Thermochemistry of calcium rare‐earth silicate oxyapatites

Abstract

The calcium rare‐earth (RE) silicate oxyapatite, Ca2RE8(SiO4)6O2 (RE = Yb, Er, Y, Dy, Nd, Gd, and Sm), powders were synthesized by the solid‐state reaction method and characterized by X‐ray diffraction (XRD), Raman spectroscopy, and elemental composition analysis. The thermodynamic properties of the oxyapatites have been investigated using high‐temperature oxide melt calorimetry in molten 2PbO–B2O3 solvent at 805°C. The energetics of the oxyapatites related to ionic substitution on two crystallographic sites, M(1) and M(2), are discussed. The enthalpy of formation from the oxides becomes more exothermic as the ionic potential decreases or the ionic radius of the REs increases, which indicates increasing energetic stability in this order.

Rare-earth pyrosilicate solid-solution environmental-barrier coating ceramics for resistance against attack by molten calcia–magnesia–aluminosilicate (CMAS) glass

“…Most recently, Stokes et al [38] have confirmed that the propensity of RE-Ca-Si apatite formation decreases with the increasing difference between RE 3+ and Ca 2+ radii in RE 2 Si 2 O 7 powders that have interacted with different CMASs (at 1400°C for 1 h), where RE = Nd (1.163 Å), Gd (1.107 Å), Dy (1.083 Å), or Er (1.062 Å), in that order. Also, these RE 2 Si 2 O 7 have been shown to have lower enthalpies for apatite formation compared to Yb 2 Si 2 O 7 , where Yb 3+ has the smallest ionic radius [39]. Thus, the combination of CMAS with the highest in Ca content (Ca/Si = 0.76, NAVAIR) and pellet with the highest Y-content (x = 2, Y 2 Si 2 O 7 ) shows the greatest propensity for apatite formation [ Fig.…”

High-Temperature Thermodynamics of Cerium Silicates, A-Ce2Si2O7, and Ce4.67(SiO4)3O

“…By this way, we obtained H f, ox to be 421.4 ± 8.7 kJ/mol and -445.6 ± 11.6 kJ/mol ( As it was stated earlier, one of the main expressed interest into rare earth silicate materials is their potentials as TBC or EBC against high temperature degradation commonly encountered in the aeronautical applications. 1,2,[10][11][12][13] Desirable TBC and EBC materials must have both structural and chemical stabilities. 2,4,[10][11][12][71][72][73] The structural stability includes congruent CTEs as the material that they are coating and phase stability at the operating temperature, which can minimize potential strains that promote mechanical failure.…”

“…As many next generation high-temperature alloys and CMC for aeronautical application offer superior thermal and mechanical properties at elevated temperature, 71,76 they often rapidly degrade in the presence of high-temperature air, steam, or mineral debris. [10][11][12]71 For this reason, the EBC or TBC must be chemically stable and resist corrosion, to sustain the operating lifespan. Hence, to fully assess the suitability of the materials for aeronautical applications, we evaluated the high temperature stability of both A-Ce 2 Si 2 O 7 and Ce 4.67 (SiO 4 ) 3 O in terms of both structural and chemical stability.…”

“…2,3 However, during the take-off and landing of aircraft, various rare-earth oxyapatites (M 2+ RE 4 (SiO 4 ) 3 O) are known to form as corrosion products between the rare-earth disilicates EBC and molten aerosol mineral dust, composed primarily of CaO-MgO-Al 2 O 3 -SiO 2 (CMAS). 10,11 Such rare-earth oxyapatite corrosion products are thermodynamically stable and can serve as an effective passivating layer to reduce or prevent further pitting and erosion of EBC/CMC on the engines of the aircraft. 10,12,13 Conversely, the potential mismatch of CTEs of the rareearth oxyapatite and the EBC/CMC could also cause increased internal stresses to build promoting mechanical failure.…”

“…17,18,21 As a result, structural and thermochemical investigations have been detailed at both low and high temperatures. 1,2,[21][22][23][24][25][26][27]11,[14][15][16][17][18][19][20] However, the thermophysical and thermochemical understanding of the low temperature polymorph of cerium disilicate (A-Ce 2 Si 2 O 7 ; space group P4 1 ) and the cation-vacant cerium oxyapatite (Ce 4.67 (SiO 4 ) 3 O; space group P6 3 /m) are still largely absent (Figure 1). Additionally, as Ce is the most abundant rare earth element 28 and an effective surrogate for Pu in the solid state system, [29][30][31][32][33] the thermodynamic properties of A-Ce 2 Si 2 O 7 and Ce 4.67 (SiO 4 ) 3 O are fundamentally needed for geochemical modeling and desirable for helping evaluate the thermodynamics of Pu in silicate solid systems.…”