ZnAl 2 O 4 :Cr 3+ crystals were synthesized by using sol–gel method at 1000 °C. The results showed that with the concentration of Cr 3+ in spinel lattices increasing, the x-ray diffraction peaks shifted toward lower diffraction angles, and the grain size revealed a rapid increase and became invariable when the Cr 3+ dopant concentration was above 0.7 mol%. All the samples showed a series of emission peaks ranging from 660 to 730 nm assigned to spin-forbidden 2 E g → 4 A 2g transition of Cr 3+ ions in spinel lattices. The intensity of emission peaks revealed a strong dependence on the Cr 3+ concentration. An additional emission peak at 740 nm was present when the Cr 3+ ions concentration was above 0.5%. All emission peaks were quenched when the Cr 3+ concentration reaches 1.3 mol%. The results of x-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) suggested that the Cr 3+ ions occupied the octahedral sites in spinel lattices, and with the Cr 3+ concentration increasing, the exchange interaction and coupling interaction between Cr–Cr were enhanced and the antisite defects related to Zn in octahedral sites increased. Graphical abstract
The Bi 2 O 3 –B 2 O 3 –SiO 2 –ZnO–Al 2 O 3 (BBSZA) glass/Al 2 O 3 composites are prepared by quenching and quenching-free methods and conducted a comparative analysis of their microstructures and properties. The results reveal that the BBSZA/Al 2 O 3 composite obtained by quenching free method has smaller pores and superior performance. Specifically, at a BBSZA content of 50 wt%, the composite exhibits a thermal conductivity of 5.352 W/m·K and a flexural strength of 193.54 MPa, which are 1.38 times and 2.08 times those obtained using the quenching method, respectively. XRD and TEM analyses , along with parallel experiments using other glasses, suggest that presintering crystallization may be the prerequisite for achieving denser composites via the quenching-free method. BBSZA glass reacts with Al 2 O 3 at the presintering process to generate a great amount of ZnAl 2 O 4 crystals, which enhances grain boundary energy and increases sintering density. This work provides insights into the low-cost and high-performance development of low temperature co-fired ceramics (LTCC).
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