, Tr the KINC transition. In the temperature range between Tc and T c or T the aphase is coexisting with the INC phase due to thermal inhomogeneities of the sample.
738 the furnace was obtained. This time the 7 x 7 x 7 mm3 sample of quartz was wrapped in Alfoil in order to reduce the thermal gradient over the sample. The temperature was controlled with a thermocouple in direct contact with the sample. The intensity of several satellite reflections of the incommensurate phase (INC), which covers the range of approximately 1.3 K between the oc and the pphase, was measured as a function of temperature. A typical series of intensity measurements is shown in figure 4. The range of coexistence of 1 K observed for the oc and the INCphase set an upper limit on the thermal gradient over the whole sample of a few 0.1 K/cm ; the strain induced by the cement between pin and sample may have contributed to widen the coexistence range. This was further confirmed by comparing this expérimental results with results from a bigger furnace for tripleaxis instruments [2] with a gradient of 0.1 K/cm [1]. The long term stability achieved with the fourcircle set up was + 0.02 K over a period of a few days. 3. Routine opération. The furnace is routinely operating between 330 and 1 200 K for samples up to 500 mm3. For the lowest temperatures it is preferable to work under poor vacuum to compensate partially for the lack of radiation heating, especially for samples, which are not in direct contact with the pin. The furnace allows for great freedom in all movements of a fourcircle diffractometer. Good long term stability is achieved by controlling the temperature at the sample position and small gradients are obtainable by wrapping the sample into metal foil. Some care has to be taken in adjusting the PID parameters for every given setup (a tunable microprocessorbased controller is preferable in this case). Obviously controlling at the heater unit gives less problems, but drifting vacuum, changes in the reflectivity of the shields or in the thermal contact (e.g. due to chemical reaction of sample and cement) normally do not allow for a stability better than 1 K/day at higher temperatures. One major problem in routine operation is the fact that the high vacuum prevents the measurement of freely mounted samples with noticeable vapour pressure. Such samples have to be sealed in a vacuum tight metal of quartz container and then inserted on a special holder. Furnaces of this type are operational since almost 4 years at different laboratories (Kernforschungszentrum Karlsruhe, ILL Grenoble, C.E.N. Grenoble, C.E.N. Saclay) without any major incident. Quite a number of experiments have shown their high reliability and simple handling. References [1] DOLINO, G., BACHHEIMER, J. P., BERGE, B. and ZEYEN, C. M. E., J. Physique 45 (1984) in press. [2] DOLINO, G., BACHHEIMER, J. P., BERGE, B., ZEYEN, C. M. E., VAN TENDELOO, G., VAN LANDUYT, S. and AMELINCKX, S., J. Physique 45 (1984) submitted.