When Frank Hawthorne (1988) edited the Reviews in Mineralogy volume on “Spectroscopic Methods in Mineralogy and Geology,” all the experiments presented had been performed at room pressure and room temperature because, at that time, vibrational and X-ray techniques were already quite difficult at ambient conditions so more sophisticated sample environments were not a priority. However, it has now become somewhat easier to perform experiments in situ at high temperatures (HT), high pressures (HP) or under combined high temperature and pressure (HP-HT). These types of experiments are becoming routine on crystals, glasses and liquids (see Shen and Wang 2014, this volume).
High-temperature experiments are important because most of the physical properties of high-temperature liquids, such as magmas and melts, are related to their atomic structure. Consequently, it is important to probe the local environment of the atoms in the sample under the conditions noted above (e.g., HT). However, at very high temperatures (~≥ 1200 °C) it is difficult to use conventional furnaces because of a number of experimental difficulties associated with their use: temperature regulation, thermal inertia and spatial obstruction of the sample. Due to the progress made in the development of lasers and X-ray, neutron and magnetic sources it is now possible to perform experiments in situ at HT, HP and HT-HP on samples of millimeter or micron size.
INTRODUCTION
In this chapter, we discuss some of these noncommercial methods used in performing ex-periments at HT, and outline the best choices for heating systems with regard to the experimental requirements. Different commercial heating systems are available such as the systems available from Linkam® (http://www.linkam.co.uk/) or Leica® (http://www.leica-microsystems.com/) for example. These two systems are well adapted to performing experiments at HT including
780 Neuville, Hennet, Florian, de Ligny
Raman (Neuville et al. 2014, this volume) and IR spectroscopy (Della Ventura et al. 2014, this volume) and X-ray absorption near-edge structure (XANES) spectroscopy (Henderson et al. 2014, this volume) in fluorescence mode. Commercial cells are capable of working at high and low temperatures, with controlled atmospheres and are relatively easy to use. But they need relatively large samples (typically 3-4 mm diameter and 1-2 mm height) and it is not possible to use them for EXAFS experiments in transmission mode or to perform HT Brillouin spectros-copy. Furthermore, it is not possible to investigate very high temperature >1800 K. There has also been a number of HT apparatus developed for use under specialized experimental condi-tions. For example, Berry et al. (2003) developed a controlled atmosphere furnace for X-ray absorption spectroscopy experiments under imposed oxygen fugacities at temperatures up to 1773 K. This furnace is well adapted for XANES experiments under controlled atmospheres but not suitable for investigating light elements. Eeckhout et al. (2008) have also developed a similar furnace.
Here we focus this chapter on two main heating methods: (i) levitation coupled with laser heating (Coutures et al. 1986) and (ii) heating using the wire system originally developed by Mysen and Frantz (1992). Both systems have been used with X-ray absorption and diffraction on several synchrotron beamlines, with neutron diffraction, and with Raman, IR and Brillouin spectroscopies.
LEVITATION TECHNIQUES
Introduction
The principle of levitation is to apply a force that counteracts gravity so that the sample is essentially suspended in air without a container. There are basically three main techniques widely used today: electromagnetic levitation, EML, (Jacobs et al. 1996), electrostatic levitation, ESL, (Paradis et al. 2001) and aerodynamic levitation. Some other techniques exist such as gas film levitation, GFL, (Haumesser et al. 2002), or combinations of the techniques above. In addition, we should also mention acoustic levitation (Trinh 1985), which is mostly used with liquids at lower temperatures.
EML is restricted to conducting samples (generally metals) that can be relatively large (up to 1-2 cm in diameter). ESL has the advantage of working both under vacuum preventing contamination by a surrounding gas and also under a few atmospheres gas pressure. However it requires a complex setup that complicates its use with many spectroscopies. In principle, all types of materials can be levitated using GFL technique and it is possible to use large quantities of sample (up to 200 g for oxide glasses). Up to now, these techniques have been used to study thermophysical properties such as viscosity, mass densities and surface tension in oxide melts (Grishchenko and Piluso 2011). A recent review of these techniques and their applications can be found in Price (2010). Among all containerless techniques, the CEMHTI “Conditions Extrêmes et Matériaux: Haute Température et Irradiation” laboratory in Orléans, France, has chosen to work with aerodynamic levitation in which the sample is levitated using a gas stream that flows through a convergent-divergent nozzle. In this chapter, we will mainly focus on this method, which is well adapted for studying liquid oxide melts.
The most widely used technique of this kind employed today is conical nozzle levitation, CNL, in which the levitator has a convergent-divergent nozzle. It was first used in combination with electromagnetic heating by Winborne et al. (1976). Coutures et al. (1986) introduced a CO2 laser heating and adapted the technique for Nuclear Magnetic Resonance (NMR) experi-ments on HT liquids (Winborne et al. 1976; Coutures et al. 1986). The advantages of CNL are the simplicity and compactness of the levitation devices, making it easy to integrate them into different experimental setups and it has subsequently been installed in various experiments
In situ High-Temperature Experiments 781
at large scale facilities (synchrotron and neutron sources). A recent review of applications of aerodynamic levitation at synchrotron and neutron sources can be found in Hennet et al. (2011). Besides gas flow, sound waves, electrostatic or magnetic fields can also be used to levitate sample.
Acoustic levitation
Acoustic levitation can suspend small objects via the acoustic radiation force that results from an impedance difference between the suspension medium, normally a gas, and a solid or liquid sample. A single-axis acoustic levitator, SAL, consists mainly of two parts: a vibrat-ing source (or transducer) generating frequencies of a few tens of kHz (i.e. ultrasound) and a reflector with a concave surface to improve the efficiency. The vibrating source-reflector axis is parallel to the direction of gravity and an acoustic wave is generated along this direction in order to counteract gravity. SAL can be used to study low melting point alloys and non-conducting materials as well as room temperature liquids. Trinth (1985) developed a device enabling work up to 700 °C. Resistive heating coils were used to heat the sample. The acces-sible temperature range is thus limited and this technique is not suitable for very HT studies.
Electromagnetic levitation
Electromagnetic levitation (Fig. 1) is mainly suitable for electrically con-ductive materials. It will be discussed here only briefly since a detailed de-scription of EML can be found else-where (Eckler 1992; Jacobs et al. 1996; Holland-Moritz et al. 2005). With EML, a radiofrequency electromagnetic field is generated by a coil and Foucault currents are induced in the sample. This leads to inductive heating of the sample, and at the same time, the interaction of the Fou-cault currents with the magnetic field of the coil leads to a force that counteracts the gravitational force and makes it pos-sible to levitate the sample. The electro-magnetic force applied on the levitated sample depends on the power absorbed by the latter. It is essentially a function of the square of the magnetic field strength and the electrical conductivity of the sample material. So by varying the heat-ing power, one can control the tempera-ture of the sample. The levitation coil is situated in a vacuum chamber, which is first evacuated and then filled with a very high purity gas, usually helium (He) or a mixture of He plus a few percent of hydrogen (H2).
Electrostatic levitation
With ESL (Paradis et al. 2001), the sample is electrically charged and levitated in a verti-cal electro-static field between two electrodes (Fig. 2). Two pairs of smaller side electrodes are used to position the sample horizontally. When it levitates, the sample is melted using lasers. With this method, it is possible to study all samples that can be electrically charged. This method has various advantages. First, it works under vacuum, preventing contamination, and allows the study of poor electrical conductors or materials with low melting points. One
Figure 1. A typical electromagnetic levitation setup developed at the German Space Center (DLR) within the “Institut für Materialphysik im Weltraum” (Institut for Materials Physics in Space). It is used for neutron diffraction on the D20 instrument at the ILL (Holland-Moritz et al. 2005).
782 Neuville, Hennet, Florian, de Ligny
drawback is the complexity of the setup that limits its combination with various spectroscopies. While some insulating oxides can be studied with this technique, it is used mostly with metals.
Aerodynamics levitation
Aerodynamic levitation was first developed in the USA by D. A. Winborne and P. C. Nordine (Win-borne et al. 1976) then later adopted at the CEMHTI by J.-P. Coutures (Coutures et al. 1990) for NMR stud-ies. It has proven to be a powerful and versatile technique for studying highly reactive liquids in the equi-librium melt and the supercooled liquid state several hundred Kelvin below the melting point (Hennet et al. 2007a). In particular, it is now widely used with neutron and X-ray diffraction techniques to investigate the structural properties of liquids (Hennet et al. 2006, 2007a,b). The working principle of aerodynamic levitation is well known and is described in detail elsewhere (Lan-dron et al. 2000). Figure 3 shows a standard aerodynamic levitation setup. The basic idea is to circulate a levitation gas (usually argon) through a nozzle onto the sample from below in order to counteract gravity and lift it above the nozzle. The sample is then heated to the desired temperature by means of one or two focused CO2 lasers. The levitation gas flow is accurately regulated and monitored by a mass flow controller to enable the sample to be maintained in a sufficiently stable aerodynamic state for long counting times. Lasers (CO2 or YAG) provide a natural choice of heating system for aerodynamic levitation and are also used with ESL. CO2 infrared lasers are particularly suitable for studying oxide materials and to some extent, can also be used for melting metals in conjunction with an oxygen free environment. Heating with a single laser leads to significant temperature gradients especially with insulating samples and generally, two lasers are used to heat and melt the samples. The lasers are focused onto the sample using eithers mirrors or ZnSe lenses from the top (vertically or not) and from below through the hole (gas path) in the conical nozzle. In both systems, the gas flow is precisely regulated using mass flow controllers and video cameras are used to monitor the sample during the levitation process.
With metals, it is also possible to use electromagnetic heating as used with ELM levitation. In particular, the Deutsches zentrum für Luft-und Raumfahrt, DLR, in Cologne and CEMHTI developed a hybrid system combining a RF heating coil with the conical nozzle (Mathiak et al. 2005). With the ESL technique, metals are melted using YAG lasers or CO2 lasers with wavelength of ~1 µm. With levitation techniques, optical pyrometry is the ideal method for temperature measurements. This technique is very easy to use but requires emissivity corrections to derive the true temperature, T, from the apparent temperature Ta measured by the pyrometer:
( )2
1 1ln
aT T Cλ
λ− = e
where C2 is the second radiation constant of Planck, C2 = 1.4388 cm K and eλ is the material emissivity at the working wavelength λ of the pyrometer.
Figure 2. Levitated sample between the electrodes of an electrostatic levitator at DLR. Redrafted from Kozaily (2012).
In situ High-Temperature Experiments 783
Figure 3. CNL environments developed at the CEMHTI for high-temperature experiments with NMR (top) and high energy X-ray diffraction at ID11 (ESRF) (bottom).
784 Neuville, Hennet, Florian, de Ligny
Experimental techniques
In this part, we mainly focus on selected experimental techniques that employ levitation methods to probe the structure of materials. All these applications are well described in Price (2010). However, we should point out that other properties can also be measured while using levitation techniques. For example, electrical and magnetic properties can be obtained by placing the levitating sample inside a RF coil and observing changes in the impedance and quality factors of the coil. Various systems have been developed to be used with the CNL and EML techniques (Enderby et al. 1997; Lohöfer 2005; Skinner et al. 2006). Some thermophysical properties can be also determined with levitated samples. This includes density, surface tension and viscosity using video observations (Egry et al. 1995; Millot et al. 2002; Paradis and Ishikawa 2005). With more difficulty, it is also possible to determine the specific heat (Paradis et al. 2001) and the thermal conductivity (Kobatake et al. 2007).
Finally, we can also point out that liquid dynamics can be studied using NMR experiments with the determination of relaxation times (Massiot et al. 1995; Gruener et al. 2001; Florian et al. 2007) and atomic diffusion coefficients (Rollet et al. 2009) in conjunction with inelastic X-ray and neutron techniques (Pozdnyakova et al. 2007; Meyer et al. 2008; Hennet et al. 2011). Figure 4 shows a comparison of the correlation time of liquid CaAl2O4 derived from viscosity measurements (Urbain 1983) and from NMR measurements (Massiot et al. 1995) and Quasi Elastic Neutron Scattering (QENS) (Kozaily et al. 2011).
APPLICATIONS OF AERODYNAMICS LEVITATION
NMR experiments
NMR at high temperature can be performed with a CO2 laser heating system using boron nitride, BN, containers or without containers by using a levitation system.
With containers, the samples are packed in pyrolitic BN crucibles. The sample container is sustained inside the radiofrequency coil of an axially symmetric multi-nuclear NMR probe with a ceramic support which also acts as a thermal shield, preventing the coil from being overheated. The samples are heated with two computer-controlled high-power CO2 lasers heating the top and bottom of the crucible to minimize thermal gradients. Temperatures up to, typically, 1400 °C can be obtained with these settings, sample temperatures being calibrated as a function of laser power using a thermocouple and the exact same experimental conditions (no NMR measurements are possible during calibration), leading to a temperature uncertainty of approximately ±10 °C. Different results can be obtained using this technique (Sen 1999; Le Losq et al. 2014; see also Stebbins and Xue 2014, this volume for a complete review of NMR at HT). More recently, using new NMR high field developments, it is now possible to measure at the same time and on the same sample two NMR signals, such as 23Na and 27Al (Fig. 5). For example, see in Figure 5, with this method 23Na and 27Al NMR spectra have been obtained on a NaAlSi3O8 composition in different states (crystalline, glass and liquid) with a high field NMR spectrometer (Brucker 750MHz, 17.6T). For these experiments, heating was provided by a CO2-laser irradiating the sample from the bottom in a UHT Brucker probe modified at the CEMHTI laboratory to handle crucibles. Samples (approx. 100 mg) were filled in tight-closed BN crucibles and heated by steps of approximately 100 °C under dried argon. A small and large 27Al background signal was also recorded at each temperature using an empty crucible and further subtracted from the actual spectra. One can also point out that this design is very well suited to the study of corrosive or air-sensitive melts such as fluorides. Using more conventional magnetic fields (i.e., 9.0 T), double CO2 irradiation conditions and temperature calibration recording the melting point of external standards, it has led to successful investigations of fluoride molten salts (see e.g., Lacassagne et al. 2002).
In situ High-Temperature Experiments 785
Figure 4. Correlation times of the aluminum movements obtained by NMR (Massiot et al. 1995) and Quasi Elastic Neutron Scattering, QENS (Kozaily et al. 2011) reported together with those derived from macro-scopic shear viscosity (Urbain 1983).
Figure 5. In situ high-temperature NMR measurements of (a) 23Na and (b) 27Al nuclei in crystallized albite NaAlSi3O8, and (c) 23Na and (d) 27Al nuclei in a glass of albite composition. All experiments were performed using a crucible design.
786 Neuville, Hennet, Florian, de Ligny
Without containers, the sample is first melted in an external levitation system to shape it as a sphere. It is then inserted in a convergent-divergent nozzle located inside the radio-frequency coil of a multi-nuclear probe (identical to the one used with the crucible design). Levitated samples are heated and melted in the NMR experiment using two focused CO2 lasers irradiating the top and bottom part of the sample, (i.e., Cote et al. 1992; Poe et al. 1992; Florian et al. 1995, 2007; Le Losq et al. 2014). Temperatures up to 2500 °C can be obtained with this design. The diameter of sample sizes varies but is commonly between 3 to 5 mm, and total NMR acquisition times of less than 50 ms are sufficient to produce spectra with excellent signal to noise ratios. This allows observation of sample cooling obtained by shutting off the laser and repeatedly acquiring NMR spectra every 25 ms. This has been used to determine the time-resolved evolution of the structure and dynamics of aluminate and aluminosilicate liquids (Massiot et al. 1995; Florian et al. 1995, 2007; Gruener et al. 2001). In this cooling approach, the sample first equilibrates its temperature within 0.3 ms, leading then to the observation of a liquid without significant thermal gradients during the course of its cooling. It also allows, in most cases, exploration of the super-cooled region (i.e., the liquid below the melting point Tm) since no heterogeneous nucleation can take place under this containerless melting.
Figure 6 shows a typical 27Al NMR spectra of liquid alumina made in 1990 at CEMHTI in Orléans, and the chemical shift of 50 ppm can be in-terpreted as a mixing of Al in four and six fold coordination. This is in good agreement with NMR experiments by Florian et al. (1995) and also with finite difference method near-edge structure (FDMNES) simulations and XANES spectroscopy at the Al K-edge by Neu-ville et al. (2009). However, some influ-ence of the temperature and levitation gas has been noted using this NMR set-up leading to non-stoichiometric Al2O3 melts (Coutures et al. 1994).
X-ray absorption spectroscopy (XAS)
X-ray absorption spectroscopy, XAS, is an element-specific method used to investigate the local structural order in materials (see Newville 2014, this volume; Henderson et al. 2014, this volume). As for solids, various types of information can be obtained with this technique. When a pre-edge peak exists, its intensity and position can give information on the Coordination Number, CN, and the oxidation state of the studied element. The detailed energy dependence of the XANES spectrum gives information about the electronic structure and from the EXAFS region it is possible to determine structural parameters such as bond lengths and CN. Compared to XRD measurements, XAS has the advantage that the determined structural information is element-specific and related to the absorbing atoms. A disadvantage is that in liquids and glasses, it is difficult to get structural information beyond the nearest neighbors of the absorbing atoms. With levitation techniques, the sample is too thick (mm range) for XAS measurements in transmission mode, and so experiments are performed by measuring the fluorescence produced by the absorption. Jacobs et al. (1996) demonstrated that it was possible to combine XAS measurements with EML techniques and to obtain information
Figure 6. 27Al NMR spectrum of a 66 mg levitated alu-mina melt at 2320 °C along with the image retrieved from the camera at a temperature of 2270 °C.
In situ High-Temperature Experiments 787
about the structure of liquid and supercooled metallic melts. XAS was then combined with the CNL technique by Landron et al. (1998) to study liquid oxides with the possibility of combining XAS and XRD experiments on the same sample. XAS experiments have also been combined with the hybrid aerodynamic electromagnetic levitator mentioned previously (Egry et al. 2008).
SAXS and SANS
Only a few experiments have used X-ray or neutron small angle scattering techniques with the CNL technique. Greaves et al. (2008) combined CNL with SAXS/XRD experiments to study polyamorphism in supercooled yttrium aluminates. Aerodynamic levitation and CO2 laser heating was also coupled with SANS experiments. This technique can be used to study phase separation in the sample, for instance. Taking advantage of the sensitivity of neutrons to magnetic moments, Fischer et al. (2007) measured the magnetic critical scattering from solid Co80Pd20 in the vicinity of the ferromagnetic transition temperature Tc.
X-ray and neutron diffraction
The first XRD experiments on levitated liquids were performed by Ansell et al. (1997) on liquid Al2O3 using the CNL setup developed by Krishnan et al. (1997). Following this work, the EML method was combined with energy-dispersive XRD (Notthoff et al. 2000; Kimura et al. 2001). The first XRD experiments using ESL were performed by Kelton et al. (2003) and a similar apparatus was further developed by other groups (Masaki et al. 2007). Since the first XRD experiments, CNL environments at synchrotron sources have been improved with the use of better detectors. In particular, several setups are installed at the APS (Mei 2008) and Spring 8 (Matsumura et al. 2007). The first XRD experiments with CNL using the CEMHTI levitation setup were conducted at the H10 beamline at LURE (Orsay, France) (Price 2010), and additional CNL setups have been installed at the Diffabs beamline at SOLEIL (Gif-sur-Yvette) and at the ID11 beamline at the ESRF (Bytchkov et al. 2010). The first neutron diffraction, ND, measurements on aerodynamically levitated liquids were performed by Landron et al. (2001) on liquid alumina at the SANDALS instrument at ISIS (Rutherford Appleton Laboratory, UK). A detailed description of the setup can be found in Landron et al. (2001). A similar CNL environment was developed by Weber et al. at the Intense Pulsed Neutron Source, (IPNS, Argonne, USA) (Weber et al. 2003). During the same period, Holland-Moritz team developed an EML environment for neutron investigation of metallic melts at the Institut Laue-Langevin, ILL, in Grenoble, France (Schenk et al. 2002). The ESL technique was further combined with neutron experiments by Paradis et al. (2002).
High-intensity synchrotron radiation sources have enabled the development of more selective methods such as anomalous X-ray scattering, AXS, which is not often used with levitation techniques. The first experiment combining AXS with CNL to study the liquid structure of Y2O3 were performed by Hennet et al. (2007b). For a polyatomic system, neutron diffraction with isotopic substitution, NDIS, is a powerful element-specific method to experimentally determine the partial structure factors. The pioneering NDIS experiment was performed by Enderby et al. (1997) who determined the partial structure factors of liquid Cu6Sn5. The NDIS technique consists of measuring the diffraction intensity for several samples of identical structure and chemical composition but having different isotopic compositions for one or more of the species. The use of NDIS is then dependent on the availability of the isotopes. Holland-Moritz et al. (2009) recently combined NDIS with EML to determine the partial structure factors and pair distribution functions of liquid and supercooled Ni36Zr64 alloy. The NDIS technique requires good counting statistics and EML is well adapted since it is possible to levitate relatively large samples. In spite of the small sample size currently used with the CNL method, it has also been combined with NDIS by Gruner et al. (2009) to study the structure of liquid Ni-Si alloys above the melting temperature.
788 Neuville, Hennet, Florian, de Ligny
Figures 7 and 8 show S(Q) and g(r) determined from X-ray and neutron diffraction at the liquidus temperature for calcium aluminosilicate melts (Kozaily et al. 2011; Kozaily 2012). These structural data are in good agreement with those expected from knowledge of the glass structure at room temperature (Cormier et al. 2000, 2005) and also from XANES experiments at the Al and Ca K-edges for CaAl2O4 (Neuville et al. 2008).
WIRE OR PLATE HEATING SYSTEM
Description, temperature and atmosphere control
The heating-wire technique was first developed by Ohashi and Hadidiacos (1976) to develop a HT microscope at the Geophysical Laboratory, Carnegie Institution of Washington. Mysen and Frantz (1992) popularized this idea, using it for high temperature Raman spectroscopy. The heating-wire, HW, was a welded junction of 1-mm-diameter thermocouple junction in Pt and Pt-Rh 10%. It serves as both a heating element and a thermocouple. The output of the thermocouple is used to regulate a 50 Hz pulse input-power supply. The thermocouple e.m.f, is measured by the same home-made apparatus between the pulses. The sample is placed within a 1 mm hole drilled mechanically at the flattened junction. This hole corresponds to the hot spot. This heating system has low thermal inertia and it is possible to change the
Figure 7. Typical average structure factors S(Q) measured on liquid calcium aluminosilicates using X-ray (left) and neutron scattering (right) (data from Kozaily 2012). Note that X-ray experiments are faster (typi-cally minutes) than neutron ones (>3 hours), and chemical compositions richer in silica can be investigated without SiO2 loss during the experiments.
Figure 8. Average pair correlation functions g(r) calculated for liquid calcium aluminosilicates using X-ray (left) and (right) neutron scattering (data from Kozaily 2012).
In situ High-Temperature Experiments 789
temperature between room temperature up to temperatures >1600 °C in few seconds. At temperature the heating system is relatively stable and can stay at the selected temperature as long as needed during the experiments. This device is remarkably simple and efficient. In the original design of Mysen and Frantz (1992) the wire was in a U-shape, and the position of the hole could slowly move vertically and laterally due to thermal expansion. This induced deformation of the wire and movement of the sample. To eliminate this it is possible to work with a stiffer wire. However, the principal problem of the first design of the heating device was the non-commercial power supply developed by Hadidiacos at the Geophysical Lab. A first modification of this heating system was made by using a wire in Pt-Rh10% heated by a direct current produced by a commercial power supply (Neuville and Mysen 1996). Furthermore, to avoid the movement of the sample due to dilatation, a new linear heating wire was made with a large variety of different alloys PtRh10%, PtIr10%, Ir and W (Richet et al. 1993). Platinum and its alloys are good wires to heat systems in air but to work at very high temperature, it is necessary to use Ir, W, Re or Ta which require vacuum or controlled atmospheres. Figure 9 shows drawings of the original device and a recent modified system made to work under air or controlled atmosphere conditions (Gonçalves Ferreira et al. 2013). The latest version (Fig. 9) has a double confinement box to safely heat radioactive materials like UO2, PuO2 and others radioactive actinides elements. This new set-up was developed with the French Atomic Energy Commission, CEA. Figure 10 shows the electric power needed to heat different wires at high temperature under different atmospheres. Ta is a good candidate to work at very high temperature without oxygen, and Ta is easily workable compared with Ir, Re or W, which cannot be flattened and need to be drilled using electro-erosion or a laser apparatus. The
Thermocouple junction
0.8 mm Pt
0.8 mm Pt-Rh10
~1 mm melt sample
Figure 9. (left) The original HW setup proposed by Mysen and Frantz (1992). (below) New HW configuration developed with Saint-Gobain Recherche, this fur-nace works with controlled atmo-spheres (Air, N2, O2, Ar, ArH2).
790 Neuville, Hennet, Florian, de Ligny
maximum size of the hole varies between 100 µm in Ir or W wire up to 2 mm in Pt or Pt-Ir10%, Pt-Rh10%.
Temperature accuracy is ±5 K at high temperature (Neuville and Mysen 1996) and reproducibility is bet-ter than 10 K if the wire does not change due to vola-tilization of its constituent alloys at high temperature, for instance Ir. Most important is to use the appropriate wire alloy for the experimental temperature and atmo-sphere conditions (Table 1).
Each wire is different:
• PtIr10% or PtRh10% are very useful, easy to make, can be used in air or with many different atmospheres, and can be put in HF to be cleaned.
• Pt with higher Ir or Rh content can increase the maximum temperature used but the wires become harder and more difficult to drill.
• Rh wire has a very high melting point, is easy to drill but very expensive.
• Ir, Re or W wire are perfect to work at very high temperature up to 3000 K and more but need controlled atmospheres with very low oxygen content. These metals are very hard and very difficult to drill a hole in; Re has better mechanical properties than Ir and a higher melting point.
• Ta wire is easy to drill, very good at high temperature, needs controlled atmospheres without oxygen, and need a very high power supply due to its low electrical resistivity.
Raman spectroscopy
Several Raman spectroscopy studies at high temperature have used this technique on silicate melts (Mysen and Frantz 1992, 1993, 1994a; McMillan et al. 1994; Frantz and Mysen 1995), on aluminosilicate melts (Mysen and Frantz 1994b; Neuville and Mysen 1996; Daniel et al. 1995) on titanosilicate melts (Mysen and Neuville 1995; Reynard and Webb 1998); germanosilicate melts (Henderson et al. 2009), borate melts (Cormier et al. 2006); borosilicate melts (Manara
500
1000
1500
2000
2500
3000
3500
0 20 40 60 80 100 120 140
T, K
P (W)
Ir Air
Ir Vacuum
Ta ArPtIr1
0% a
ir
PtRh10% air
Rh air
PtIr10% vaccum
Re Ar
Figure 10. Electric power versus temperature for different wires and atmosphere.
Table 1. Liquidus temperature for some metals.
Metal Liquidus, K
Platinum, Pt 2041
Rhodium, Rh 2236
Iridium, Ir 2720
Tantalum, Ta 3293
Rhenium, Re 3455
Tungsten, W 3695
In situ High-Temperature Experiments 791
et al. 2007; Lenoir et al. 2010) and also on crystal melting (Richet et al. 1998). Raman spectra of potassium silicate melts collected using a heating wire device (Mysen and Frantz 1992) are similar to those obtained on glass using a Leitz® furnace (McMillan et al. 1992). The only precaution to be taken every time consists to verify that the chemical composition of the sample does not change during the thermal cycle. This verification can be easily made by comparison of Raman spectra at the beginning and at the end of experiment as shown by Neuville and Mysen (1996). The results obtained by HT Raman spectrometry correspond to the changes which takes place during melting, phase transitions, the glass transition, or to structural changes with changing temperature. These are developed in more detail in the Raman chapter (Neuville et al. 2014, this volume). The heating wire can also be used for Brillouin measurements at high temperature (Vo-Thanh et al. 2005).
X-ray diffraction
Crystalline materials. The heating system has been used at different light sources to investigate the structure of crystals, glasses and melts. One of the first investigations was made at the LURE light source to investigate the structure of simple compounds such as MgO, CaO, Al2O3, and MgAl2O4, by X-ray diffraction (Fig. 11) (Fiquet et al. 1999). In Figure 11, the lattice parameters, a and c of the four components, MgO, CaO, Al2O3 and MgAl2O4, are plotted and compared (Fiquet et al. 1999). For this kind of experiment, it is easy to determine the thermal expansion coefficients as a function of temperature and chemical composition.
Glasses and liquids. X-ray diffraction on glasses provides important knowledge about their structures (see Taylor and Brown 1979a,b) but we have had to wait for progress on light sources to conduct similar experiments on melts that require HT conditions and short data acquisition times. Recently, with developments at beamlines such as those at SOLEIL (DIFFABS) and ESRF (ID11) it is possible to investigate directly the structure of melts at high temperature. The wire heating device has been adapted for the DIFFABS beamline (Fig. 12) to investigate the structure of melts and glasses at the glass transition temperature (X-ray Absorption Spectroscopy was recorded in the same time, see below). The scattering measurements are carried out in transmission. In Figure 13 is shown an example of the X-ray scattering experiments recorded at 17.5 keV on a glass composition (33%SiO2-
4
4,2
4,4
4,6
4,8
5
5,2
500 1000 1500 2000 2500 3000 3500
Latt
ice
para
met
ers,
Å
T, K
aMgO
a/2MgAl2O4
aCaO
aAl2O3
c/3Al2O3
Figure 11. Lattice parameters a and c for MgO, CaO, Al2O3 and MgAl2O3 from Fiquet et al. (1999). The c parameter of Al2O3 has been divided by 3 and a of MgAl2O3 by 2.
792 Neuville, Hennet, Florian, de Ligny
33%Al2O3-33%CaO, with a 0.1 of ZrO2 in mol%). Several temperatures were studied from room temperature to the glass transition and to the liquid state at 1873 K (Fig. 13). The unusual geometry of the heater makes it rather complicated to acquire these spectra. Some adjustments are required in order to eliminate all of the wire diffraction peaks. Its takes two hours to acquire a X-ray diffraction pattern, which means that the apparatus must have excellent thermal stabil-ity for HT experiments, in this case at 1873 K. The spectra of the samples annealed for 15 and 50 minutes at 1173 K were obtained at ambient temperature after quenching. The spectra at different temperatures show a major peak at 15° followed by two more diffuse peaks at 20° and 30° before the intensity gradually decreases. The spectrum of the liquid at 1873 K is again simi-lar to that of the glass at ambient temperature, in agreement with the homogeneous amorphous nature of the two samples. The spectrum of the undercooled liquid annealed for 15 minutes at
Figure 12. Heating wire assembly on the X-ray scattering set-up at the DIFFABS beamline.
20 40 60 80 100 120
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itra
ry u
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Figure 13. X-ray scattering diagram at 17.5 keV for the glass at 300 K, the stable liquid at 1873 K, and the partially annealed glass at 1173 K. The Bragg peaks visible in the annealed liquid diagram at 1173 K correspond to ZrO2 crystals.
In situ High-Temperature Experiments 793
1173 K does not display any significant change with respect to those of glass and liquid. On the other hand, the product obtained after annealing for 50 minutes at 1173 K shows Bragg peaks, which are characteristic of the ZrO2 phase which crystallizes at this temperature. The pair cor-relation functions g(r) are determined from the diffraction diagrams of the glass and the liquid. These diagrams provide some insight into the structural changes occurring in the liquid in the neighborhood of the glass transition temperature and in the nucleation domain. Figure 14 shows these correlation functions g(r) for the glass, the undercooled liquid annealed for 15 minutes at 1173 K and, for the first time a stable liquid at 1873 K. Although it is not easy to interpret all the g(r) functions, in Figure 14 we provide a suggested assignment for the various peaks. Some significant changes are observed between the glass and the liquid. The 2.38 Å peak, which represents the Ca-O distances, is visible in the g(r) for the glass, but disappears at higher temperatures. The shoulder at 2.09 Å, which represents the Zr-O distance, is more visible in the liquid than the glass, even when it overlaps the 1.74 Å peak assigned to Si-O and Al-O. The annealed liquid at 1173 K also displays significant changes, especially with the Ca-O and Zr-O distances. These changes are the result of a reorganization of the liquid before the nucleation/crystallization phenomena shown in Figure 13 for the liquid annealed at 1173 K for 50 minutes.
To understand the results on the glass and melt, we need to consider three points:
• Glass is disordered, but each atom has a stable local environment, which can be char-acterized by pair distances such as Si-O, Al-O, Zr-O, Ca-O, O-O (arrows in Fig. 14).
• The liquid is disordered but the atoms can move, and by performing X-ray diffraction, a statistic average is taken (red curve, Fig. 14) where T-O pair can be observed (T = Si,Al).The second peak corresponds to cation-cation pairs. But the detailed pairs are lost in the anharmonic distribution of the liquid state.
• By cooling the liquid slowly down to the glass transition temperature, atomic motion decreases and each atom reaches a minimum configurational state. At 1173 K, which is higher than the glass transition temperature (1100 K), the X-ray diffraction pattern (Fig. 13) looks similar to those of the liquid state, but by computing the pair correlation function we observe the presence of the Ca-O peak in a fixed configuration. This implies that the calcium atoms are the first to equilibrate into their crystalline positions, followed later by Al and Si.
2 4 6 8 10
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Figure 14. g(r) at different temperatures for glass, liquid, and liquid annealed for 15 minutes at 1173 K, the peaks are assigned to the different atomic pair distances.
794 Neuville, Hennet, Florian, de Ligny
X-ray absorption
By using different types of heating wire, it is possible to measure X-ray absorption on heavy and light elements. Some examples on different K-edge or L-edge (Magnien et al. 2008; Neuville et al. 2008, 2009; Cochain et al. 2009; de Ligny et al. 2009) are discussed below. From X-ray absorption at the Fe K-edge, Magnien et al. (2008) obtain the redox state of Fe at high temperature as a function of time. This was measured by looking at the pre-edge fol-lowing Wilke et al. (2001) and it was possible to demonstrate that different redox processes take place depending on the temperature. By looking at the XANES of light elements at high temperature, Neuville et al. (2008) linked changes in the Al K-edge in calcium aluminate com-positions to viscosity, and proposed a model to understand atomic dynamics in depolymerize melt compositions.
On the same composition investigated by X-ray diffraction on the DIFFABS beamline X-ray absorption spectra at the Zr K-edge were recorded in transmission or fluorescence modes. Figure 15 shows the different spectra obtained on the glass or melt at different temperatures.
Figure 15. Absorption spectra at the K-edge of Zr for the glass and for the annealed liquid at 1373 K in transmission and fluorescence (a), and at different temperatures up to the liquidus (b).
(a)
(b)
In situ High-Temperature Experiments 795
The XANES spectrum at the K-edge of Zr in the glass has a significant white line and few EXAFS oscillations, which characterizes a lack of long-range order. At 1373 K, the spectra contain the same white line, but it is followed by a series of very distinct oscillations, which imply a change of order in the distribution of Zr in the liquid, due to partial crystallization of the sample or structural reorganization around Zr. The spectral differences visible in Figure 15b, between transmission and fluorescence is explained by the fact that the whole sample is analyzed in transmission, while in fluorescence only the near surface is probed over approximately ten microns. The fact that the oscillations observed in transmission mode were more distinct than those in fluorescence means that the sample is more crystallized at its core than near the surface.
The white line varies little with temperature, while the oscillations increase after the glass transition because of partial crystallization of the sample. The spectrum of the liquid beyond the liquidus is similar to that of glass, and is therefore characteristic of a homogeneous phase in which long-range order is absent.
ADVANTAGES, DIFFERENCES AND CONCLUSIONS
Levitation techniques and heating wire devices are complementary tools, which can be used with a variety of diffraction and spectroscopic techniques to investigate materials at high temperatures for heating samples. Figure 16 shows X-ray diffraction patterns obtained using both systems. It clearly demonstrates that both heating methods give similar results within ex-perimental error. However experimentally there are some differences between the two heating techniques which one should be aware of before deciding upon the most suitable method for specific experiments.
The conical nozzle levitation, CNL, laser heating, is easy to implement at very high temperatures, (>2000 K) and for samples from 10 mg up to 100 mg (1 mm to 5 mm diameter), allows very fast quench and a glass sample can be obtained with ease. CNL works with different atmospheres, pO2 can be controlled and it does not use containers, which avoids any contacts with the heating system and potential surface effects on crystallization. The technique can be adapted to perform experiments using a wide variety of spectroscopic techniques at high temperature e.g., neutron and X-ray diffraction, X-ray absorption, small angle scattering, Raman spectroscopy, etc. Disadvantages are that CNL can induce vaporization of light elements and the chemical composition of the sample can change during the experiment because of the gas used as levitator.
The Wire Heating, WH, device, is easy to use at low temperature with Pt-Ir10% alloy, but can go to higher temperatures with appropriate heating wires (like W, Ir, Re, Ta…). However they are more difficult to mill and need to be used with controlled atmospheres. WH can work with less than 10 mg although the heating wire is in contact with the sample. This method is however ideal for working at lower temperatures with high thermal stability and enables studies of nucleation processes around the glass transition temperature for example. It can be placed in a vacuum chamber to investigate light elements using X-ray absorption spectroscopy but is not suitable for use in with neutron techniques, which require large sample volumes. WH is ideal for working on volatile elements, because the vaporization of the elements is very small, and the sample is contained within the hole in the wire. New developments now allow work with controlled atmosphere and radioactive elements such as U, Pu, and other radioactive actinides to investigate their structure with X-ray diffraction or absorption.
796 Neuville, Hennet, Florian, de Ligny
Figure 16. (a) X-ray diffraction measurements on calcium aluminosilicate (33% SiO2-33% Al2O3-33% CaO in mol%), (b) S(Q) and (c) g(r) obtained using levitation or a heating wire.
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In situ High-Temperature Experiments 797
ACKNOWLEDGMENTS
Thanks to Eozen Strukelj and Mathieu Roskosz for help during measurement on DIFFABS beamline at SOLEIL. Paula Gonçalves Ferreira (Saint-Gobain Recherche), Philippe Martin (CEA-Cadarache), Florent Lebreton (CEA-Marcoule), Thibaud Delahaye (CEA-Marcoule), Dominique Thiaudière (SOLEIL) and Grant Henderson for constructive reviews.
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