International
Tables for
Crystallography
Volume H
Powder diffraction
Edited by C. J. Gilmore, J. A. Kaduk and H. Schenk

International Tables for Crystallography (2018). Vol. H, ch. 2.6, pp. 150-151

Section 2.6.4. General system setup of non-ambient chambers

C. A. Reissa*

aNoordikslaan 51, 7602 CC Almelo, The Netherlands
Correspondence e-mail: Celeste.Reiss@PANalytical.com

2.6.4. General system setup of non-ambient chambers

| top | pdf |

2.6.4.1. Sample stage

| top | pdf |

The main requirement for a good non-ambient chamber is that the specimen is cooled/heated homogeneously at a controllable rate. The temperature of the goniometer and other parts of the diffractometer should not be affected while operating the temperature stage. Different sample-stage designs are possible: direct heating/cooling via a strip or plate, or surround heating/cooling with an oven or gas convection for a capillary. The advantage of an environmental heater/cooler is the good temperature homogeneity, as there is heat transfer from all sides by radiation and gas convection around the sample or capillary. In contrast, when using a direct heater such as a heating/cooling strip or plate only one-side heat transfer to the sample is possible through the contact surface. An advantage of direct heaters/coolers is the ability to achieve very high and low temperatures and rapid temperature changes. Other differences are the more accurate sample-temperature measurement in an oven compared with a strip heater, where temperature gradients can be present in the strip and the sample attached to the strip heater. For high-temperature measurements with capillaries, the best choice is fused silica (`quartz') glass with a melting point of ∼1973 K; for low-temperature measurements borosilicate glass capillaries are used. A typical hardware setup for a non-ambient X-ray diffraction experiment is shown in Fig. 2.6.1[link].

[Figure 2.6.1]

Figure 2.6.1 | top | pdf |

Typical hardware setup for a non-ambient X-ray diffraction experiment as described in Section [link]2.6.4; non-ambient chamber, temperature/process-control unit, vacuum/gas equipment, cooling water and goniometer with height-alignment stage connected to a PC.

2.6.4.2. Temperature-control unit, process controller

| top | pdf |

To control the temperature, a temperature controller with an integrated process controller is needed. For controlled heating/cooling it is necessary to continuously measure the actual temperature and compare it with the set temperature. Often, a standard industrial process controller is used to convert the signal from the temperature sensor into a temperature value, to display the value, to send it to the control PC and to control the power for heating/cooling. In addition to controlling the sample conditions, the temperature-control unit (TCU) usually monitors other instrument components such as the cooling of the sample-stage housing and safety devices.

2.6.4.3. Vacuum equipment, gas supply

| top | pdf |

High-temperature X-ray diffraction measurements are often performed in vacuum or in an inert-gas atmosphere to avoid oxidation of the specimen or the sample support. Systems with a rotary pump typically achieve a vacuum of 10−2 mbar (where 1 mbar = 100 Pa); when adding a turbo molecular pump to the rotary pump, a vacuum of about 10−4 mbar can be reached. A low vacuum or a completely dry gas atmosphere, e.g. pure nitrogen (or helium, which has the advantage of a lower background in the diffraction patterns), is also needed for low-temperature experiments to avoid icing problems. Best practice is not to vent the flow of inert gas into the diffractometer enclosure or the laboratory atmosphere, but into the ventilation system (fume hood). Some local safety authorities may require such venting.

2.6.4.4. Water cooling

| top | pdf |

The housing of the sample stage must be kept close to room temperature to avoid heat transfer to the diffractometer and to ensure user safety. In most cases, water is used for this purpose, and the cooling water can be shared with the diffractometer.

2.6.4.5. Diffractometer and height-compensation mechanism

| top | pdf |

The non-ambient chamber has to be interfaced to the gonio­meter. Interfaces are available without and with a height-compensation mechanism; the latter can be manual or motorized.

When heating/cooling a specimen in an environmental heater, sample displacement is virtually unavoidable, mainly owing to the thermal expansion/contraction of the sample holder. It is possible to correct the temperature-dependent change of the sample position with a height-compensation mechanism (motorized z stage) or to model the displacement in the refinement software. When using a z stage that is controlled via software, the shifts in peak positions are only caused by the thermal lattice expansion/contraction of the sample under study. If no such mechanism is available, a parallel X-ray beam (which is not sensitive to sample displacement) can be used, but the resolution may be worse compared with measurements in para-focusing Bragg–Brentano geometry, and granularity may be significant. For strip heaters the displacement of the sample due to the strip is not so pronounced. If a peak of the material of the strip is visible in the diffractogram this can be used as a reference for height compensation if the thermal expansion of the strip material is also taken into account.








































to end of page
to top of page