Abstract
Nanostructured segregates of alkaline earth oxides exhibit bright photoluminescence emission and great potential as components of earth-abundant inorganic phosphors. We evaluated segregation engineering of Ca2+- and Ba2+-admixtures in sintered MgO nanocube-derived compacts. Compaction and sintering transform the nanoparticle agglomerates into ceramics with residual porosities of Φ = 24–28%. Size mismatch drives admixture segregation into the intergranular region, where they form thin metal oxide films and inclusions decorating grain boundaries and pores. An important trend in the median grain size evolution of the sintered bodies with dCa(10 at. %) = 90 nm < dBa(1 at. %) = 160 nm < dMgO = 250 nm ∼ dCa(1 at. %) = 280 nm < dBa(10 at. %) = 870 nm is rationalized by segregation and interface energies, barriers for ion diffusion, admixture concentration, and the increasing surface basicity of the grains during processing. We outline the potential of admixtures on interface engineering in MgO nanocrystal-derived ceramics and demonstrate that in the sintered compacts, the photoluminescence emission originating from the grain surfaces is retained. Interior parts of the ceramic, which are accessible to molecules from the gas phase, contribute with oxygen partial pressure-dependent intensities to light emission.
Highlights
Internal interfaces and boundary regions between nanocrystalline grains have a key influence on the physico-chemical material properties
On alkaline earth metal oxides (AEOs) doped with different cationic admixtures, the study of interface modification utilizing admixture segregation has previously been performed on micro- and single crystalline materials for multiple reasons: first, the fundamental investigation of bulk and interface phenomena benefit from its simple structure and the pronounced ionicity of AEOs.[11−17] Second, research in heterogeneous catalysis has revealed a number of key reactions where perfectly dissolved impurity ions and surface segregates act as active surface sites of the catalyst particles.[15,18−22] Magnetism, which arises from the incorporation and interface segregation of transition-metal ions, has great potential for applications in spintronics and represents another example for functional properties that emerge from engineering the solid− solid interface.[23−25]
With grain sizes below 12 nm (Figure S1, Supporting Information) the particles do exhibit a uniform distribution of the respective admixtures over the entire ensemble.[40−42] The advantage of small average grain size, narrow particle size distribution, and uniform distribution of admixtures is - compared to micro- and single-crystalline materials - related to the significantly smaller diffusion paths inside the nanocrystals the admixed ions have to overcome to reach the nanocrystal surface
Summary
Internal interfaces and boundary regions between nanocrystalline grains have a key influence on the physico-chemical material properties Their design and formation are critical for the optimization of the structural and functional performance of sintered structures.[1−4] Grain boundary (GB) engineering in combination with ion exsolution and strain effects has successfully been employed to achieve improved electrode materials.[5] Another example for related development of functional materials is the adjustment of nanostructured SrTiO3 thermoelectrics via the addition of extrinsic elements into the GB phase together with control over oxygen partial pressure and temperature.[6] The enhanced solubility and diffusion of transition-metal cations in and through GBs inside acceptor-doped CeO2, as the third example, were found to have a significant impact on the performance and durability of ceria-containing solid-state electrolytes.[7−10]. We have addressed for the first time and in the context of ceramics production two key questions: (i) how do these admixture segregates affect coarsening of nanometer-sized grains during microstructure evolution and sintering upon annealing of nanoparticle-derived compacts and (ii) can the optical properties that are specific to the free grain surfaces inside powders be preserved during ceramic manufacturing. This work offers knowledge for precise control over admixture distribution between grains and their interfaces and, on grain growth during sintering
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