Transparent ultrafine Yb3+:Y2O3 laser ceramics fabricated by spark plasma sintering

Conventional ceramic processing techniques do not produce ultrafine‐grained materials. However, since the mechanical and optical properties are highly dependent on the grain size, advanced processing techniques are needed to obtain ceramics with a grain size smaller than the wavelength of visible light for new laser sources. As an empirical study for lasing from an ultrafine‐grained ceramics, transparent Yb³⁺:Y₂O₃ ceramics with several doping concentrations were fabricated by spark plasma sintering (SPS) and their microstructures were analyzed, along with optical and spectroscopic properties. Laser oscillation was verified for 10 at.% Yb³⁺:Y₂O₃ ceramics. The laser ceramics in our study were sintered without sintering additives, and the SPS produced an ultrafine microstructure with an average grain size of 261 nm, which is about one order of magnitude smaller than that of ceramics sintered by conventional techniques. A load was applied during heating to enhance densification, and an in‐line transmittance near the theoretical value was obtained. An analysis of the crystal structure confirmed that the Yb³⁺:Y₂O₃ ceramics were in a solid solution. To the best of our knowledge, this study is the first report verifying the lasing properties of not only ultrafine‐grained but also Yb‐doped ceramics obtained by SPS.     

Effect of Yb doping concentration on crystallization kinetics in yttrium aluminum garnet nano-powders

Different ytterbium concentration–doped yttrium aluminum garnet (Yb:YAG) nanopowders were synthesized using coprecipitation with nitrate salts as the starting materials. The phase evolutions, morphologies, and microstructures of the powders synthesized from various ytterbium-doped precursors were investigated. Ytterbium doping concentration was discovered to have a crucial effect during the YAG phase formation from the precursor. Crystallized Yb:YAG powders were directly obtained at temperatures as low as 900 °C without the formation of any intermediate phase. The crystallization kinetics of the Yb:YAG precursors were analyzed using non-isothermal differential scanning calorimetry. Avarami parameters of 0.97, 1.00, 1.13, and 1.21 were obtained for Yb doping concentrations of 0, 3, 6, and 9 at% respectively, and crystallization activation energies of 1506±40, 1342±36, 1171±31, and 978.1±26 kJ/mol were calculated. The activation energy for YAG crystallization was lower when a high Yb doping concentration was used because the presence of Yb³⁺ prohibited the presence of anionic SO₄²⁻ in the precursors, thus enhancing the elemental diffusion between particles. Both the average grain size and particle size of Yb: YAG decreased when Yb doping concentration was increased, and at various calcination temperatures.     

Highly transparent Yb:Y2O3 ceramics obtained by solid-state reaction and combined sintering procedures

(Y_0.87-xLa_0.1Zr_0.03Yb_x)₂O₃ (x?=?0.02, 0.04, 0.05) transparent ceramics were obtained by solid-state reaction and combined sintering procedures with La₂O₃ and ZrO₂ as sintering additives. A method based on two-step intermediate sintering in air followed by vacuum sintering was applied in order to control the densification and grain growth of the samples during the final sintering process. The results indicate that La₂O₃ and ZrO₂ co-additives can improve the microstructure and optical properties of Yb:Y₂O₃ ceramics at relatively low sintering temperature. On the other hand, the addition of Zr⁴⁺ ions leads to the formation of dispersed scattering volumes in the ceramic bodies. Transmittance of 78.8% was measured for the 2.0?at% Yb:Y₂O₃ ceramic sample at the wavelength of 1100?nm. The spectroscopic properties of Yb:Y₂O₃ ceramics were investigated at room temperature. The obtained results show that the absorption cross-section at 978?nm is in the range of 2.08?×?10^–20 to 2.36?×?10^–20 cm², whereas the emission cross-section at 1032?nm is ~1.0?×?10^–20 cm².     

Preparation and characterizations of Yb:YAG-derived silica fibers drawn by on-line feeding molten core approach

Three Yb:YAG transparent ceramics with Yb₂O₃ doping concentrations of 1, 10, and 15 at%, respectively were made into silica-clad hybrid fibers using an on-line feeding molten core approach. The diffusion of silica was mitigated such that the lowest SiO₂ concentration was 36.4 wt%, and consequently, the Yb₂O₃ content could reach 8.93 wt% in the fiber core. The fiber core transformed from a YAG ceramic to an yttrium aluminosilicate glass, and the formation of abundant Q² silicate species implied that the structure of the core glass maintained some environments similar to that of YAG with Q²–AlO₄ tetrahedra. The absorption and emission spectra of the obtained fibers were compared to those of Yb:YAG ceramics, and the self-absorption effect was analyzed in detail. All three fibers could output lasers under 940 or 970 nm pumping. The maximum output power of the Yb:YAG-derived fibers was higher than that of ceramic wafers owing to the cladding pump technology, which offered a new method to improve the application of ceramics.     

Sintering of Yb3+:Y2O3 transparent ceramics in hydrogen atmosphere

Highly transparent Yb³⁺:Y₂O₃ ceramics with doping concentration up to 40.0 at.% had been fabricated successfully via hydrogen atmosphere sintering, where the raw powders were synthesized by co-precipitation method. The sintering temperature is about 600 °C lower than its melting temperature. SEM investigation revealed the average grain size of Yb³⁺:Y₂O₃ ceramics sintered at 1850 °C for 9 h was about 7 μm. The highest transmittance of as-prepared 1 mm thickness samples around wavelength of 1050 nm reached 80%, which is close to the theoretical value of Y₂O₃. The optical spectroscopic properties of Yb³⁺:Y₂O₃ transparent ceramics have also been investigated, which shows that it is a very good laser material for diode laser pumping and short pulse mode-locked laser.     

Fabrication,microstructure, and optical properties of Yb:Y3ScAl4O12 transparent ceramics with different doping levels

Transparent Yb:Y₃ScAl₄O₁₂ (Yb:YSAG) ceramics with different ytterbium doping concentrations such as 5, 10, 15, 20 at.% have been successfully fabricated by solid-state reactive sintering. All the obtained ceramics are in dense and homogeneous structure after sintering at 1820°C for 30 hours and with a posttreatment by hot isostatic pressing at 1750°C for 3 hours with 200 MPa pressure. We systematically analyzed the influence of Yb³⁺ doping concentration on the microstructure and optical properties of the ceramics. The 10 at.% Yb:Y₃ScAl₄O₁₂ ceramics with a thickness of 3.2 mm show the best transparency as high as 80.9% at 1100 nm. The laser emission of the 10 at.% Yb:YSAG ceramics was tested, resulting in a maximum slope efficiency of 67.6% and a maximum output power of 11.3 W under quasi-continuous wave pump conditions. The tuning range spanned from 990 to 1071 nm.     

Fabrication and microstructure development of Yb:YAG transparent ceramics from co-precipitated powders without additives

Sintering additives are generally considered to be important for improving densification in fabrication of transparent ceramics. However, the sintering aids as impurities doped in the laser materials would decrease the laser output power and produce additional heat during laser operation. In this work, Yb:YAG ceramics were vacuum-sintered without additives at different temperatures for various soaking time through using ball-milled powders synthesized by co-precipitation route. The densification behavior and grain growth kinetics of Yb:YAG ceramics were systematically investigated through densification curves and microstructural characterizations. It was determined that the densification in the 1500°C-1600°C temperature range was controlled by a grain-boundary diffusion. It is revealed that the volume diffusion is the main mechanism controlling the grain growth between 1600°C and 1750°C. Although SiO₂ additives can promote densification during low-temperature sintering, the optical transmittance of Yb:YAG ceramic with no additives, sintered at 1800°C for 15 hours, reaches a maximum of 83.4% at 1064 nm, very close to the measured transmittance value of Yb:YAG single crystal. The optical attenuation loss was measured at 1064 nm in Yb:YAG transparent ceramic, to be 0.0035 cm⁻¹, a value close to that observed for single crystals.     

Submicron-grained Yb:Lu2O3 transparent ceramics with lasing quality

We report on our recent progress of fabricating Yb³⁺-doped Lu₂O₃ transparent ceramics for 1 μm solid-state laser application. Well-dispersed 3.3 at.% Yb:Lu₂O₃ nanopowders were synthesized using a co-precipitation method. Without using any sintering aids, the Yb:Lu₂O₃ nanopowders could be densified by vacuum sintering at 1500°C/10 hours followed by HIPing at 1480°C/4 hours. Such obtained Yb:Lu₂O₃ ceramics had not only dense microstructure and submicron grain size of about 0.6 μm, but also in-line transmission of 80.0% at 600 nm. Preliminary continuous wave (CW) laser experiments with an uncoated Yb:Lu₂O₃ ceramic slab have demonstrated highly efficient CW laser oscillation at 1079.8 nm.     

Comparative study of Yb:Lu3Al5O12 and Yb:Lu2O3 laser ceramics produced from laser-ablated nanopowders

We present a comparative study of two Lu-based oxide ceramics doped with Yb³⁺ ions, namely Yb:Lu₃Al₅O₁₂ (garnet) and Yb:Lu₂O₃ (sesquioxide), promising for thin-disk lasers. The ceramics are fabricated using nanopowders of 3.6 at.% Yb:Lu₂O₃ and Al₂O₃ produced by laser ablation: Yb:Lu₃Al₅O₁₂ – by vacuum sintering at 1800 °C for 5 h with the addition of 1 wt% TEOS as a sintering aid, and Yb:Lu₂O₃ – by vacuum pre-sintering at 1250 °C for 2 h followed by Hot Isostatic Pressing at 1400 °C for 2 h under Ar gas pressure of 207 MPa. The comparison includes the structure, Raman spectra, transmission, optical spectroscopy and laser operation. The crystal-field splitting of Yb³⁺ multiplets is revealed for Lu₃Al₅O₁₂. A continuous-wave (CW) Yb:Lu₃Al₅O₁₂ ceramic microchip laser generates 5.65 W at 1031.1 nm with a slope efficiency of 67.2%. In the quasi-CW regime, the peak power is scaled up to 8.83 W. The power scaling for the Yb:Lu₂O₃ ceramic laser is limited by losses originating from residual coloration and inferior thermal behavior.     

Influences of Solid Loadings on the Microstructures and the Optical Properties of Yb:YAG Ceramics

Transparent Y_2.85Yb_0.15Al₅O₁₂ ceramics were prepared using an aqueous tape casting and vacuum sintering method. The rheological properties were measured by a rheometer. The results indicate high quality tapes, and ceramics can be obtained by increasing the solid loading of the corresponding slurries. The densities of the tapes increase from 2.42 to 2.69 g/cm³ by increasing the solid loadings from 35 to 50 vol%. The corresponding green body densities range from 52.7 to 57.1% of the theoretical. The solid loading suitable for fabricating transparent Yb:YAG ceramics should be higher than 45 vol%.     

Microstructural evolution and enhanced mechanical properties of 95Al2O3–Yb/Mg/Si ceramics by optimizing sintering temperature

Designed component of 0.95Al₂O₃–0.015Yb₂O₃–0.01MgO–0.025SiO₂ (95Al₂O₃–Yb/Mg/Si) ceramics were prepared by the traditional mixed-oxide method in the sintering temperature range of 1450–1700 °C. The influence of sintering temperature on the crystal structure, densification, microstructure, mechanical, friction and wear properties of 95Al₂O₃-YbMgSi ceramics were systematically investigated. XRD and SEM analysis results revealed that the increase in sintering temperature was very beneficial to eliminate the pores, increase the density and grain size, which obeys the common grain growth law. Both the flexural strength and hardness of obtained samples were increased almost linearly when the sintering temperature increased from 1450 °C to 1700 °C. The ceramics sintered at 1650 °C showed the optimum properties: H_v = 1812.3, σ = 151.3 MPa, μ = 0.41, ρ = 3.72 g/cm³ and K_c = 8.06e^?5 mm³/N·m, respectively. Furthermore, the results of friction and wear experiments suggested that the 95Al₂O₃–Yb/Mg/Si ceramic prepared at the optimizing sintering process exhibited more stable friction state and lower wear degree under non-lubricated conditions. The enhanced mechanical properties could be attributed to their structure densification, pore elimination and gain growth with the increase of sintering temperature.     

Carbonate-precipitation synthesis of Yb:Y2O3 nanopowders and its characteristics

Ytterbium-doped yttria (Yb³⁺:Y₂O₃) nanopowders for transparent ceramics were synthesized by using a carbonate-precipitation method. The characteristics of precursor and powders calcined at different temperatures were investigated. The pure yttria phase can form through calcining at 700 °C. The Yb³⁺:Y₂O₃ nanopowders calcined at 1100 °C were well dispersed with a spherical morphology, and had a narrow particle size distribution with a mean particle size of about 70 nm. By using 1100 °C-calcined powders, nearly full dense Yb³⁺:Y₂O₃ ceramics were fabricated at 1750 °C for 8 h without any additives under vacuum conditions. The fluorescence spectrum of the sintered ceramics illustrates that there are two emission peaks locating at 1028 and 1071 nm respectively, all corresponding to the ²F_5/2 → ²F_7/2 transitions of Yb³⁺ ion. Homogeneous Yb³⁺:Y₂O₃ nanopowders synthesized by carbonate-precipitation method are suitable for the fabrication of IR-transparent ceramics.     

Effect of NaF doping on the transparency,microstructure and spectral properties of Yb3+: CaF2 transparent ceramics

Yb³⁺:CaF₂ transparent ceramics are promising laser gain media with outstanding performance. However, low transmittance in the visible range is the main challenge that restricts the application of Yb³⁺:CaF₂ ceramics in the laser system. In this paper, a new scheme to eliminate the residual pores in the Yb³⁺:CaF₂ transparent ceramics based on doping of NaF as a sintering aid is proposed. Microstructural characterization indicated that NaF could inhibit the grain growth and increase the transmittance in the visible range significantly. The corresponding transmittance was measured to be 85% at the wavelength of 400 nm. The spectra results showed that co-doped with Na⁺ ions could break the clusters of Yb³⁺ ions and modulate the spectroscopy properties of Yb³⁺: CaF₂ lattice efficiently. This paper proved that doping with NaF is an efficient strategy to improve the transmittance and fluorescence quantum efficiency of Yb³⁺:CaF₂ transparent ceramics.     

Pump laser induced photodarkening in ZrO2-doped Yb:Y2O3 laser ceramics

5 at.% Yb:Y₂O₃ transparent ceramics were fabricated using vacuum sintering plus HIP. The ceramics doped with 1 at.% ZrO₂ as the sintering additive were densified at 1700 °C in vacuum followed by HIPing at 1775 °C, while those without sintering additives were densified at 1520 °C in vacuum followed by HIPing at 1450 °C. After sintering, both ceramics had relatively high in-line transmittance. However, during laser experiments, the ZrO₂-doped Yb:Y₂O₃ (Zr-YbY) ceramics were photodarkened when irradiated by 940 nm pump light. The discoloration might be attributed to the formation of Zr³⁺ color centers during lasing. In contrast, no photodarkening effect was detected in the pure Yb:Y₂O₃ ceramics without sintering additives (P-YbY). The P-YbY ceramics exhibited much higher lasing efficiency (17%) than the Zr-YbY ceramics (9%). To our best knowledge, it is the first time that the photodarkening effect was detected in rare-earth doped sesquioxide laser ceramics.     

Phase stability and thermal conductivity of RE2O3 (RE=La,Nd, Gd,Yb) and Yb2O3 co-doped Y2O3 stabilized ZrO2 ceramics

Y₂O₃ stabilized ZrO₂ (YSZ) has been considered as the material of choice for thermal barrier coatings (TBCs), but it becomes unstable at high temperatures and its thermal conductivity needs to be further reduced. In this study, 1 mol% RE₂O₃ (RE=La, Nd, Gd, Yb) and 1 mol% Yb₂O₃ co-doped YSZ (1RE1Yb–YSZ) were fabricated to obtain improved phase stability and reduced thermal conductivity. For 1RE1Yb–YSZ ceramics, the phase stability of metastable tetragonal (t′) phase increased with decreasing RE³⁺ size, mainly attributable to the reduced driving force for t′ phase partitioning. The thermal conductivity of 1RE1Yb–YSZ was lower than that of YSZ, with the value decreasing with the increase of the RE³⁺ size mainly due to the increased elastic field in the lattice, but 1La1Yb–YSZ exhibited undesirably high thermal conductivity. By considering the comprehensive properties, 1Gd1Yb–YSZ ceramic could be a good potential material for TBC applications.     

Vacuum sintering of highly transparent La1+xYb1+yZr2O7 ceramics with excess La and Yb contents

In this study, a series of transparent ceramics with chemical composition of La_1+xYb_1+yZr₂O₇ (x, y = 0.1?0.5) were successfully prepared by vacuum sintering using combustion synthesized powders. The effects of excess contents on the phase composition, microstructure and in-line transmittance have been studied. The detailed results indicate that the in-line transmittance increases at first and then decreases as La content be elevated. It was also determined that the highest in-line transmittance of La_1+xYb_1+yZr₂O₇ (x, y = 0.1?0.5) ceramics is 84.1 % at 1100 nm when the excess amount of co-doped La-Yb is 30 %. Compared with stoichiometric LaYbZr₂O₇ ceramic, the nonstoichiometric La_1+xYb_1+yZr₂O₇ (x, y = 0.1?0.5) ceramics exhibit much higher transparency. In addition, the high excess amount of La, Yb and co-doped La-Yb also shows effects on the phase composition and crystal structure.     

Silica reactivity during reaction‐sintering of Nd:YAG transparent ceramics

Silica (SiO₂) is widely used as sintering aid during vacuum sintering of YAG (Y₃Al₅O₁₂)‐based transparent ceramics. These ceramics are mainly used for laser applications when they are doped with rare‐earth luminescent elements such as Yb³⁺ or Nd³⁺. By means of microstructural, chemical, dilatometry, and thermogravimetry analyses, this study has evidenced that sufficiently high amount of silica (ie above the solubility limit in YAG) forms intergranular transient liquid phase of mixed composition Y‐Al‐Si‐O that vaporizes rapidly for temperatures higher than 1350°C. As a result, silica content after sintering remains always lower than the solubility limit in YAG ceramics (ie lower than 900 ppm). Finally, vacuum sintering with an external source of gaseous Si was proven to be suitable to manufacture highly transparent Nd:YAG ceramics.     

Fabrication and characterisation of porous silicon nitride ceramics using Yb2O3 as sintering additive

Porous Si₃N₄ ceramics were fabricated by liquid-phase sintering with a Yb₂O₃ sintering additive, and the microstructure and mechanical properties of the ceramics were investigated, as a function of porosity. Low densification was achieved using a lower Yb₂O₃ additive content. Fibrous β-Si₃N₄ grains developed in the porous microstructure, and the grain morphology and size were affected by different sintering conditions. A high porosity, ∼40–60%, with β-Si₃N₄ grain development, was obtained by adjusting the additive content. Superior mechanical properties, as well as strain tolerance, were obtained for porous ceramics with a microstructure of fine, fibrous grains of a bimodal size distribution.     

Effects of Yb3+ doping on phase structure,thermal conductivity and fracture toughness of (Nd1-xYbx)2Zr2O7

To investigate the effects of Yb³⁺ doping on phase structure, thermal conductivity and fracture toughness of bulk Nd₂Zr₂O₇, a series of (Nd_1-xYb_x)₂Zr₂O₇ (x?=?0, 0.2, 0.4, 0.6, 0.8, 1.0) ceramics were synthesized using a solid-state reaction sintering method at 1600?°C for 10?h. The phase structures were sensitive to the Yb³⁺ content. With increasing doping concentration, a pyrochlore-fluorite transformation of (Nd_1-xYb_x)₂Zr₂O₇ ceramics occurred. Meanwhile, the ordering degree of crystal structure decreased. The substitution mechanism of Yb³⁺ doping was confirmed by analyzing the lattice parameter variation and chemical bond of bulk ceramics. The thermal conductivities of (Nd_1-xYb_x)₂Zr₂O₇ ceramics decreased first and then increased with the increase of Yb³⁺ content. The lowest thermal conductivity of approximately 1.2?W?m^?1 K^?1 at 800?°C was attained at x?=?0.4, around 20% lower than that of pure Nd₂Zr₂O₇. Besides, the fracture toughness reached a maximum value of ~1.59?MPa?m^1/2 at x?=?0.8 but decreased with further increasing Yb³⁺ doping concentration. The mechanism for the change of fracture toughness was discussed to result from the lattice distortion and structure disorder caused by Yb³⁺ doping.     

Optimizing liquid phase promotion effect by modifying powder specific surface area in Si4+/Mg2+ co-doped transparent Yb:YAG ceramics

Utilizing the Si⁴⁺/Mg²⁺ co-doping has been considered an effective approach to fabricate highly transparent ceramics. However, the optimum doping concentration has been reported with considerable uncertainties. In this work, highly transparent Yb:YAG ceramics were obtained via the solid-state method and the sintering behavior is discovered to be closely related to both the doping concentration of Si⁴⁺/Mg²⁺ and the specific surface area (S_BET) of powders. S_BET is effectively modified by setting the ball-milling time, where the maximum S_BET (30.914 m²/g) is achieved with 24 h ball-milling time. With increasing S_BET, less Mg²⁺ is required for better optical properties. When S_BET equals 30.914 m²/g, the highest in line transmittance @ 1100 nm of 84.85% is obtained with Si⁴⁺/Mg²⁺ doping concentrations of 0.50 wt% and 0.05 wt%, respectively. The relation between S_BET and optimum doping concentration is explained by the different magnitudes of liquid phase promotion required for different contact areas between powder particles.