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Along with extensive research on the three-dimensional (3D) printing of polymers and metals, 3D printing of ceramics is now the latest trend to come under the spotlight. The ability to fabricate ceramic components of arbitrarily complex shapes has been extremely challenging without 3D printing. This review focuses on the latest advances in the 3D printing of ceramics and presents the historical origins and evolution of each related technique. The main technical aspects, including feedstock properties, process control, post-treatments and energy source–material interactions, are also discussed. The technical challenges and advice about how to address these are presented. Comparisons are made between the techniques to facilitate the selection of the best ones in practical use. In addition, representative applications of the 3D printing of various types of ceramics are surveyed. Future directions are pointed out on the advancement on materials and forming mechanism for the fabrication of high-performance ceramic components.

“Sandwich” structure YAG/Nd:YAG/YAG ceramics with different core (Nd 3+ doped area) lengths of 1 mm, 5 mm and doping concentrations of 1 at.%, 2 at.% were prepared by dry pressing and vacuum sintering of oxide powder mixture. Smaller average grain size was found in the specimen with the larger core and higher doping concentration. With the increase of core length, the optimum transmission of output coupler increases from 10% to 19% and better laser performance can be obtained. However, longer core length causes the noticeable thermal lens effect, which influences the beam quality significantly. It is also reflected in the thermally induced depolarized beam pattern, which becomes more obvious with the longer core length and higher Nd 3+ doping concentration.

Highly transparent Nd:YAG ceramics were fabricated by non-aqueous gelcasting and vacuum sintering for the first time, using epoxy resin and polyamine curing agent as gelling system. The influences of dispersant, solid loading and the organics of gelling system on the viscosity of suspensions and the stability of suspensions were discussed. The optimized nonaqueous slurry was of 54 vol% solid loading, comprising the commercial α-Al 2 O 3 , Nd 2 O 3 and Y 2 O 3 powders. The slurry with low viscosity and good fluidity was obtained by using 0.45 wt% PEI as dispersant. The homogenous and dense green body was obtained through solidificating the slurry in-situ. The green body after burnout had a relative density of 55.0% with a narrow pore size distribution. The transmittance of the Nd:YAG ceramics sintered at 1775 °C for 30 h with the thickness of 1.5 mm reached 83% at 1064 nm. The average grain size of the sample was about 71 μm.

We reported on the fabrication and properties of 2.5 at.% Yb:LuAG ceramics prepared by a solid-state reactive sintering method using commercial available powders as starting materials. The Yb:LuAG ceramics sintered at 1850 °C for 50 h have a dense and homogeneous structure with the average grain size of about 60 μm. The transmittances of the Yb:LuAG ceramics with the 3 mm thickness are 82.6% at 400 nm and 84.0% at 1100 nm. The maximum luminescence peak is located at about 1029 nm and the fluorescence lifetime is 1.3 ms. It is demonstrated that the Yb:LuAG ceramics have excellent mechanical properties, furthermore the thermal expansivity and thermal conductivity are even better than those of Yb:YAG ceramics. Continuous-wave (CW) laser operation is obtained with the maximum output power of 2.1 W and a slope efficiency of 41.7% for non-coated Yb:LuAG ceramics with the output coupler of T = 10%.

3D printing has attracted a lot of attention in recent years. Over the past three decades, various 3D printing technologies have been developed including photopolymerization-based, materials extrusion-based, sheet lamination-based, binder jetting-based, power bed fusion-based and direct energy deposition-based processes. 3D printing offers unparalleled flexibility and simplicity in the fabrication of highly complex 3D objects. Tactile sensors that emulate human tactile perceptions are used to translate mechanical signals such as force, pressure, strain, shear, torsion, bend, vibration, etc. into electrical signals and play a crucial role toward the realization of wearable electronics and electronic skin. To date, many types of 3D printing technologies have been applied in the manufacturing of various types of tactile sensors including piezoresistive, capacitive and piezoelectric sensors. This review attempts to summarize the current state-of-the-art 3D printing technologies and their applications in tactile sensors for wearable electronics and electronic skin. The applications are categorized into five aspects: 3D-printed molds for microstructuring substrate, electrodes and sensing element; 3D-printed flexible sensor substrate and sensor body for tactile sensors; 3D-printed sensing element; 3D-printed flexible and stretchable electrodes for tactile sensors; and fully 3D-printed tactile sensors. Latest advances in the fabrication of tactile sensors by 3D printing are reviewed and the advantages and limitations of various 3D printing technologies and printable materials are discussed. Finally, future development of 3D-printed tactile sensors is discussed.

Nd:LuAG transparent ceramics were fabricated by a solid-state reaction method using commercial high-purity oxide powders as raw materials. The microstructure, optical transmittance, fluorescence spectrum and laser performance of 0.8 at.% Nd:LuAG transparent ceramics were investigated. It is found that the relative density and grain size for Nd:LuAG ceramics increase with the increase of sintering temperature. The specimen sintered at 1830 °C for 30 h is dense and homogeneous in structure with an average grain size of about 39.6 μm, whose in-line transmittances are 82.8% at 1064 nm and 81.8% at 400 nm. The main fluorescence emission peak is centered at about 1063.5 nm and the fluorescence lifetime is 240 μs. The optimum laser properties are obtained with the 300 μm pumping beam and 25% output coupler, corresponding to a slope efficiency of 43.4% and an optical–optical efficiency of 39.3%. In addition, the depolarization caused by the thermally-induced birefringence was also investigated.

Highly transparent 2 at.% Nd:YAG ceramics were prepared by a solid-state reaction sintering method using commercial high-purity powders of α-Al 2 O 3 , Y 2 O 3 and Nd 2 O 3 . For the Nd:YAG ceramics sintered at 1750 °C for 50 h, a dense and pore-free microstructure with average grain size of about 15 μm was obtained, whose in-line transmittances reached 84.2% at 1064 nm and 81.7% at 400 nm, respectively. As the surface conditions of Nd:YAG ceramic influence the laser behavior significantly, especially the laser-induced damage, the interaction between laser and Nd:YAG ceramics was mainly investigated. Three kinds of Nd:YAG ceramics with different surface roughness were prepared by different polishing processes. It was found that the surface roughness influenced the laser-induced damage threshold significantly. In addition, we also calculated the laser-induced damage threshold and observed the damage morphology of ceramics.

Highly transparent polycrystalline 2.0 at.% Nd:YAG ceramics were fabricated by a solid-state reactive sintering method using commercial α-Al2O3, Y2O3 and Nd2O3 powders as starting materials. The in-line transmittances of the Nd:YAG ceramics vacuum sintered at 1750 °C for 50 h with the thickness of 5.8 mm are 83.9% at 1064 nm and 82.5% at 400 nm. The thermal effects in the Nd:YAG ceramics were mainly investigated in detail. It is found that the thermal focal length decreases with the increase of pump power. The experimental results of thermal focal lengths are in accordance with the theoretical calculations. The observed depolarized beam patterns and depolarization phenomena illustrate the detailed change of thermally induced birefringence in Nd:YAG ceramics. The depolarization shows a obvious nonlinear change tendency at low pump power.

Yb:YAG ceramic slab with Cr 4+ :YAG edge cladding was fabricated by vacuum sintering technology using commercial powders as raw materials and CaO as a charge compensator. After annealing, a portion of Cr 3+ ions transform into Cr 4+ . The lasing wavelength of 1030 nm of Yb 3+ lies within the Cr 4+ board absorption band range of 900–1100 nm. The intensity of the Cr 4+ absorption peak is proportional to the Cr concentration. In order to keep the Cr:Ca molar ratio of 1:3, the amount of CaO grows accordingly with the increasing of the Cr concentration. If adding excessive CaO, abnormal grain growth and residual pores are easily generated. When the Cr concentration is 0.35 at%, the comprehensive properties of the composite ceramic are the best among three samples. The interface region between Cr:YAG and Yb:YAG regions shows a uniform microstructure, indicating that there is no interface effect presence in the composite ceramic. The transmission of the sample is comparable to that of single crystal.

2.0at.% Nd:YAG transparent ceramics were prepared by a solid-state reactive sintering method. The as-sintered specimens were annealed at different temperatures for different time. The ceramics with average grain size of 15μm are dense and homogeneous in structure. 1450°C is determined as the optimum annealing temperature and the annealing time longer than 5h is considered as the appropriate time by the optical spectra and laser performance. At this temperature and holding time, the concentration of color centers in the specimen is relatively low and the laser slope efficiency is the highest. It is concluded that less color centers and lower optical loss in Nd:YAG transparent ceramics are necessary to obtain the excellent laser performance.