Co:MgAl₂O₄ is an efficient saturable absorber for passive Q-switching solid-state laser in eye safety laser. Co:MgAl₂O₄ single crystals produced by the Czochralski^[1,2] or Verneuil^[3] method were extensively reported^[4,5,6]. However, polycrystalline Co:MgAl₂O₄ ceramics have attached much attention for its outstanding advantages over single crystals in the lower cost, higher doping level and scalable production.

For transparent ceramics, the transparency depends largely on the numbers of optical scattering centers from secondary phase, grain boundaries and pores^[7,8,9]. Generally, secondary phase can be reduced by controlling the composition of starting materials and the whole fabrication process. And the reflection and refraction in the grain boundary can be averted by choosing proper matrix materials without birefringence or other anisotropy. Therefore, cubic crystalline materials with high symmetry are usually used to prepare transparent ceramics. Many works have been devoted to the study on how to eliminate pores to the greatest extent. For the highly transparent Co:MgAl₂O₄ ceramics, the fully dense microstructure without pores is rather important. In general, transparent ceramics can be produced by pressure-assistant sintering methods^[10,11,12]. Compared with pressureless sintering, pressure-assisted sintering can provide more driving force to eliminate pores and promote the densification of the ceramics^[13], such as hot pressing (HP) and hot isostatic pressing (HIP). It is known that by using three dimensionally gas-assisted pressure, HIP shows much higher efficiency in fabricating dense ceramics with homogeneous microstructure in comparison to HP^[11]. Most importantly, HIP post-treatment is an effective method to eliminate the close pores in ceramics by transmitting the external pressure in argon. And most of the open pores in green bodies can be removed by vacuum pre-sintering, thus the two-step sintering method of vacuum pre-sintering and HIP post-treatment is most popular and utilized. Additionally, ceramics pre-sintered with certain densities of 92%-95% have a better influence after HIP post-treatment^[12]. Therefore, the pre-sintering mechanism of the Co:MgAl₂O₄ transparent ceramics should be systematically investigated to obtain the proper density for the HIP post-treatment.

At present, many studies on the preparation of highly transparent Co:MgAl₂O₄ ceramics have been reported. Firstly, using AlF₃ and MgF₂ as sintering aids, Ikesue^[13] fabricated highly transparent Co:MgAl₂O₄ ceramics through vacuum sintering and HIP post-treatment. However, the addition of a large number of additives might has a bad influence on the optical qualities due to the scatting centers formed by the secondary phases^[14]. Goldstein, et al^[15] and Luo, et al^[16] obtained transparent Co:MgAl₂O₄ ceramics by a combination of vacuum sintering and HIP post-treatment without any sintering aids, while the optical quality of the ceramic was not good. In our previous work ^[17], transparent Co:MgAl₂O₄ ceramics with high optical quality were fabricated by lab-made Co:MgAl₂O₄ nanopowders. However, the influences of pre-sintering temperature on the quality of Co:MgAl₂O₄ ceramics were not investigated systematically. Thus, further research on Co:MgAl₂O₄ ceramics are urgently needed.

In this study, the pure Co:MgAl₂O₄ nanopowders were synthesized by the co-precipitation method, and then transparent Co:MgAl₂O₄ ceramics were prepared by vacuum pre-sintering combined with sequent HIP post-treatment. This work provided a systematic investigation of the pre-sintering temperature on the densification, microstructure evaluation, and optical transparency of Co:MgAl₂O₄ ceramics. Meanwhile, absorption cross- section of the ceramic sample with best optical property was also discussed.

The starting materials used were Mg(NO₃)₂·6H₂O, Al(NO₃)₃·9H₂O and Co(NO₃)₂·6H₂O, and the ammonium carbonate was chosen as the precipitating agent. The detail procedure is similar with the previous work^{[17,18,19,20]}. Firstly, the multi-cation solutions were obtained by dissolving the above nitrates with deionized water and the cationic stoichiometric ratio of Co:Mg:Al was set at 0.005:0.995:2. For the precipitation solution, the concentrati on of ammonium carbonate was 1.5 mol/L. Then, the precursor was obtained at room temperature by adding the mixed solution drop-wise into the precipitation solution at a speed of 4 mL/min under mild agitation. After agitating for half hour and aging for 1 h, the precursor precipitate was washed with deionized water and ethanol both for two times, following by drying at 70 ℃ for 48 h. Then the precursor was sieved with 200-mesh (74 μm) screen and calcined at 1100 ℃ for 4 h in air, and 0.05at% Co:MgAl₂O₄ nanopowders were obtained. Finally, the calcined powders were uniaxially dry-pressed into the pellet at 20 MPa followed by cold isostatically pressed at 250 MPa. After that, the green bodies were pre-sintered at 1500-1700 ℃ for 5 h in the tungsten mesh-heated vacuum furnace and HIP post- treatment at 1800 ℃ for 3 h in 200 MPa of argon. At last, the obtained Co:MgAl₂O₄ ceramics were mirror- polished to 1.2 mm in thickness.

The field emission scanning electron microscope (FESEM, SU8220, Hitachi, Japan) was used to test the morphologies and microstructure of the powders and ceramics thermally etched at 1300 ℃ for 1 h. Grain sizes of the sintered samples were measured by the linear intercept method and the average grain size was calculated by multiplying the average linear intercept distance by 1.56^[21]. The in-line transmittance and absorption coefficient of ceramics were measured with a UV-Vis-NIR spectrophotometer (Model Cary-5000, Varian, USA).

The information of the as-synthesized precursor, including the XRD and FESEM results was presented in our previous report^[17]. The FESEM micrograph of the Co:MgAl₂O₄ powders is displayed in Fig. 1. It can be seen that the calcined powders consist of near club-shaped particles and flaky particles with some holes, and the pores resulted from the decomposition of precursor.

To observe the microstructures of obtained Co:MgAl₂O₄ ceramics, the samples were thermally etched at 1300 ℃ for 1 h. Fig. 2 shows the FESEM microstructures of Co:MgAl₂O₄ ceramics pre-sintered in vacuum at 1500- 1700 ℃ for 5 h. The ceramics show homogeneous structures, and no secondary phase existed. It can be noted that there are quite a few pores in the ceramics pre-sintered at 1500 and 1550 ℃ while the number of pores decreases obviously with the further increase of pre-sintering temperature, indicating the increase of the relative density. Notably, more pores can be found in the ceramic sample pre-sintered at 1700 ℃, and the phenomenon is consistent with the results reported in previous reports, which shows that many intragranular pores appear in the ceramics with high pre-sintering temperature^[22]. These pores resulting from the high grain growth rate at high sintering temperatures^[23] and are difficult to be eliminated by the following HIP post-treatment^[24].

The relative density and average grain size curves of the Co:MgAl₂O₄ ceramics pre-sintered at 1500-1700 ℃ as a function of sintering temperature are shown in Fig. 3. The average grain size increases with the increase of pre-sintering temperature while the relative density firstly increases and reaches the maximum at 1650 ℃. The decrease of the density is mainly caused by pores wrapped in grains or at grain boundaries. Moreover, the relative density of 98.5% at 1650 ℃ is still not enough to make the ceramics transparent. With the sintering temperature increasing, the average grain size of Co:MgAl₂O₄ ceramics calculated by the linear intercept method increase as expected, as shown in Fig. 3. A rapid grain growth occurs in the sintering temperature range of 1500-1700 ℃, the average grain size increases from 2.3 to 11.3 μm.

The pre-sintered Co:MgAl₂O₄ ceramics were HIP post-treated at 1800 ℃ for 3 h to obtain fully dense ceramics. The microstructures of the thermally etched surfaces of the HIP post-treated samples are shown in Fig. 4. After the HIP post-treatment, no obvious pores can be found in ceramics pre-sintered at 1550-1700 ℃. However, for the sample pre-sintered at 1500 ℃ with HIP post-treatment, there are still some pores remained.

Fig. 5 displays the average grain size of 0.05at% Co: MgAl₂O₄ ceramics pre-sintered at different temperatures for 5 h with HIP post-treatment. The grain grows greatly after HIP post-treatment and the average grain size of ceramics pre-sintered at 1500-1700 ℃ increases with the increasing pre-sintering temperature, which is 6.2, 12.3, 13.5, 16.8 and 20.3 μm, respectively.

Fig. 6(a) shows the photo of mirror polished Co:MgAl₂O₄ ceramics pre-sintered at 1500-700 ℃ for 5 h and HIP post-treated at 1800 ℃ for 3 h with 200 MPa Ar atmosphere. It can be seen that all pre-sintered samples are opaque due to the low relative densities. After HIP post-treatment, the ceramic samples pre- sintered at the temperature above 1500 ℃ become transparent and the letters under the ceramics can be seen clearly, whereas the ceramic pre-sintered at 1500 ℃ with HIP post-treatment is still opaque. In addition, the blue color of the ceramics results from the absorption band of Co²⁺ at 500-700 nm. The in-line transmission 1500-1700 ℃ for 5 h and HIP post-treated at 1800 ℃ for 3 h is presented in Fig. 6(b). All transmission spectra of the ceramics pre-sintered at different temperatures show the similar features. The main absorption bands are located at 500-700 nm and 1200-1600 nm, which are the typical absorption bands of Co²⁺ ions located in tetrahedral Td sites^[25,26]. Consistent with Fig. 6, the in-line transmittance of the sample pre-sintered at 1500 ℃ with HIP post-treatment is relatively low. In addition, the transmittance increases with the increase temperature of 1500-1650 ℃. The ceramic sample pre-sintered at 1650 ℃ shows the optimal optical transparency, with the transmittance of 81.4% at 400 nm and 85.9% at 900 nm, which is close to the theoretical transmittance. However, the transmittance of ceramics pre-sintered at 1700 ℃ decreases obviously. The theoretical transmittance can be calculated by the followed formula:

where n is the refractive index of MgAl₂O₄ and it can be calculated by the Sellmeier equation^[27]:

where λ is the wavelength of light, and A and B are equal to 0.0066 and 0.3659 μm ², respectively. Thus the refractive index at 400 and 700 nm are 1.737 and 1.705, respectively, and the theoretical transmittance are calculated to be 86.5% and 87.3% at 400 and 900 nm. Moreover, the decrease of in-line transmittance within 200-400 nm wavelength can be found, meaning that there are still some nano-scale pores (less than 1 μm) ^[28] at the grain boundaries or entrapped in the grains as optical scattering centers in samples.

The absorption coefficient spectrum in 1000-1800 nm of 0.05at% Co:MgAl₂O₄ ceramic pre-sintered at 1650 ℃ and HIP post-treated at 1800 ℃ is shown in Fig. 7, which is the band intended to be used for the passive Q-switching in eye safe lasers. The broad absorption band in 1200-1600 nm is attributed to the transition from the⁴A₂(⁴F_9/2) ground-state to the ⁴T₁(⁴F) excited multiplet. The absorption cross-section (σ_gas) is calculated by the following equations (3) and (4).

where σ_gas is the ground state absorption cross section, α_a is the absorption coefficient, N is the concentration of Co²⁺ ions, N_A is Avogadro’s number andM denotes relative molecular mass while C_S is molar concentration of Co²⁺ in the ceramic. For the ceramics pre-sintered at 1650 ℃ and HIP post-treated at 1800 ℃, the value of αat 1540 nm is 4.68 cm^-1. Thus the σ_gas is calculated to be 6.18×10^-19 cm², which is consistent with that of Co:MgAl₂O₄ transparent ceramics^[18] and single crystal^[2,3] reported in literatures, indicating its promising application for saturable absorber in 1.5 μm.

In this work, highly transparent 0.05at% Co:MgAl₂O₄ ceramics were fabricated from the co-precipitated Co:MgAl₂O₄ nanopowders by a combination of vacuum sintering at 1500-1700 ℃ for 5 h with hot isostatic pressing at 1800 ℃ for 3 h. The pure Co:MgAl₂O₄ nanopowders were obtained by calcining the precursor at 1100 ℃ for 4 h. The ceramics pre-sintered at 1500- 1700 ℃ are opaque because many intergranular pores were remained inside ceramics. The average grain sizes increase from 2.3 to 11.3 μm at the presintering temperature of 1500-1700 ℃. Pores located at grain boundary could be removed efficiently by hot isostatic pressing, which causes the relative density increasing. The in-line transmittance of the ceramic pre-sintered at 1650 ℃ for 5 h and HIP post-treated at 1800 ℃ for 3 h is close to the theoretical value, which are 81.4% at 400 nm and 85.9% at 900 nm. Besides, the ground absorption area is calculated to be 6.18×10 ^-19 cm² at 1540 nm, and the high optical quality and σ_gas make it an attractive material in passive Q-switching at 1.5 μm.