Design, calibration, and operation of a system for drop‐and‐catch (DnC) calorimetry on oxides from temperature above 1500°C are described. This system allows the measurement of heat contents and ...heats of fusion by drop calorimetry of small (100 mg or less) samples held by containerless levitation at high temperature and dropped in a calorimeter at room temperature. The spheroids, 2‐3 mm in diameter, prepared by laser melting of powders, are aerodynamically levitated in a splittable nozzle levitator and laser heated to the desired temperature monitored by radiation thermometry. The sample is dropped by splitting the nozzle and caught by splittable water‐cooled calorimetric plates at 25°C, which provide complete enclosure of the sample to avoid heat loss by radiation. The drop time is ~0.1 seconds, calorimeter equilibration time after the drop is ~15 minute. DnC experiments are automated with software‐controlled laser power and programmable delay between splitting the nozzle and catching the sample. The fusion enthalpy of Al2O3 measured by DnC calorimeter, 120 ± 10 kJ/mol, agrees well with previously reported values. The system can be used for measurements of fusion enthalpies of refractory oxides amenable to laser heating as well as for splat quenching of oxide melts.
Abstract
Structure and thermodynamics of pure cubic ZrO
2
and HfO
2
were studied computationally and experimentally from their tetragonal to cubic transition temperatures (2311 and 2530 °C) to their ...melting points (2710 and 2800 °C). Computations were performed using automated
ab initio
molecular dynamics techniques. High temperature synchrotron X-ray diffraction on laser heated aerodynamically levitated samples provided experimental data on volume change during tetragonal-to-cubic phase transformation (0.55 ± 0.09% for ZrO
2
and 0.87 ± 0.08% for HfO
2
), density and thermal expansion. Fusion enthalpies were measured using drop and catch calorimetry on laser heated levitated samples as 55 ± 7 kJ/mol for ZrO
2
and 61 ± 10 kJ/mol for HfO
2
, compared with 54 ± 2 and 52 ± 2 kJ/mol from computation. Volumetric thermal expansion for cubic ZrO
2
and HfO
2
are similar and reach (4 ± 1)·10
−5
/K from experiment and (5 ± 1)·10
−5
/K from computation. An agreement with experiment renders confidence in values obtained exclusively from computation: namely heat capacity of cubic HfO
2
and ZrO
2
, volume change on melting, and thermal expansion of the liquid to 3127 °C. Computed oxygen diffusion coefficients indicate that above 2400 °C pure ZrO
2
is an excellent oxygen conductor, perhaps even better than YSZ.
The oxidation of boride and carbide-based ultra-high-temperature ceramics is the primary limiting factor for their use as aerodynamic surfaces. Understanding the behaviour of the oxides that can ...result from oxidation of metal borides and carbides at very high temperatures is essential to optimise and tailor the performance of these materials; yet experimental thermodynamic and structural data for refractory oxides above 2000°C are mostly absent. The following techniques that can be applied to fill this gap are discussed: (i) commercial ultra-high-temperature differential thermal analysis for investigation of phase transformations and melting in inert environments to 2500°C, (ii) a combination of laser heating with a splittable nozzle aerodynamic levitator for splat quenching and drop calorimetry from temperatures limited only by sample evaporation, (iii) synchrotron X-ray and neutron diffraction on laser-heated aerodynamically levitated oxide samples for in situ observation of phase transformations in variable atmospheres, refinement of high-temperature structures and thermal expansion. Recent experimental findings include anomalous thermal expansion of the defect fluorite phase of YSZ, thermodynamics of pyrochlore-fluorite transformation from high-temperature structure refinements, and measurement of thermal expansion to the melting temperatures and fusion enthalpies of Zr, Hf, La, Yb and Lu oxides. These methods provide temperatures, enthalpies and volume change for phase transformations above 2000°C, which are required for thermodynamic assessments and calculation of phase diagrams of multicomponent systems.
The previously unknown experimental HfO2–Ta2O5‐temperature phase diagram has been elucidated up to 3000°C using a quadrupole lamp furnace and conical nozzle levitator system equipped with a CO2 ...laser, in conjunction with synchrotron X‐ray diffraction. These in‐situ techniques allowed the determination of the following: (a) liquidus, solidus, and invariant transformation temperatures as a function of composition from thermal arrest experiments, (b) determination of equilibrium phases through testing of reversibility via in‐situ X‐ray diffraction, and (c) molar volume measurements as a function of temperature for equilibrium phases. From these, an experimental HfO2–Ta2O5‐temperature phase diagram has been constructed which is consistent with the Gibbs Phase Rule.
HfO2–Ta2O5‐Temperature phase space built based on the observed equilibrium phases from in situ X‐ray powder diffraction and the Gibbs Phase Rule.
The thermodynamic stability of ceramic coatings with respect to their reaction products is crucial to develop more durable coating materials for gas‐turbine engines. Here, we report direct ...measurements using high‐temperature solution calorimetry of the enthalpies of reaction between some relevant ceramic coatings and a corrosive molten silicate. We also report the enthalpy of mixing between the coatings and molten silicate after combining the results measured by high‐temperature solution calorimetry with enthalpies of fusion measured by drop‐and‐catch calorimetry and differential thermal analysis. The enthalpies of solution of selected silicate and zirconia‐based coatings and apatite reaction products are moderately positive except for 7YSZ, yttria‐stabilized zirconia. Apatite formation is only favorable over coating dissolution in terms of enthalpy for 7YSZ. The enthalpies of mixing between the coatings and the molten silicate are less exothermic for Yb2Si2O7 and CaYb4Si3O13 than for 7YSZ, indicating lower energetic stability of the latter against molten silicate corrosion. The thermochemical results explain and support the very corrosive nature of CMAS melts in contact with ceramic coatings.
Abstract
The previously unknown experimental HfO
2
–Ta
2
O
5
‐temperature phase diagram has been elucidated up to 3000°C using a quadrupole lamp furnace and conical nozzle levitator system equipped ...with a CO
2
laser, in conjunction with synchrotron X‐ray diffraction. These in‐situ techniques allowed the determination of the following: (a) liquidus, solidus, and invariant transformation temperatures as a function of composition from thermal arrest experiments, (b) determination of equilibrium phases through testing of reversibility via in‐situ X‐ray diffraction, and (c) molar volume measurements as a function of temperature for equilibrium phases. From these, an experimental HfO
2
–Ta
2
O
5
‐temperature phase diagram has been constructed which is consistent with the Gibbs Phase Rule.
The previously unknown experimental HfO2-Ta2O5-temperature phase diagram has been elucidated up to 3000 degrees C using a quadrupole lamp furnace and conical nozzle levitator system equipped with a ...CO2 laser, in conjunction with synchrotron X-ray diffraction. These in-situ techniques allowed the determination of the following: (a) liquidus, solidus, and invariant transformation temperatures as a function of composition from thermal arrest experiments, (b) determination of equilibrium phases through testing of reversibility via in-situ X-ray diffraction, and (c) molar volume measurements as a function of temperature for equilibrium phases. From these, an experimental HfO2-Ta2O5-temperature phase diagram has been constructed which is consistent with the Gibbs Phase Rule.
Ceramic equilibrium phase diagrams have proven to be difficult to produce for materials above 1500 °C, We demonstrate that in-situ X-ray powder diffraction on laser-heated, levitated samples can be ...used to elucidate phase diagrams up to 3000 °C. In these experiments, solid spherical samples were suspended and rotated by a gas stream through a conical nozzle levitator, heated by a 400 W CO2 laser at beamline 6-ID-D of the Advanced Photon Source at Argonne National Laboratory, Lemont, IL, USA. X-ray diffraction patterns suitable for Rietveld refinement were collected at 100 °C temperature intervals and were used to determine the phase fields of phases present. The temperature of each phase was determined based on thermal expansion data collected by powder diffraction in conjunction with the Quadrupole Lamp Furnace (QLF) at beamline 33-BM-C of the Advanced Photon Source. The liquidus temperatures were determined from levitation and laser heated recoalescence experiments. The crystal structures of new phases were solved using the charge flipping method from powder diffraction data. The HfO2-Ta2O5 binary was investigated as an example system due to its high melting points and application in refractories and electronics.
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