Structural Effects of Lanthanide Dopants on Alumina

Ketan Patel, Victoria Blair, Justin Douglas, Qilin Dai, Yaohua Liu, Shenqiang Ren, Raymond Brennan
2017 Scientific Reports  
Lanthanide (Ln 3+ ) doping in alumina has shown great promise for stabilizing and promoting desirable phase formation to achieve optimized physical and chemical properties. However, doping alumina with Ln elements is generally accompanied by formation of new phases (i.e. LnAlO 3 , Ln 2 O 3 ), and therefore inclusion of Ln-doping mechanisms for phase stabilization of the alumina lattice is indispensable. In this study, Ln-doping (400 ppm) of the alumina lattice crucially delays the onset of
more » ... transformation and enables phase population control, which is achieved without the formation of new phases. The delay in phase transition (θ → α), and alteration of powder morphology, particle dimensions, and composition ratios between αand θ-alumina phases are studied using a combination of solid state nuclear magnetic resonance, electron microscopy, digital scanning calorimetry, and high resolution X-ray diffraction with refinement fitting. Loading alumina with a sparse concentration of Ln-dopants suggests that the dopants reside in the vacant octahedral locations within the alumina lattice, where complete conversion into the thermodynamically stable α-domain is shown in dysprosium (Dy)-and lutetium (Lu)-doped alumina. This study opens up the potential to control the structure and phase composition of Ln-doped alumina for emerging applications. Aluminum oxide (Al 2 O 3 ), also known as alumina, is a common ceramic material of interest for advanced applications. Apart from the most thermodynamically stable α -phase, alumina also exists in several metastable/ transitional phases 1 . These metastable phases are classified into two major categories, including face centered cubic (FCC) and hexagonal closed packing (HCP) anionic arrangements 2 . These polymorphs depend on the arrangement of cations within each oxygen sub-group. The existence of several crystalline polymorphs and vacant octahedral sites enables alumina to be adopted for a variety of doping studies and technological applications 3-5 . Lanthanide (Ln 3+ ) ion-doped Al 2 O 3 is particularly appealing, since the local environment of the doped ions is known to critically influence the optical and mechanical properties of Al 2 O 3 due to the large size mismatch between Ln dopants and aluminum 6-8 . For example, the presence of dopants (ytterbium, gadolinium or lanthanum) strengthens the grain boundaries of alumina, largely affecting the mechanical properties 9-11 . A study conducted on doping alumina with lanthanum (La) showed a reduction in the phase transformation (γ → α ) temperature by 100 °C, thus increasing the thermal stability of γ -Al 2 O 3 12 . Although Ln doping has shown favorable effects on alumina, understanding the dopant location within the alumina matrix remains a challenge. Due to the large size of the Ln-ions compared to Al 3+ (0.054 nm for Al 3+ and 0.103 nm → 0.0861 nm for Ln 3+ series), which results in an ionic size mismatch between the dopants (Ln 3+ ) and the Al 3+ cations, the solubility of lanthanide cations in alumina is a significant challenge 13,14 . Additionally, there is a limited understanding of the local structures and distributions of Ln-ions within the alumina matrix [15] [16] [17] [18] [19] . Previous reports on Ln-doping of alumina have generally been conducted at doping concentrations of 0.5-5 wt% 20-25 . The formation of a new phase, LnAlO 3 or Ln 2 O 3 , is observed in previously reported cases, which complicates understanding of the role Ln-doping plays on structure and phase transition of the alumina lattice 26,27 . This also makes it intriguing to assess the role of other Ln-elements as "structural promoters" due to their phase transformation delays and resulting effects on the structure of doped alumina. lattice. The formation of 100% α -phase was also observed in the case of Dy-and Lu-doped alumina. This study revealed a new perspective on the significance of Ln-doping in alumina. Experimental Details Synthesis of Ln-doped alumina powders was achieved by starting with an in-situ nano-precipitation method to synthesize ammonium aluminum hydroxide carbonate (NH 4 Al 1−x RE x (OH) 2 CO 3 ) in an aqueous environment. The resulting powder was calcined at 1300 °C for 30 minutes in air. The detailed synthesis method is described in the Supporting Information section. The structural and chemical composition characterization was carried out by using an FEI Quanta 450FEG Scanning Electron Microscope. High resolution synchrotron powder diffraction data were collected using beamline 11-BM at the Advanced Photon Source (APS), Argonne National Laboratory. Discrete detectors covering an angular range from − 6 to 16° 2θ were scanned over a 34° 2θ range, with data points collected every 0.001° 2θ at a scan speed of 0.01°/s. All NMR data was recorded on a Bruker AVIII 400 MHz NMR Spectrometer with a 4 mm, two-channel MAS probe. Approximately 75 mg of material was packed into a 4 mm Zirconia rotor with Kel-F drive cap (Wilmad Lab Glass) and spun at the magic angle using a rate denoted in the text. The magic angle was confirmed using a KBr sample. 27 Al NMR pulses, offset, and chemical shift references were calibrated and check periodically during the data acquisition run using an alumina (powder) sample. For 1D 27 Al MAS spectra, the Bruker pulse program, excitation pulse, interscan delay, acquisition time, number of scans, sweep width, and experiment duration were "onepulse", 0.6 μ s at 100 W, 0.5 s, 16 ms, 20480, 600 ppm and 3 h, respectively. The data were processed by zero filling FID to 4096 points, Fourier Transform, phase correction, and referencing. For 2D 27 Al MQ-MAS spectra, the precise acquisition parameters were optimized for each individual sample. A representative example of the Bruker pulse program, excitation, conversion and selective pulses, interscan delay, acquisition time in F2 and F1, sweep width in F2 and F1, and experiment time were 7 μ s at 73 W, 2.5 μ s at 73 W, 30 μ s at 350 mW, 0.5 s, 24 ms, 10 ms, 400 ppm, the MAS spin rate (10 or 12 kHz, see figure caption) and three days, respectively. The data were processed by zero filling the matrix to 4096 × 512 complex points, 100 Hz line broadening in F2, Fourier Transform in both F2 and F1, phase correction, and shearing transform.
doi:10.1038/srep39946 pmid:28059121 pmcid:PMC5216414 fatcat:bry437qaibgonjz4kba7v23kny