A mirror furnace for neutron diffraction up to 2300 K
Abstract: This paper describes a mirror furnace that has been developed for neutron diffraction work at temperatures up to 2300 K. It is based on a reflecting rotational ellipsoid, in which the heating element, a halogen lamp, is placed at one focus and the sample at the other. It works in a normal, i.e. oxidizing, atmosphere, but can also be used in a vacuum. It has been developed for experiments of long duration. The stability and reproducibility of the temperature are better than 1\% of the setting temperature. Further main characteristics are applicability to single-crystal and powder work, very low background, low power consumption and very easy and cheap handling. Several experiments have been carried out with the furnace.
Summary (2 min read)
- Structural work at (very) high temperatures is important.
- Many naturally occurring compounds, i.e. minerals, have structures or microstructures or, more generally, exhibit order/disorder phenomena that reflect the thermal prehistory of the rock-forming process itself.
- In particular, the relatively low absorption of thermal neutrons by most elements makes the use of neutron furnaces particularly easy, whereas high-temperature studies at, say, T > 1300 K with X-rays are usually cumbersome, at least if aimed at structure refinement.
- The basic principle of mirror heating was developed for crystal-growth applications (Eyer, Zimmermann & Nitsche, 1975; Watanabe & Shimazu, 1976) .
- It should be mentioned that the same principle was also used for a new X-ray powder furnace (Schneider, 1992; Schneider, Frey, Johnson & Laschke, 1993) .
II. Specimen mounting and alignment
- Sample support and adjustment are usually provided by a thin ceramic tube or rod of AI203 or ZrO2, to which the sample may be glued with a ceramic cement.
- Other possibilities are platinum cans for powder samples or platinum/rhodium wires sintered around a ceramic sample.
- A particularly sophisticated sample holder for single-crystal work was used by Neder, Frey & Schulz (1990) .
- Approximately 80% of reciprocal space can be set into diffraction position (within the limiting sphere).
- Alternatively, the furnace can be placed in a large off-centre Eulerian cradle.
III. Temperature control
- The thermocouples are placed in holes drilled in the sample or are even sintered within a ceramic sample.
- This procedure provides a precise temperature measurement, except for gradients within the sample.
- It should be mentioned that reflections from the Pt/Rh elements cause some unwanted contamination of powder patterns, which might be cumbersome in the analysis of minority phases.
- In many cases, this influence may be ruled out by excluding regions from these diagrams.
- The furnace was checked in a series of powder and single-crystal experiments studying either Bragg reflections or diffuse phenomena.
- To check the background scattering by the furnace, an empty scan was compared to a scan without the furnace.
- With a platinum can, correspondingly spurious platinum reflections become visible that do not affect the powder-data analysis in most cases.
- Their great success with this type of furnace encouraged the ILL at Grenoble to rebuild and sell mirror furnaces of this type.
V. Further developments
- Further developments of the mirror furnace towards higher temperatures and various gas atmospheres were investigated recently by Mursic (1992)(see also Mursic, Vogt, Boysen & Frey, 1992) .
- One essential concern is the quality and the tolerable heat load of the reflecting mirror surfaces.
- Coating with silver, gold or even SiO2 is desirable.
- The main limiting factors towards higher temperatures are, however, the mechanical stability of the crystal support and the cooling of the glass bodies of the light bulbs.
- In an oxidizing atmosphere, measurements by thermocouples are restricted to temperatures below 2100K (PtRh30/PtRh6); higher temperatures can only be measured by pyrometric methods.
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Cites background from "A mirror furnace for neutron diffra..."
...Mirror, or imaging furnaces which utilise high power quartz-halogen bulbs and double ellipsoidal reflectors of the type developed for crystal growth in the Space-lab project have also been developed for neutron use,  see Fig....
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