Isotropic sintering shrinkage of 3D glass-ceramic nanolattices: backbone preforming and mechanical enhancement

There is a perpetual pursuit for free-form glasses and ceramics featuring outstanding mechanical properties as well as chemical and thermal resistance. It is a promising idea to shape inorganic materials in three-dimensional (3D) forms to reduce their weight while maintaining high mechanical properties. A popular strategy for the preparation of 3D inorganic materials is to mold the organic–inorganic hybrid photoresists into 3D micro- and nano-structures and remove the organic components by subsequent sintering. However, due to the discrete arrangement of inorganic components in the organic-inorganic hybrid photoresists, it remains a huge challenge to attain isotropic shrinkage during sintering. Herein, we demonstrate the isotropic sintering shrinkage by forming the consecutive –Si–O–Si–O–Zr–O– inorganic backbone in photoresists and fabricating 3D glass–ceramic nanolattices with enhanced mechanical properties. The femtosecond (fs) laser is used in two-photon polymerization (TPP) to fabricate 3D green body structures. After subsequent sintering at 1000 °C, high-quality 3D glass–ceramic microstructures can be obtained with perfectly intact and smooth morphology. In-suit compression experiments and finite-element simulations reveal that octahedral-truss (oct-truss) lattices possess remarkable adeptness in bearing stress concentration and maintain the structural integrity to resist rod bending, indicating that this structure is a candidate for preparing lightweight and high stiffness glass–ceramic nanolattices. 3D printing of such glasses and ceramics has significant implications in a number of industrial applications, including metamaterials, microelectromechanical systems, photonic crystals, and damage-tolerant lightweight materials.

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Introduction
Establishing the structural nanolattice glass-ceramics at the micro-and nanoscale has drawn tremendous attention, owing to its high biocompatibility, outstanding mechanical properties, and chemical resistance [1][2][3][4][5][6][7][8].In comparison with organic materials, dielectric and ceramic materials are difficult to fabricate into complex structures, particularly at microscales with high resolution.The process of manufacturing 3D inorganic structures requires inorganic materials to be tightly bonded at the micro-and nanoscale by sintering, which remains a huge limitation for improving resolution and degree of freedom [9,10].In addition, the introduced porosity and inhomogeneity during traditional sintering may lead to the formation of cracks, making the architecture integrity susceptible to damage [3].
Recently, most advanced technologies for fabricating 3D inorganic complex microstructures based on femtosecond laser two-photon polymerization (fs-TPP) have been extensively investigated to achieve the goal of compactness, lightweight, and miniaturization in the manufacturing process [11][12][13].The fs-TPP is a cutting-edge direct writing technology, which can surpass the optical diffraction limit to achieve subwavelength spatial resolution [14][15][16][17][18][19][20][21].It stands as a novel approach for high-precision fabrication.Structural glass and ceramics fabrication by fs-TPP can be categorized into two main approaches.The first one is the hollow-beam-based strategy, which involves the following steps: creating solid polymer skeletons by TPP, depositing inorganic materials by methods such as atomic layer deposition or magnetron sputtering deposition, and then etching out of the polymer core [22,23].However, this method is limited to producing hollow structures, and the obtained inorganic components are constrained by the target materials.The second approach is the polymer-based strategy, which is recognized to be a technique with tremendous potential for fabricating real 3D inorganic microstructures [24][25][26].Organic-inorganic hybrid photoresists are one of the most commonly used photosensitive resins and can be synthesized by sol-gel method to provide the inorganic components in the cross-linked networks [27].After sintering, the inorganic framework will be consolidated into micro-structural ceramics, while the organic components are eliminated through thermal processes [3,28].However, during sintering, microstructures are prone to anisotropic shrinkage owing to the discrete arrangement of inorganic components in the organic-inorganic hybrid photoresists, which has a negative effect on the mechanical properties of the structures.Thus, pre-formed inorganicbased cross-linked networks are necessary to realize the desired isotropic shrinkage of 3D inorganic microstructures [29,30].
In this work, we innovatively propose the idea of constructing an ordered arrangement inorganic backbone in organicinorganic photoresist to achieve isotropic shrinkage of microstructures during sintering [31][32][33].Based on this molecular composition of SZ2080, the lightweight glass-ceramic microstructures with high stiffness were established by the TPP method and sintering process, maintaining almost perfectly intact and smooth morphology.The hybrid polymer transforms into an amorphous phase doped with the crystalline tetragonal zirconia (t-ZrO 2 ) at 1000 • C, forming the glassceramic.In-depth research revealed the appearance of a dense layer composed of t-ZrO 2 nanoparticles on the surface, ensuring that the prepared microstructures possess desirable stiffness.It was found that the glass-ceramic with nanolattice structures showed a promising lightweight and high stiffness due to the isotropic shrinkage.This strategy of constructing preformed inorganic main chains was applied to the 3D printing of well-designed glass and ceramic structures with high resolution, which has diverse applications in mechanical metamaterials.

3D glass-ceramic nanolattices fabrication
A homebuilt femtosecond laser direct writing system was used for TPP.A femtosecond laser (Pharos-10 W, light conversion) with a center wavelength of 515 nm and pulse duration of around 260 fs was installed, and a three-axis positioning stage with galvometer scanning system (ES112166, Aerotech Inc., Pittsburgh, PA, USA) was integrated.An attenuator (2-EWP-R-0515-M, Altechna) was placed in the optical path to adjust the effective laser intensity.A 100× oil immersion objective lens (NA = 1.4) was used to fabricate the complex 3D microstructures.In this work, the main fabrication parameters of the scanning speed, laser power and pulse repetition frequency are 0.2 mm s −1 , 275 µW and 1 MHz, respectively.After the polymerization, samples were immersed in ethanol for 50 min, followed by drying at 60 • C for 5 min.Then, samples were sintered in a muffle furnace at ambient pressure in air.

Characterization
To investigate the composition, thermal properties and phase during sintering, samples were prepared with an ultraviolet (UV) curing lamp for 30 min.The molecular structure of SZ2080 was determined by the Fourier transform infrared (FTIR) spectroscopy (Nicolet6700) and Laser confocal micro Raman spectrometer (LabRAM Odyssey). 1 H nuclear magnetic resonance (NMR) spectroscopy of SZ2080 was recorded using Bruker Avance NEO 400 MHz in DMSO-d6 solution.
Thermogravimetric analysis (TG), derivative TG and differential scanning calorimetry (DSC) were carried out with a simultaneous thermal analyzer (STA449F3, Netzsch).The XRD patterns were measured with a powder XRD (PAN alytical-Empyrean) using a Cu Kα radiation.To investigate the morphology and the crystalline structure of the microstructure fabricated by TPL after sintering, a scanning electron microscope (SEM) (ProX, Phenom) and a TEM (Talos F200S) were used.We performed in-situ SEM compressive tests using FT-NMT03 nanomechanical testing system with maximum force: 200 mN, flat punch size: 50 × 50 µm.Finite element simulations were carried out by using COMSOL multiphysics.

Results and discussion
The fabrication of glass-ceramic microstructures starts with writing the polymeric resin SZ2080 by fs laser TPP (figure 1(a)).We manufactured a Pegasus-like sculpture (the landmark building of Wuhan University of Technology) with a length of 90 µm.The microstructure was subsequently sintered in a muffle furnace at 1000 • C. The isotropic shrinkage was achieved and accompanied by shrinkage of about 33%, the structure retained its shape without any distortion as shown in figure 1(b).No significant distortion after sintering treatment is attributed to the formation of M-O-M (M = Si or Zr) inorganic framework (figure 1(c)) onto which there are covalently attached organic groups.This critical backbone framework is configured by the condensation reaction [27] between silicon alkoxide and zirconium n-propoxide in SZ2080, as shown in figure S1.As shown in figure 2(a), the spectral region from 1250 cm −1 -1000 cm −1 is associated with the deformation vibration of Si-O-Si bonds [34].Additionally, the three bands observed around 613 cm −1 , 900 cm −1 , and 940 cm −1 are assigned to Si-O-Zr bonds in the backbone of SZ2080 [34,35].Furthermore, the appearance of Si-O-Zr bonds can be attributed to the asymmetric stretching ranging between 950 cm −1 and 1100 cm −1 , as shown in figure 2(b) [36].The terminal bonds of Si-OH in inorganic backbones are corresponding to the wide bands observed at 3500 cm −1 in the FTIR spectrum and the narrow peak at 1377 cm −1 in the Raman spectrum, respectively [34,[37][38][39].This inorganic backbone is formed almost in the photoresist by the condensation reaction between inorganic alkoxide, investigated by the comparison of 1 H NMR spectroscopy at different states as shown in figure S2 [27].The functional groups associated with hydrogen can be found in figure 2(c), and the inorganic main chains can be identified by 29 Si-NMR spectrum as shown in figure 2(d) [40,41], proving the molecular structure of pre-formed SZ2080 as shown in figure 2(e).Thus, the isotropic shrinkage can be derived from the -Si-O-Si-O-Zr-O-inorganic framework.Benefiting from the pre-formed inorganic backbone, the axial and radial sizes of the cylindrical structure undergo a remarkable reduction, shrinking from 18.8 µm to 12.0 µm and from 38.3 µm to 26.2 µm, respectively, exhibiting a nearly uniform decrease of approximately 33%, as shown in figures 3(a) and (b).During the sintering treatment, the organic components decompose, leaving a pure inorganic framework.With the increasing sintering temperature and considering the composition of SZ2080, an amorphous SiZrOC phase is initially generated.Consequently, the emergence of the crystalline zirconia phase follows (figure S3) as the formation of the crystalline silica phase requires a higher temperature.These excellent characteristics of SZ2080 mentioned above make it particularly suitable for creating low-density, lightweight glass-ceramic microstructures with high stiffness.
The TG analysis and the DSC (TG-DSC) were carried out simultaneously with the x-ray diffraction (XRD) (figures 3(c) and (d)) to explore the phase composition in sintering process.It can be concluded from TG-DSC analysis that the sintering process comprises approximately three stages.Initially, the evaporation of solvents occurs, as evidenced by the weight decrease and the broad endothermic peak observed below 200 • C. Consequently, the combustion of the organic components transpires within the temperature range between 400 • C and 600 • C, as evidenced by the large exothermic peak and the rapid mass reduction recorded in the TG-DSC curves.Lastly, the appearance of two exothermic peaks at 930 • C and 1170 • C indicates the crystallization of t-ZrO 2 and SiO 2 , respectively, which is confirmed by XRD analysis of sintered samples at 1000 • C and 1400 • C. Compared with figures S4 and 3(c), this process will be prolonged with increasing silicon content.As presented in figure 3(d), at the sintering temperature of 600 • C, a broad peak is evident on the XRD curve at low angles, indicating the formation of an amorphous phase.Upon ascending beyond 600 • C, crystalline phases gradually precipitate.The XRD results also demonstrate the initial emergence of crystalline t-ZrO 2 at 1000 • C, facilitating the forming of the glassceramic.It is only when the temperature reaches 1400 • C that cristobalite precipitates [42].It is worth noting that the microstructures will suffer severe deformation or even complete dissolution due to the melting of the amorphous phase at around 1200 • C. Therefore, our investigations were focused on the mechanical properties of glass-ceramic microstructures via sintering at 1000 • C.
Nanomechanical uniaxial compression of fs laser TPPderived glass-ceramic micropillar experiments reveal that the Young's modulus of intrinsic glass-ceramic material is 1.06 GPa (figures S5(a) and (b)).Transmission electron microscopy (TEM) studies were performed to further understand the reason for the high Young's modulus when treated with 1000 • C. Figures 4(a) and (b) show the bright-field image and corresponding diffraction patterns of the rods on microstructures, which confirm the main crystalline phase to be t-ZrO 2 .Interestingly, we found in the high-angle annular dark field (HAADF) image that the rods on the microstructures are clearly layered, as shown in figure 4(c).The energy dispersive spectroscopy results (figures 4(d) and (f)) display that the dashed box is the Zr-rich region, and the solid is the Si-rich region.It can be seen from the high-resolution TEM images (figures 4(e) and (g)) that in the Zr-rich region, the distribution of t-ZrO 2 nanoparticles is relatively uniform, and at the same time, the amorphous Si-rich region is dispersed with larger precipitated t-ZrO 2 nanoparticles.The element enrichment can be attributed to the metallic migration, which is related   to the sintering temperature and activation energy.This phenomenon is commonly known as metallic element leaching [43].The dynamics of this process can be illustrated by the Arrhenius law: where K is the reaction rate, determined by the temperature T and activation energy E a .At room temperature, the E a of SiO 2 and ZrO 2 are around 430 kJ mol −1 and 246 kJ mol −1 , respectively, thus indicating that ZrO 2 exhibits a relatively higher reaction rate to migrate outside the structure [44,45].
With the sintering temperature increasing, a larger number of particles will collide, leading to the termination of the reaction, and resulting in the layered distribution of inorganic dioxides.In ceramics, zirconia possesses excellent fracture toughness, strength, as well as other intrinsic physical properties, which have been widely applied for composite ceramics reinforcement [46][47][48].The appearance of the dense t-ZrO 2 layer, coupled with the emergence of nanoparticles in the amorphous region, ensures the glass-ceramic material with good stiffness jointly.
To achieve complex microstructures with high resolution, we first investigated the influence of laser parameters on processing accuracy during TPP, as shown in figure S6.After that, we further processed three types of periodically arranged nanolattices: the truss structure (truss), the octahedral structure (oct), and the octahedral truss structure (oct-truss).By adjusting the size of the rods on the microstructures through digital design, we attained varying relative densities (ρ = ρ/ρ s ) across the configurations.It is well-known that the laser focal intensity distribution is close-to-Gaussian profile [49], thus, the axial and radial dimensions of the rods exhibit inherent inconsistencies, resulting in a deviation from the real processing size and the intended designed size (figure S7).As shown in figures 5(a)-(c), all periodic nanolattices shrink isotropically after sintering at 1000 • C, with a shrinkage rate of approximately 37%.Remarkably, these nanolatties maintain their structural integrity and smooth almost perfectly compared to the initial microstructures.In order to access the behavior of the nanolattices under compression, in-suit compression experiments by applying an axial load along the vertical direction (supplementary movies S1-S3).Figures 5(d)-(i) shows the stress-strain curves of these three structures, along with their corresponding finite-element simulations.For Truss, the Young's moduli experience an increment from 175 MPa to 229 MPa, accompanied by a simultaneous increase in deformation from 7.7% to 20.7%.The simulation result demonstrates the stress concentration on the vertical rods, which causes the structural susceptibility to bending.This was demonstrated by the compression movie (supplementary movie S4).As shown in figure S8, the numerical finiteelement simulations reveal that the experimental values are much larger than the theoretical calculations.This discrepancy can be explained by the enhancement of the t-ZrO 2 layer.Previous research revealed the impressive mechanical properties of the Oct structures in metamaterials.Figure S9 summarizes SEM images of the deformation of each nanolattice structure with continuous applied stresses.It can be found that the truss was susceptible to destruction under 22.6 MPa as in figures S9(a)-(d).Combining with the truss and the oct structures, the oct-truss exhibits more excellent stiffness, in which the vertical rods can withstand major stresses while the oct rods can resist bending, which is consistent with the trend of stress simulation depicted in figures 5(g)-(i).According to figures S9(i)-(l), the oct-truss is able to maintain structural integrity under high stress up to 27.1 MPa.To clarify the stiffness of these structures, the stress-strain curves of them were shown in figure S10, demonstrating that the outstanding oct-truss shows a remarkable ability to resist highly applied stress and subsequently exhibit a notable capacity for recovery.As a result, the oct-truss shows the highest Young's modulus among the three structures, reaching 243 MPa with the relative density of 0.195.The resulting mechanical performance of our glass-ceramic nanolattices, together with previously reported structures and materials is summarized in figure 6.This figure illustrates the relationship between relative Young's modulus and relative mass density, providing a comprehensive overview of the comparative analysis.Additionally, as shown in figure S11, glassceramic nanolattices have stiffness-weight ratios, implying that the Young's modulus does not substantially degrade as density reduces.This type of glass-ceramic nanolattices shows promising applications for lightweight and stiffness metamaterials.

Conclusion
We advance a 3D printing methodology to fabricate lightweight and stiff glass-ceramic microstructures based on TPP of photosensitive resin SZ2080, achieving isotropic shrinkage by establishing the inorganic backbone in the photoresist.The resulting 3D glass-ceramic nanolattices are of high quality with feature sizes down to 500 nm and shrinkage upon sintering of only around 35%.The glass ceramics were formed in microstructures with the appearance of a t-ZrO 2 layer at 1000 • C, which attaches good stiffness and high relative Young's modulus to the prepared microstructures.In-suit compression experiments and finite-element simulations were carried out to determine the stiffness of the microstructures and the force distribution under compression.It is found that the oct-truss structure exhibits notable power in withstanding stress concentration and maintains the structural integrity to resist rod bending, which is a promising candidate structure for preparing lightweight and high-stiffness glass-ceramics.Our findings demonstrate the accessibility of structural inorganic materials with low density and high stiffness, which is of great significance to the design and manufacture of microand mesoscale engineering systems.

Figure 1 .
Figure 1.Preparation of the glass-ceramic Pegasus-like sculpture and principle of isotropic shrinkage.(a) Schematic diagram of 3D manufacturing glass-ceramic microstructures based on SZ2080 resin.(b) The SEM images of the Pegasus-like sculpture before and after sintering.(c) Illustration of the inorganic framework of M-O-M (M = Si or Zr) onto which there are covalently attached organic groups.

Figure 2 .
Figure 2. Characterization of the inorganic backbone in photoresists.(a) The FTIR spectrum and (b) the Raman spectrum associated with photoresist of SZ2080 film.(c) The fine 1 H-NMR spectrum of SZ2080 film with corresponding hydrogen-containing functional groups.(d) The fine 29 Si-NMR spectrum of SZ2080 film with corresponding inorganic main chains.(e) The chemical molecular structural formula of SZ2080.

Figure 3 .
Figure 3. Characterization of isotropic shrinkage and phase transition during sintering process.SEM images of the micro cylinder structures fabricated by SZ2080 (a) before and (b) after the sintering step at 1000 • C of femtosecond laser direct writing process.The scanning angle is 45 • and scale bars are 20 µm and 10 µm, respectively.(c) TG-DSC results of the photocured SZ2080 resin.(d) XRD analysis of the photocured resin treated with 600 • C, • C, and 1400 • C for one hour in the air at ambient pressure.

Figure 4 .
Figure 4. TEM characterization of the nanostructures after sintering.(a) and (b) The bright-field image and the corresponding diffraction patterns of the rods on the microstructures sintering at 1000 • C. (c) The HAADF image with obvious stratification.(d) and (f) The EDS maps of Si and Zr corresponding to the HAADF images of the Zr-rich region and Si-rich region.The high-resolution TEM images of the (e) Zr-rich region and (g) Si-rich region.

Figure 5 .
Figure 5. Characterization of different unit cells and corresponding in-situ uniaxial compression experiments and finite-element simulations.(a)-(c) SEM images of truss, oct, and oct-truss before and after sintering at 1000 • C. (d)-(f) stress-strain curves of nanolattices with different ρ. (g)-(i) Finite element simulations of force distribution of three structures show that the vertical rods on truss and oct-truss introduce local stress concentrations.

Figure 6 .
Figure 6.Summary of relative Young's modulus versus relative density for our glass-ceramic nanolattices mechanical performance, together with previous reported structures and materials.