Transparent polycrystalline alumina using spark plasma sintering: Effect of Mg, Y and La doping

Abstract

Transparent polycrystalline alumina (PCA) is a promising replacement for sapphire. Its optical properties however are highly dependent on the grain size and residual porosity which need to be controlled for real inline transmittances (RIT), that are high enough for possible applications.To achieve high RITs, doping as well as pressure assisted sintering is often used. In this study spark plasma sintering (SPS) and doping are investigated. A systematic experimental design is used to study the influence of Mg, Y and La single or co-doping (75-450 ppm) as well as the SPS sintering pressure and temperature on the RIT and grain size of PCA.Using optimized sintering parameters, RITs of >50% were attained in the visible wavelength (640 nm) for 0.8 mm thick samples for almost all doping strategies. The best RIT of 57% was for triple-doped samples at a total dopant level of 450 ppm. These results are significantly better than previously published SPS studies and illustrate that SPS sintered alumina can attain high and reproducible optical transmittances under various doping and sintering conditions.

Atomistic modeling of dopant segregation in α-alumina ceramics: Coverage dependent energy of segregation and nominal dopant solubility

“…Experimental studies however indicate that below the solubility limit and at the same dopant concentration and sintering conditions, La and Y doped samples have approximately the same grain size. 43,46,47 The effect on creep resistance is also reported to be either very similar 45,46 or even less for La than for Y. 47 From the present results two possible indications as to why La is not more effective than Y can be gained.…”

“…Both dopants are known to decrease grain growth during sintering 42,43 and increase the high-temperature creep resistance [44][45][46][47][48][49] of ␣-alumina. Grain boundary segregation rather than second phase precipitation is the cause of these effects, as they occur below the solubility limit.…”

“…Grain boundary segregation rather than second phase precipitation is the cause of these effects, as they occur below the solubility limit. 43,46,47,50 Initial suggestions that La dopants promote the formation of low-energy, low-Σ boundaries could not be substantiated 38 and the prevalent opinion in recent literature is that both dopants reduce grain boundary diffusion 21,38,47,48 and grain boundary dislocation climb/slide 33,49 due to site blocking. Due to the higher segregation energy calculated here and elsewhere 35,36 the interface concentration of La at the same overall dopant concentration and grain size is expected to be higher than for Y.…”

Effects of the nature of the doping salt and of the thermal pre-treatment and sintering temperature on Spark Plasma Sintering of transparent alumina

“…For both mechanical and economical reasons, numerous studies have been performed over recent years to replace sapphire by transparent polycrystalline alumina (PCA) in various optical applications such as discharge lamp envelopes and optical windows or armour [1][2][3][4][5]. The light transmission properties of fine-grained PCA can be described by the Apetz and van Bruggen model [6], based on the Rayleigh-GansDebye approximation.…”

“…SPS is a processing technique that has been widely used over the past decades [1,5,[17][18][19][20]. It was shown that dense, finegrained materials can be developed at low temperatures and short sintering times compared to conventional techniques such as hot isostatic pressing (HIP).…”