Impacts of substituting magnesium with zinc on crystallization behaviors in an aluminosilicate glass.

A series of zinc-magnesium mixed aluminosilicate glasses with the molar composition (1-r)MgO·rZnO·Al2O3·2.5SiO2, where r = 0.00, 0.25, 0.50, 0.65, 0.75, and 1.00, were fabricated to probe the effects of substitution of magnesium with zinc on crystallization behaviors. Based on the evolution of phase compositions as revealed by calorimetric behaviors and X-ray diffraction patterns, a series of transparent surface crystallized glasses ranging from high transparency for the pure Zn-end member to heavy translucency for the pure Mg-end member were fabricated through heat treatment at the first crystallization peak temperature for 20 min. With the substitution of Mg with Zn, the evolution of morphology unveiled by optical microscopy is ascribed to the alteration of crystal phases formed from the sole metastable Zn-ß quartz solid solution to the coexistence of polycrystal phases containing Zn-ß quartz solid solution, µ-cordierite, or α-cordierite. These findings are very helpful for optimizing the performance of crystallized aluminosilicate glasses.

The glass transition in the high-density amorphous Zn/Co-ZIF-4.

The high-density amorphous phases (HDAs) of bimetallic zeolitic imidazolate frameworks (Zn/Co-ZIF-4) were prepared. The temperature dependence of the isobaric heat capacity (Cp) of ZIF-4 HDAs was measured to determine the glass transition temperature (Tg) of HDAs. The Tg non-linearly decreases with the molar ratio R, where R is Co/(Co + Zn), indicating the presence of a mixed-metal node effect. This effect arises from the non-linear increase of the degree of configurational freedom in the HDA as R increases. The degree of configurational freedom is inversely correlated with the network connectivity, which is, in turn, affected by variations in the MN4 (M: Zn or Co; N: nitrogen) tetrahedral symmetry in the ZIF-4 HDA. Overall, this work offers valuable insights into the glass transition of metal-organic frameworks.

Tuning the hardness and crack resistance through liquid-liquid phase separation in an aluminosilicate glass.

Here, spinodal decomposition is used as a strategy to enhance the mechanical properties of the 30Al2O3·70SiO2 glass. The melt-quenched 30Al2O3·70SiO2 glass exhibited a liquid-liquid phase separation with an interconnected snake-like nano-structure. Through further heat treatment at 850 °C for different durations of up to 40 hours, we observed a continuous increase of up to about 0.90 GPa in hardness (Hv) together with a drop in the slope for Hv rise at 4 hours. However, the crack resistance (CR) achieved a maximum value of 13.6 N when the heat treatment time was 2 hours. Detailed calorimetric, morphological and compositional analyses were conducted to elucidate the effect of tuning the thermal treatment time on hardness and crack resistance. These findings pave the way to utilize the spinodal phase-separated phenomena to enhance the mechanical properties of glasses.

An Upper Bound Visualization of Design Trade-Offs in Adsorbent Materials for Gas Separations: CO2 , N2 , CH4 , H2 , O2 , Xe, Kr, and Ar Adsorbents.

The last 20 years have seen many publications investigating porous solids for gas adsorption and separation. The abundance of adsorbent materials (this work identifies 1608 materials for CO2 /N2 separation alone) provides a challenge to obtaining a comprehensive view of the field, identifying leading design strategies, and selecting materials for process modeling. In 2021, the empirical bound visualization technique was applied, analogous to the Robeson upper bound from membrane science, to alkane/alkene adsorbents. These bound visualizations reveal that adsorbent materials are limited by design trade-offs between capacity, selectivity, and heat of adsorption. The current work applies the bound visualization to adsorbents for a wider range of gas pairs, including CO2 , N2 , CH4 , H2 , Xe, O2 , and Kr. How this visual tool can identify leading materials and place new material discoveries in the context of the wider field is presented. The most promising current strategies for breaking design trade-offs are discussed, along with reproducibility of published adsorption literature, and the limitations of bound visualizations. It is hoped that this work inspires new materials that push the bounds of traditional trade-offs while also considering practical aspects critical to the use of materials on an industrial scale such as cost, stability, and sustainability.

Percolative Channels for Superionic Conduction in an Amorphous Conductor.

All-solid-state batteries greatly rely on high-performance solid electrolytes. However, the bottlenecks in solid electrolytes are their low ionic conductivity and stability. Here we report a new series of amorphous xAgI·(1-x)Ag3PS4 (x = 0∼0.8 with interval of 0.1) conductors, among which the sample with x = 0.8 exhibits the highest ionic conductivity (about 1.1 × 10-2 S cm-1) and ultrahigh chemical stability. We discovered the existence of mixed disordered Ag3PS4 and AgI clusters in the amorphous conductors using solid-state nuclear magnetic resonance spectroscopy. The high ionic conductivity was ascribed to the formation of the interconnecting AgI clusters, i.e., the percolative channels for superionic conduction. The composition dependence of the ionic conductivity for this series of amorphous conductors was clarified by a continuum percolation model. These findings provide fundamental guidance for designing and fabricating high-performance amorphous solid electrolytes for all-solid-state batteries.

Mixed metal node effect in zeolitic imidazolate frameworks.

We synthesized two series of bimetallic (zinc and cobalt) zeolitic imidazolate frameworks (ZIF-62) under different solvothermal conditions. It is found that the structure of the derived ZIF crystals is highly sensitive to synthesis conditions. One series possesses the standard ZIF-62 structure, whereas the other has a mixed structure composed of both the standard structure and an unknown one. The standard series exhibits a slight negative deviation from linearity of melting temperature (T m) and glass transition temperature (T g) with the substitution of Co for Zn. In contrast, the new series displays a stronger negative deviation. These negative deviations from linearity indicate the mixed metal node effect in bimetallic ZIF-62 due to the structural mismatch between Co2+ and Zn2+ and to the difference in their electronic configurations. The new series involves both cobalt-rich and zinc-rich phases, whereas the standard one shows one homogeneous phase. Density functional theory calculations predict that the substitution of Co for Zn increases the bulk modulus of the ZIF crystals. This work indicates that the structure, melting behaviour, and mechanical properties of ZIFs can be tuned by metal node substitution and by varying the synthetic conditions. Both series of ZIFs have higher glass forming abilities due to their higher T g/T m ratios (0.77-0.84) compared to most good glass formers.

Deformation mechanism of a metal-organic framework glass under indentation.

Revealing the deformation mechanism of brittle materials under sharp contact loading (indentation) is important for their applications since this knowledge is crucial for identifying the origin of flaw and scratch formation on their surfaces. As a newly emerged glass family, metal-organic framework (MOF) glasses have not been studied concerning the mechanism of their indentation-induced deformation. Here, we explore this mechanism for ZIF-62 glass (a typical MOF glass system). The fractions of densification and shear flow during indentation were determined by atomic force microscopy, while the elastic deformation was measured via nanoindentation. The results show that ZIF-62 glass deforms primarily through densification and elastic deformation under the sharp contact loading. Significant pile-ups around indents were not observed, indicating that no or limited shear flow occurs in the glass during indentation. This behavior could be attributed to three structural factors, namely, high free volume, easily densified glass structure, and limited translational mobility of structural units.

Fracture toughness of a metal-organic framework glass.

Metal-organic framework glasses feature unique thermal, structural, and chemical properties compared to traditional metallic, organic, and oxide glasses. So far, there is a lack of knowledge of their mechanical properties, especially toughness and strength, owing to the challenge in preparing large bulk glass samples for mechanical testing. However, a recently developed melting method enables fabrication of large bulk glass samples (>25 mm3) from zeolitic imidazolate frameworks. Here, fracture toughness (KIc) of a representative glass, namely ZIF-62 glass (Zn(C3H3N2)1.75(C7H5N2)0.25), is measured using single-edge precracked beam method and simulated using reactive molecular dynamics. KIc is determined to be ~0.1 MPa m0.5, which is even lower than that of brittle oxide glasses due to the preferential breakage of the weak coordinative bonds (Zn-N). The glass is found to exhibit an anomalous brittle-to-ductile transition behavior, considering its low fracture surface energy despite similar Poisson's ratio to that of many ductile metallic and organic glasses.

Observation of indentation-induced shear bands in a metal-organic framework glass.

Metal-organic framework (MOF) glasses are a newly emerged family of melt-quenched glasses. Recently, several intriguing features, such as ultrahigh glass-forming ability and low liquid fragility, have been discovered in a number of zeolitic imidazolate frameworks (ZIFs) that are a subset of MOFs. However, the fracture behavior of ZIF glasses has not been explored. Here we report an observation of both cracking pattern and shear bands induced by indentation in a representative melt-quenched ZIF glass, that is, ZIF-62 glass (ZnIm1.68bIm0.32). The shear banding in the ZIF glass is in strong contrast to the cracking behavior of other types of fully polymerized glasses, which do not exhibit any shear bands under indentation. We attribute this anomalous cracking behavior to the easy breakage of the coordinative bonds (Zn-N) in ZIF glasses, since these bonds are much weaker than the ionic and covalent bonds in network glasses.

Ultrahigh-field 67Zn NMR reveals short-range disorder in zeolitic imidazolate framework glasses.

The structure of melt-quenched zeolitic imidazole framework (ZIF) glasses can provide insights into their glass-formation mechanism. We directly detected short-range disorder in ZIF glasses using ultrahigh-field zinc-67 solid-state nuclear magnetic resonance spectroscopy. Two distinct Zn sites characteristic of the parent crystals transformed upon melting into a single tetrahedral site with a broad distribution of structural parameters. Moreover, the ligand chemistry in ZIFs appeared to have no controlling effect on the short-range disorder, although the former affected their phase-transition behavior. These findings reveal structure-property relations and could help design metal-organic framework glasses.

From Molten Calcium Aluminates through Phase Transitions to Cement Phases.

Crystalline calcium aluminates are a critical setting agent in cement. To date, few have explored the microscopic and dynamic mechanism of the transitions from molten aluminate liquids, through the supercooled state to glassy and crystalline phases, during cement clinker production. Herein, the first in situ measurements of viscosity and density are reported across all the principal molten phases, relevant to their eventual crystalline structures. Bulk atomistic computer simulations confirm that thermophysical properties scale with the evolution of network substructures interpenetrating melts on the nanoscale. It is demonstrated that the glass transition temperature (T g) follows the eutectic profile of the liquidus temperature (T m), coinciding with the melting zone in cement production. The viscosity has been uniquely charted over 14 decades for each calcium-aluminate phase, projecting and justifying the different temperature zones used in cement manufacture. The fragile-strong phase transitions are revealed across all supercooled phases coinciding with heterogeneous nucleation close to 1.2T g, where sintering and quenching occur in industrial-scale cement processing.

Phenol Abatement by Titanium Dioxide Photocatalysts: Effect of The Graphene Oxide Loading.

Hetero-photocatalytic graphene-TiO2 materials have, in the literature, been found to possess better photocatalytic activity for environmental applications compared to pure TiO2. These types of materials can be prepared in different ways; however, their photocatalytic performance and quality are not easily controlled and reproduced. Therefore, we synthetized graphene oxide-TiO2 nanoparticles by sol-gel reaction from TiCl4, as precursor, with two different methods of synthesis and with a graphene oxide (GO) loading ranging from 0 to 1.0. This approach led to a good adhesion of GO to TiO2 through the Ti-O-C bonding, which could enhance the photocatalytic performances of the materials. Overall, 0.05 wt % GO loading gave the highest rate in the photodegradation of phenol under visible light, while higher GO loadings had a negative impact on the photocatalytic performances of the composites. The 0.05 wt % GO-TiO2 composite material was confirmed to be a promising photocatalyst for water pollutant abatement. The designed synthetic approach could easily be implemented in large-scale production of the GO-TiO2 coupling materials.

Optical properties of a melt-quenched metal-organic framework glass.

Metal-organic framework (MOF) glasses are characterized by the possession of both inorganic and organic components, linked in a continuous network structure by coordination bonds. To the best of our knowledge, the optical properties of MOF glasses have not been reported until now. In this work, we prepared a transparent bubble-free bulk MOF glass, namely, the ZIF-62 glass (ZnIm2-xbImx), using our newly developed hot-pressing technique, and measured its optical properties. The ZIF-62 glass has a high transmittance (up to 90%) in the visible and near-infrared wavelength ranges, which is comparable to that of many oxide glasses. Using the Becke line nD method, we found that the ZIF-62 glass exhibits a refractive index (1.56) similar to most inorganic glasses, though a lower Abbe number (∼31).

Flux melting of metal-organic frameworks.

Recent demonstrations of melting in the metal-organic framework (MOF) family have created interest in the interfacial domain between inorganic glasses and amorphous organic polymers. The chemical and physical behaviour of porous hybrid liquids and glasses is of particular interest, though opportunities are limited by the inaccessible melting temperatures of many MOFs. Here, we show that the processing technique of flux melting, 'borrowed' from the inorganic domain, may be applied in order to melt ZIF-8, a material which does not possess an accessible liquid state in the pure form. Effectively, we employ the high-temperature liquid state of one MOF as a solvent for a secondary, non-melting MOF component. Differential scanning calorimetry, small- and wide-angle X-ray scattering, electron microscopy and X-ray total scattering techniques are used to show the flux melting of the crystalline component within the liquid. Gas adsorption and positron annihilation lifetime spectroscopy measurements show that this results in enhanced, accessible porosity to a range of guest molecules in the resultant flux melted MOF glass.

Metal-organic framework glasses with permanent accessible porosity.

To date, only several microporous, and even fewer nanoporous, glasses have been produced, always via post synthesis acid treatment of phase separated dense materials, e.g. Vycor glass. In contrast, high internal surface areas are readily achieved in crystalline materials, such as metal-organic frameworks (MOFs). It has recently been discovered that a new family of melt quenched glasses can be produced from MOFs, though they have thus far lacked the accessible and intrinsic porosity of their crystalline precursors. Here, we report the first glasses that are permanently and reversibly porous toward incoming gases, without post-synthetic treatment. We characterize the structure of these glasses using a range of experimental techniques, and demonstrate pores in the range of 4 - 8 Å. The discovery of MOF glasses with permanent accessible porosity reveals a new category of porous glass materials that are elevated beyond conventional inorganic and organic porous glasses by their diversity and tunability.

A metal-organic framework with ultrahigh glass-forming ability.

Glass-forming ability (GFA) is the ability of a liquid to avoid crystallization during cooling. Metal-organic frameworks (MOFs) are a new class of glass formers (1-3), with hitherto unknown dynamic and thermodynamic properties. We report the discovery of a new series of tetrahedral glass systems, zeolitic imidazolate framework-62 (ZIF-62) [Zn(Im2-x bIm x )], which have ultrahigh GFA, superior to any other known glass formers. This ultrahigh GFA is evidenced by a high viscosity η (105 Pa·s) at the melting temperature Tm, a large crystal-glass network density deficit (Δρ/ρg)network, no crystallization in supercooled region on laboratory time scales, a low fragility (m = 23), an extremely high Poisson's ratio (ν = 0.45), and the highest Tg/Tm ratio (0.84) ever reported. Tm and Tg both increase with benzimidazolate (bIm) content but retain the same ultrahigh Tg/Tm ratio, owing to high steric hindrance and frustrated network dynamics and also to the unusually low enthalpy and entropy typical of the soft and flexible nature of MOFs. On the basis of these versatile properties, we explain the exceptional GFA of the ZIF-62 system.

Melt-Quenched Glasses of Metal-Organic Frameworks.

Crystalline solids dominate the field of metal-organic frameworks (MOFs), with access to the liquid and glass states of matter usually prohibited by relatively low temperatures of thermal decomposition. In this work, we give due consideration to framework chemistry and topology to expand the phenomenon of the melting of 3D MOFs, linking crystal chemistry to framework melting temperature and kinetic fragility of the glass-forming liquids. Here we show that melting temperatures can be lowered by altering the chemistry of the crystalline MOF state, which provides a route to facilitate the melting of other MOFs. The glasses formed upon vitrification are chemically and structurally distinct from the three other existing categories of melt-quenched glasses (inorganic nonmetallic, organic, and metallic), and retain the basic metal-ligand connectivity of crystalline MOFs, which connects their mechanical properties to their starting chemical composition. The transfer of functionality from crystal to glass points toward new routes to tunable, functional hybrid glasses.