The influence of dopants on the surface enthalpy of Yttrium aluminum garnet (YAG)
Introduction
Yttrium aluminum garnet (Y3Al5O12, YAG) is a synthetic garnet crystalline material with a number of attractive properties, including high chemical stability, high quantum yield, low toxicity, low level of thermal expansion, and tunable light emission wavelength [[1], [2], [3]]. This stable oxide material has high creep resistance and when doped with rare earth ions shows applications in various fields such as solid-state lasers, cathode ray tube, and back light source [[4], [5], [6]]. YAG has also been recognized as one of the best phosphor host materials so far and therefore is currently of great interest to a variety of displays applications [7,8]. Although considerable advances have been made towards the understanding of processing of YAG into single crystals and polycrystalline transparent ceramics, the description of the phase stability and overall thermodynamics of doped and undoped YAG is still limited. Such data are relevant as serve as parameters affecting crystal growth, crystallization, and coarsening.
The crystal structure of YAG is a complex cubic structure containing three different oxygen polyhedral [9]. Dodecahedral sites are occupied by Y3+ ions, while Al3+ ions occupy octahedral and tetrahedral sites in the ratio of 2:3, which is the result of the differences in the ionic radii: O2−(1.4 Å), Y3+(1.28 Å), Al3+(0.51 Å). Noteworthy, the ionic radii of Y3+ ions and rare-earth ions are relatively close; as a consequence, trivalent rare earths can easily replace Y3+, making YAG a suitable host material for optical applications[9,10].
Recently, few studies have shown the influence of additives on the sintering behavior of YAG and Nd doped YAG nanoparticles. Zamir has reviewed the influence of various cations such as 1.4 at% of La2O3, ZrO2, MgO, Nb2O5, and SiO2 on the grain boundary (GB) mobility in YAG [11]. All dopants, except for La2O3, increase the GB mobility as compared to undoped YAG (La2O3 didn’t cause any significant change in YAG’s GB mobility). Haneda et al. reported the effect of doping on the ion diffusivity in YAG [12]. The grain boundary diffusivity of oxide ions was only weakly influenced by the type and amount of dopant. In spite of the importance of doping in YAG, there is a lack of published work on the study of the effect of the additions on the thermodynamic properties of YAG, in particular regarding interfacial energies. Such data controls processes such as single crystal growth, while having impacting effects on the stability of phases at the nanoscale [13,14]. Materials such as TiO2, TiO2-SnO2, Sc2O3-ZrO2, and Y2O3-ZrO2 all have proposed nanoscale phase diagrams that take into account contributions from both bulk enthalpies and interfacial enthalpies to allow for an accurate prediction of phase stability [13,[15], [16], [17]]. Accurate measurements of surface enthalpies are challenging but among the available techniques to do so, two calorimetric approaches have been systematically used to determine the accurate surface enthalpies of various oxides: high temperature oxide melt drop solution [18,19] and water adsorption microcalorimetry [20,21]. To calculate the surface enthalpy using high temperature solution calorimetry, one needs to have multiple samples that have different grain sizes which are dissolved in a molten solvent maintained at high temperature in a calorimeter. In this method, the enthalpy is measured first and is related back to the surface enthalpy using a Hess’ Law engine [22]. On the other hand, water adsorption calorimetry requires only a single sample and is a non-destructive technique. This method relies on the Gibbs adsorption theory, using the heat involved when water molecules are adsorbed on the surface of the oxide to derive accurate surface enthalpy data [20]. There are various reports from our group where water adsorption calorimetry was used to measure the surface enthalpy of nanoparticles [[20], [21], [22], [23], [24]].
The present study aims to explore the effect of dopants on the surface enthalpy of YAG by applying water adsorption calorimetry. La3+ and Mg2+ were used as dopants, targeting an analysis of the effect of different valences and ionic radii on the surface enthalpy of YAG and in an attempt to establish design protocols for surface enthalpy control. Nanoparticles were used in the studies to assure significant surface area in the samples and allow for a direct measurement using water adsorption microcalorimetry. The surface enthalpy data acquired here are averages of all existing surface planes on the studied nanoparticles, but give important insights for process control utilizing these thermodynamic quantities.
Section snippets
Synthesis
YAG powders were synthesized using reverse strike co-precipitation, which is described elsewhere in details [25]. All the metal nitrates are from Sigma-Aldrich which were used as the starting materials. The nitrates were dissolved in stoichiometric ratios (3 mol of Y ions and 5 mol of Al ions) in deionized water to form 1 M solution, which was then added drop wise into a 5 M NH4OH aqueous solution (used as the coprecipitation agent). Excess NH4OH aqueous solution was used to maintain a high pH
Synthesis and nanoparticles’ characterization
Fig. 1 shows the thermogravimetric curve for the updoped and 2 % (La, Mg) doped YAG precursors (as precipitated from the reverse-strike precipitation method). The TG curves for all the samples show a decrease in the weight loss with increase in temperature, with basically two distinct slopes followed by saturation above 800 °C. The weight loss below 200 °C is attributed to the physisorbed water molecules in the samples, while the one in between 200 to 500 °C can be related to the transformation
Conclusions
In the present study, surface enthalpy of undoped and doped YAG nanoparticles were measured using water adsorption calorimetry. The surface enthalpy of pure YAG was directly measured as 1.29 + 0.11 J/m2 and the measured anhydrous surface enthalpies of 2%-doped La and Mg doped YAG nanoparticles are 0.98 + 0.09 and 1.11 + 0.07 J/m2, respectively. While the changes suggest it is possible to modulate YAG’s surface enthalpy for better processing control, it was observed that the presence of water
CRediT authorship contribution statement
Geetu Sharma: Investigation, Writing - original draft. Kimiko Nakajima: Investigation. Dereck N.F. Muche: Investigation. Ricardo H.R. Castro: Data curation, Writing - original draft, Validation, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was funded by NSF DMR Ceramics1609781.
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