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Description of a sample holder for ion channeling near liquid-helium temperature
290
Nuclear Instrnments and Methods in Physics Research B45 (1990) 290-292 North-Holland
DESCRIPTION OF A SAMPLE HOLDER HELIUM TEMPERATURE B. DAUDIN, CENG, D$artement
FOR ION CHANNELING
NEAR LIQUID-
M. DUBUS and F. VIARGUES de Recherche Fondamentale
SBT, BP 85 X, 38041 Grenoble C&dex, France
Ion channeling is sensitive to very small shifts (10m2 nm) of the atomic equilibrium positions. As a consequence, this technique appears to be suitable to study lattice dynamics, in particular when a displaeivephase transition occurs. As many phase transitions of interest are observed at low temperature, we developed a three-axis goniometer in order to perform channeling experiments between 5 and 30 K. As no thermal screen could be placed between the sample and the ion beam, the quantity of heat radiated onto the sample holder was very large. The technical solutions which were chosen to overcome this difficulty and ensure both an efficient cooling and a good rotational mobility of the sample are described in detail. A liquid-helium flow of - 6.5 I/h was found to be necessary to achieve a continuous refrigeration of the sample at 5 K. To conclude, proton channeling experiments in the blue bronze, K,,,MoO,,
are presented as an illustration of the device possibilities.
1. Introduction Ion channeling is a suitable technique for studying lattice dynamics or displacive phase transitions as the minimum channeling yield (xtii,) is proportional, respectively, to the square of the thermal vibration amplitude and to the atomic spacing. Consequently, we recently used ion channeling to discriminate between Debye and Einstein atomic vibration modes in LaE, [l] and to study the charge density wave behaviour in TiSe, [2] and the blue bronze K,,MoO, [3]. To further investigate this last system, experiments between 5 and 30 K were necessary. In this paper, we report the development of a cryogenic sample holder that was designed for this purpose.
A general view of the resulting device is shown in figs. 1-4. It basically consists of a copper chamber which is fed with liquid helium through a spring-like inox coil of tubing. The helium gas is evacuated through a similar coil, larger in diameter than the input one. The advantage of such an arrangement is that the outer coil, which is cooled by the gas, acts as a thermal shield for the iMer coil. For the same purpose, the input tube was wrapped in aluminium, as shown in figs. 3 and 4. The cold chamber is fixed on the goniometer using an auxiliary part made of a composite material (glass fiber + epoxy resin) which has a very low thermal conductivity and allows to minimize the heat conduction.
2. Description of the experimental setup The requirements for such a sample holder were: (1) to be cooled at liquid helium temperature; (2) to have an effective area of - 1 cm’ to allow measurements on large samples; (3) to allow a satisfactory temperature regulation to be achieved between 5 and 30 K; (4) to meet the requirements for Rutherford backscattering spectrometry (RBS) as no thermal shield can be interposed between the incoming beam and the sample surface; (5) to be adaptable to a three-axis goniometer placed in the analysis chamber. In particular, it was essential for experimental convenience that the sample could be easily changed by access through a 150 mm viewport. 0168-583X/90/$03.50 0 Elsevier Science Publishers B.V. (North-Holland)
Fig. 1. Schematic view of the liquid helium supply. (1) Transfer line; (2) sliding seal assembly; (3) cold point; (4) common vacuum with analysis chamber; (5) copper tubing; (6) helium gas output; (7,8) intake and exit spring-like inox coils of tubing; (9) cold chamber (copper).
B. Daudin et al. / Sample holder for ion channeling
291
Fig. 2. Schematic view of the cold sample holder assembly. (1) Fiber-glass/epoxy resin; (2) goniometer; (3) vertical rotation axis; (4) aluminium nut; (5) four-probe thermometer; (6) sample; (7) copper disc; (8) resistive heater; (9) copper chamber.
A resistive heater made of manganin wire is wound around the chamber to provide the heating power necessary for the temperature regulation. In order to reduce the thermal leaks by conduction along the wires, these are thermally anchored to the external coil, as shown in fig. 4. The sample is fixed with silver paint on a copper disk shown in fig. 5. The thermometer, i.e. a carbon resistor, is placed in a hole drilled in the disk in order to be thermally coupled as well as possible to the sample. To avoid direct heating of the temperature sensor through the leads, these have been wound several times around the copper disk. Silicon grease is used to achieve
Fig. 3. Photograph of the cold sample holder mounted on the goniometer, in the analysis chamber.
Fig. 4. Photograph of the cold sample holder showing the details described in figs. 1 and 2.
a good thermal coupling between the disk and the cold chamber. An aluminium nut which is screwed on the cold chamber allows the disk to be firmly fixed, due to the difference between the copper and aluminium thermal expansion coefficients. Two pins, which are fixed in the body of the cold chamber, ensure that the copper disk is correctly placed and prevent it from rotating when tightened using the aluminium nut. This arrangement ensures a good thermal coupling to the chamber, allowing a minimum temperature of 5 K to be reached, in spite of the heat flm radiated on the sample. It is worth noting that another disk, equipped with a platinum resistor, can be used for experiments performed at temperatures above 30 K. The use of coils for liquid helium supply allows a high rotational mobility, namely 30 o along the three axes of the goniometer. The translational mobility is + 1 cm. As there is no thermal shield in front of the sample, the position of the backscattered particle detector can be arbitrarily fixed.
Fig. 5. Photograph of the copper disk which is fixed on the cold chamber. III. EQUIPMENT
292
B. Daudin et al. / Sample holder for ion channeling
The experimental procedure is to orient the sample along an axis of interest at room temperature. It has been checked that, during the subsequent cooling at liquid-helium temperature, the positional stability of the sample is better than 0.06’ with respect to all three rotational axes. The pressure in the liquid helium tank can be regulated, in order to optimize the transfer, according to the temperature range of interest for the measurements. A typical value of 300 mbar allows to reach the liquid helium transfer rate of 6.5 l/h which is required to achieve a continuous refrigeration of the sample at 5 K. In this case, the chamber is full of liquid helium. For a lower flow, temperature oscillations are observed, due to the coexistence of liquid and gas in the cold chamber. This phenomenon results from the thermal leaks in the helium transfer line and in the input coil, which are responsible for a partial evaporation of liquid helium. The thermal leaks in the chamber itself, mainly due to the heat radiated from the surrounding goniometer and to the power carried by the ion beam, are small compared to the leaks due to the transfer line. The typical power carried by the proton beam during experiments described below was 3 mW. It was experimentally checked that the resulting temperature increase when applying such a beam was negligible. This demonstrated that the thermal coupling between the copper disk and the cold chamber was good and allowed to conclude that the actual sample temperature was limited only by the thermal diffusivity of the material. When the temperature is increased above 5 K, another regime of oscillation is observed between = 9 and 13 K. The use of a phase separation device could avoid these oscillations but this solution was discarded, as it would require a larger space near the chamber than is practically available. However, it is possible (even if difficult) to work at every temperature above 5 K by changing the helium flow. Above 13 K, there is only gas in the chamber and an accurate temperature regulation is possible. In order to reduce the liquid helium consumption, it is therefore convenient to limitate the gas flow through the exhaust using a small-size bypass. An alternative solution could be to reduce the pressure in the liquid helium tank, in order to reduce the helium transfer rate. Nevertheless, this latter possibility was ruled out as it implies that the helium tank undergoes pressurization-depressurization cycles which are responsible for an excess of consumption. The device described above has been used to measure the proton channeling yield in K,-,,MoO,, which is a quasi one-dimensional conductor undergoing a Peierls transition towards an incommensurate charge density
K0.3M00?
.-c 025
x
E
0.2 0.15 1 0.1
4 0
10
20
30
40
50
60
T (Kl
Fig. 6. 1 MeV proton channeling yield in blue bronze, as a function of temperature. The current was about 3 x 10d9 A. (For details, see ref. [5].)
wave state at 183 K. Nonlinear conductivity due to the sliding of the charge density wave is well documented [4]. In addition, from the results shown in fig. 6, it is clear that temperature hysteresis is present. This effect has been interpreted assuming that incommensurate charge density wave domains coupled to point lattice defects are present in this temperature range [3,5]. To conclude, we wish to point out that proton channeling has provided, to our knowledge, the first evidence for a structural disorder below 40 K in the blue bronze K,,,MoO,. This emphasizes the interest of further channeling experiments at low temperature, which are made possible by the versatility of the sample holder described in this paper. We wish to thank Mr. A. Marcou, Mr. M. Pogrocheff and Mr. G. Berard for their technical assistance.
References PI Y. Peysson, B. Daudin, M. Dubus and R.E. Benenson, Phys. Rev. B34 (1986) 8367. 121 M. Nunez-Regueiro, B. Dauclin, M. Dubus and C. Ayache, Solid State Comtnun. 54 (1985) 457. [31 B. Daudin, M. Dubus, J. Dumas and J. Marcus, J. Phys. (Paris) 48 (1987) 1779. [41 For a review, see C. Schlenker and J. Dumas. in: Crystal Chemistry and Properties of Materials with Quasi One-Dimensional Structure, ed. J. Rouxel (D. Reidel, Dordrecht, 1986) p. 135; R.M. Fleming, Synth. Met. 13 (1986) 241. [51 B. Daudin, M. Dubus, J. Dumas and J. Marcus, Synth. Met. 29 (1989) 5227; J. Dumas et al., to be published.
Nuclear Instrnments and Methods in Physics Research B45 (1990) 290-292 North-Holland
DESCRIPTION OF A SAMPLE HOLDER HELIUM TEMPERATURE B. DAUDIN, CENG, D$artement
FOR ION CHANNELING
NEAR LIQUID-
M. DUBUS and F. VIARGUES de Recherche Fondamentale
SBT, BP 85 X, 38041 Grenoble C&dex, France
Ion channeling is sensitive to very small shifts (10m2 nm) of the atomic equilibrium positions. As a consequence, this technique appears to be suitable to study lattice dynamics, in particular when a displaeivephase transition occurs. As many phase transitions of interest are observed at low temperature, we developed a three-axis goniometer in order to perform channeling experiments between 5 and 30 K. As no thermal screen could be placed between the sample and the ion beam, the quantity of heat radiated onto the sample holder was very large. The technical solutions which were chosen to overcome this difficulty and ensure both an efficient cooling and a good rotational mobility of the sample are described in detail. A liquid-helium flow of - 6.5 I/h was found to be necessary to achieve a continuous refrigeration of the sample at 5 K. To conclude, proton channeling experiments in the blue bronze, K,,,MoO,,
are presented as an illustration of the device possibilities.
1. Introduction Ion channeling is a suitable technique for studying lattice dynamics or displacive phase transitions as the minimum channeling yield (xtii,) is proportional, respectively, to the square of the thermal vibration amplitude and to the atomic spacing. Consequently, we recently used ion channeling to discriminate between Debye and Einstein atomic vibration modes in LaE, [l] and to study the charge density wave behaviour in TiSe, [2] and the blue bronze K,,MoO, [3]. To further investigate this last system, experiments between 5 and 30 K were necessary. In this paper, we report the development of a cryogenic sample holder that was designed for this purpose.
A general view of the resulting device is shown in figs. 1-4. It basically consists of a copper chamber which is fed with liquid helium through a spring-like inox coil of tubing. The helium gas is evacuated through a similar coil, larger in diameter than the input one. The advantage of such an arrangement is that the outer coil, which is cooled by the gas, acts as a thermal shield for the iMer coil. For the same purpose, the input tube was wrapped in aluminium, as shown in figs. 3 and 4. The cold chamber is fixed on the goniometer using an auxiliary part made of a composite material (glass fiber + epoxy resin) which has a very low thermal conductivity and allows to minimize the heat conduction.
2. Description of the experimental setup The requirements for such a sample holder were: (1) to be cooled at liquid helium temperature; (2) to have an effective area of - 1 cm’ to allow measurements on large samples; (3) to allow a satisfactory temperature regulation to be achieved between 5 and 30 K; (4) to meet the requirements for Rutherford backscattering spectrometry (RBS) as no thermal shield can be interposed between the incoming beam and the sample surface; (5) to be adaptable to a three-axis goniometer placed in the analysis chamber. In particular, it was essential for experimental convenience that the sample could be easily changed by access through a 150 mm viewport. 0168-583X/90/$03.50 0 Elsevier Science Publishers B.V. (North-Holland)
Fig. 1. Schematic view of the liquid helium supply. (1) Transfer line; (2) sliding seal assembly; (3) cold point; (4) common vacuum with analysis chamber; (5) copper tubing; (6) helium gas output; (7,8) intake and exit spring-like inox coils of tubing; (9) cold chamber (copper).
B. Daudin et al. / Sample holder for ion channeling
291
Fig. 2. Schematic view of the cold sample holder assembly. (1) Fiber-glass/epoxy resin; (2) goniometer; (3) vertical rotation axis; (4) aluminium nut; (5) four-probe thermometer; (6) sample; (7) copper disc; (8) resistive heater; (9) copper chamber.
A resistive heater made of manganin wire is wound around the chamber to provide the heating power necessary for the temperature regulation. In order to reduce the thermal leaks by conduction along the wires, these are thermally anchored to the external coil, as shown in fig. 4. The sample is fixed with silver paint on a copper disk shown in fig. 5. The thermometer, i.e. a carbon resistor, is placed in a hole drilled in the disk in order to be thermally coupled as well as possible to the sample. To avoid direct heating of the temperature sensor through the leads, these have been wound several times around the copper disk. Silicon grease is used to achieve
Fig. 3. Photograph of the cold sample holder mounted on the goniometer, in the analysis chamber.
Fig. 4. Photograph of the cold sample holder showing the details described in figs. 1 and 2.
a good thermal coupling between the disk and the cold chamber. An aluminium nut which is screwed on the cold chamber allows the disk to be firmly fixed, due to the difference between the copper and aluminium thermal expansion coefficients. Two pins, which are fixed in the body of the cold chamber, ensure that the copper disk is correctly placed and prevent it from rotating when tightened using the aluminium nut. This arrangement ensures a good thermal coupling to the chamber, allowing a minimum temperature of 5 K to be reached, in spite of the heat flm radiated on the sample. It is worth noting that another disk, equipped with a platinum resistor, can be used for experiments performed at temperatures above 30 K. The use of coils for liquid helium supply allows a high rotational mobility, namely 30 o along the three axes of the goniometer. The translational mobility is + 1 cm. As there is no thermal shield in front of the sample, the position of the backscattered particle detector can be arbitrarily fixed.
Fig. 5. Photograph of the copper disk which is fixed on the cold chamber. III. EQUIPMENT
292
B. Daudin et al. / Sample holder for ion channeling
The experimental procedure is to orient the sample along an axis of interest at room temperature. It has been checked that, during the subsequent cooling at liquid-helium temperature, the positional stability of the sample is better than 0.06’ with respect to all three rotational axes. The pressure in the liquid helium tank can be regulated, in order to optimize the transfer, according to the temperature range of interest for the measurements. A typical value of 300 mbar allows to reach the liquid helium transfer rate of 6.5 l/h which is required to achieve a continuous refrigeration of the sample at 5 K. In this case, the chamber is full of liquid helium. For a lower flow, temperature oscillations are observed, due to the coexistence of liquid and gas in the cold chamber. This phenomenon results from the thermal leaks in the helium transfer line and in the input coil, which are responsible for a partial evaporation of liquid helium. The thermal leaks in the chamber itself, mainly due to the heat radiated from the surrounding goniometer and to the power carried by the ion beam, are small compared to the leaks due to the transfer line. The typical power carried by the proton beam during experiments described below was 3 mW. It was experimentally checked that the resulting temperature increase when applying such a beam was negligible. This demonstrated that the thermal coupling between the copper disk and the cold chamber was good and allowed to conclude that the actual sample temperature was limited only by the thermal diffusivity of the material. When the temperature is increased above 5 K, another regime of oscillation is observed between = 9 and 13 K. The use of a phase separation device could avoid these oscillations but this solution was discarded, as it would require a larger space near the chamber than is practically available. However, it is possible (even if difficult) to work at every temperature above 5 K by changing the helium flow. Above 13 K, there is only gas in the chamber and an accurate temperature regulation is possible. In order to reduce the liquid helium consumption, it is therefore convenient to limitate the gas flow through the exhaust using a small-size bypass. An alternative solution could be to reduce the pressure in the liquid helium tank, in order to reduce the helium transfer rate. Nevertheless, this latter possibility was ruled out as it implies that the helium tank undergoes pressurization-depressurization cycles which are responsible for an excess of consumption. The device described above has been used to measure the proton channeling yield in K,-,,MoO,, which is a quasi one-dimensional conductor undergoing a Peierls transition towards an incommensurate charge density
K0.3M00?
.-c 025
x
E
0.2 0.15 1 0.1
4 0
10
20
30
40
50
60
T (Kl
Fig. 6. 1 MeV proton channeling yield in blue bronze, as a function of temperature. The current was about 3 x 10d9 A. (For details, see ref. [5].)
wave state at 183 K. Nonlinear conductivity due to the sliding of the charge density wave is well documented [4]. In addition, from the results shown in fig. 6, it is clear that temperature hysteresis is present. This effect has been interpreted assuming that incommensurate charge density wave domains coupled to point lattice defects are present in this temperature range [3,5]. To conclude, we wish to point out that proton channeling has provided, to our knowledge, the first evidence for a structural disorder below 40 K in the blue bronze K,,,MoO,. This emphasizes the interest of further channeling experiments at low temperature, which are made possible by the versatility of the sample holder described in this paper. We wish to thank Mr. A. Marcou, Mr. M. Pogrocheff and Mr. G. Berard for their technical assistance.
References PI Y. Peysson, B. Daudin, M. Dubus and R.E. Benenson, Phys. Rev. B34 (1986) 8367. 121 M. Nunez-Regueiro, B. Dauclin, M. Dubus and C. Ayache, Solid State Comtnun. 54 (1985) 457. [31 B. Daudin, M. Dubus, J. Dumas and J. Marcus, J. Phys. (Paris) 48 (1987) 1779. [41 For a review, see C. Schlenker and J. Dumas. in: Crystal Chemistry and Properties of Materials with Quasi One-Dimensional Structure, ed. J. Rouxel (D. Reidel, Dordrecht, 1986) p. 135; R.M. Fleming, Synth. Met. 13 (1986) 241. [51 B. Daudin, M. Dubus, J. Dumas and J. Marcus, Synth. Met. 29 (1989) 5227; J. Dumas et al., to be published.