Bulk supercrystalline ceramic-organic nanocomposites: New processing routines and insights on the mechanical behavior

In the strive to produce nature-inspired hierarchical materials with an enhanced combination of mechanical properties, supercrystalline ceramic-organic nanocomposites have been produced in bulk form and characterized from a variety of perspectives. Through an interdisciplinary collaboration at the crossroad between materials science, chemistry and mechanical engineering, a bottom-up approach has been designed. It consists of a sequence of self-assembly, pressing and heat treatment, and it leads to macroscopic poly-supercrystalline materials with exceptional mechanical properties and behavior. The crosslinking of the organic phase induced by the heat treatment does not only increase the materials’ stiffness, hardness and strength (elastic modulus up to 70 GPa, hardness up to 5 GPa and bending strength up to 630 MPa), but alters also their constitutive response. Fracture toughness values higher than theoretical predictions have emerged (~ 1 MPa·m1/2), implying the presence of extrinsic toughening mechanisms, such as the crack-path deviation observed at indents’ corners. Ex-situ nanoindentation and in-situ SAXS/microcompression studies also suggest the possibility for supercrystalline materials to accommodate compressibility and plastic-like deformation. Defects analogous to the ones typically observed in crystalline lattices, such as stacking faults, dislocations and slip bands, are detected at the superlattice scale (even if one order of magnitude larger than the atomic one, and with interactions among the nano-building blocks controlled by the organic phase). Correlations between defects, processing and mechanical properties have been drawn by adapting the classic theories of mechanical behavior of materials. These same materials are additionally being used as bricks for the development of novel hierarchical composites, via additive manufacturing or fluidized bed techniques. Please click Additional Files below to see the full abstract

Hierarchical supercrystalline nanocomposites through the self-assembly of organically-modified ceramic nanoparticles

Biomaterials often display outstanding combinations of mechanical properties thanks to their hierarchical structuring, which occurs through a dynamically and biologically controlled growth and self-assembly of their main constituents, typically mineral and protein. However, it is still challenging to obtain this ordered multiscale structural organization in synthetic 3D-nanocomposite materials. Herein, we report a new bottom-up approach for the synthesis of macroscale hierarchical nanocomposite materials in a single step. By controlling the content of organic phase during the self-assembly of monodisperse organically-modified nanoparticles (iron oxide with oleyl phosphate), either purely supercrystalline or hierarchically structured supercrystalline nanocomposite materials are obtained. Beyond a critical concentration of organic phase, a hierarchical material is consistently formed. In such a hierarchical material, individual organically-modified ceramic nanoparticles (Level 0) self-assemble into supercrystals in face-centered cubic superlattices (Level 1), which in turn form granules of up to hundreds of micrometers (Level 2). These micrometric granules are the constituents of the final mm-sized material. This approach demonstrates that the local concentration of organic phase and nano-building blocks during self-assembly controls the final material's microstructure, and thus enables the fine-tuning of inorganic-organic nanocomposites' mechanical behavior, paving the way towards the design of novel high-performance structural materials.The authors gratefully acknowledge the financial support from the German Research Foundation (DFG) via the SFB 986-M3, projects A1, A6, Z2, and Z3. We thank Dr. F. Beckmann (Helmholtz-Zentrum Geesthacht, Geesthacht, Germany) for scanning the sample with the technique SRµCT and for reconstructing the slices, and Dr. I. Greving (Helmholtz-Zentrum Geesthacht, Geesthacht, Germany) for her inputs on SRµCT. Dr. F. Brun (National Institute of Nuclear Physics, Trieste, Italy) is acknowledged for the discussion regarding quantitative analysis using Pore3d

Iron oxide-based nanostructured ceramics with tailored magnetic and mechanical properties: Development of mechanically robust, bulk superparamagnetic materials

Nanostructured iron-oxide based materials with tailored mechanical and magnetic behavior are produced in bulk form. By applying ultra-fast heating routines via spark plasma sintering (SPS) to supercrystalline pellets, materials with an enhanced combination of elastic modulus, hardness and saturation magnetization are achieved. Supercrystallinity-namely the arrangement of the constituent nanoparticles into periodic structures-is achieved through self-assembly of the organically-functionalized iron oxide nanoparticles. The optimization of the following SPS regime allows the control of organics' removal, necking, iron oxide phase transformations and nano-grain size retention, and thus the fine-tuning of both mechanical properties and magnetic response, up until the production of bulk mm-size superparamagnetic materials.Deusche Forschungsgemeinschaft (DFG

Optimization of Spark-Plasma Sintering Efficiency: Tailoring Material Structure and Advanced Tooling Design

The efficiency of Spark Plasma Sintering (SPS) is optimized by means of a temperature-based procedure. The correlations between temperature, porous material structure and tooling geometry are explored. The issue of agglomeration, cause of the formation of hierarchical porous structures in the powder compact, is addressed with an analytical and numerical approach. Densification kinetics, sintering stress, bulk and shear moduli of agglomerated compacts are modeled as functions of the hierarchy characterizing the porosity and the nonlinear viscous rheology of the material. The material nonlinearity is expressed by the strain rate sensitivity parameter, which in turn is dependent on temperature. An empirically-based explicit formulation of strain rate sensitivity as a function of temperature is provided. A subsequent tuning of the strain rate sensitivity allows the optimization of the thermal regime in order to obtain in situ deagglomeration of the SPS specimen or the production of tailored material structures. In order to guarantee that the optimized thermal conditions are experienced by the entire specimen, we study the geometry of the SPS tooling setup. The tooling influence on temperature distributions is assessed in both the radial and the axial directions by means of a combined experimental and finite-element study. The phenomena underlying the presence of thermal gradients are individuated and controlled by tailoring the design of the tooling setup. Novel geometries are proposed for the components of the tooling, capable to annihilate the temperature disparities and thus complete the optimization of the SPS procedure. A comprehensive finite-element framework, in which both agglomerated powder compacts and tooling setup can be reconstructed and improved, is therefore developed

A Fully-Coupled Finite Element Analysis of Field Assisted Sintering Techniques

Field-Assisted Sintering Techniques (FAST) have gained growing interest in the academic and industrial communities in the last decades, thanks to their outstanding characteristics in densifying powder materials with respect to the conventional sintering technologies. The fundamental contribution of the electric field to the consolidation process is still under investigation, but its role in the production of intense Joule heating inside the powder specimen is well assessed. This Joule heating is responsible for the obtainment of the temperatures necessary for the material densification, and its homogeneous distribution is a crucial requirement in order to attain satisfactory final outcomes, in terms of density and microstructure. When increasing the size of the specimen to be sintered, thermal non-uniformities issues arise and become gradually more compromising. During FAST procedures, the specimen is located inside a tooling setup constituted by a variable number of graphite components, whose significant effects on the current and temperature distributions is well known. Being such tooling axisymmetric, two main cross-sections can be individuated when studying electrical and thermal gradients: axial and radial. Both distributions have been thoroughly analyzed in our study. Problems of localized overheating and strong temperature inhomogeneities have been experimentally individuated and numerically addressed. Finite Element Methods (FEM) have been selected as an optimal tool to reconstruct experimental processes, compensate for the lack of data on local temperatures (only measurable at one or two specific points in a FAST setup), and finally optimize the graphite tooling setup in order to mitigate the undesired thermal non-uniformities. The FEM-suggested improvements led to the experimental implementation of novel tooling configurations, which successfully uniformized the axial and radial temperature distributions in a variety of macro-scale FAST configurations and finally allowed the production of sintered specimens endowed with a notable microstructural homogeneity

Nanofatigue of supercrystalline nanocomposites

Abstract Supercrystalline nanocomposites (SCNCs) are a new category of hybrid materials consisting of inorganic nanoparticles surface‐functionalized with organic ligands and with periodic nanostructures, featuring multi‐functionality and able to reach exceptional mechanical properties. Although efforts have been made to explore their mechanical behavior, their response to cyclic loading remains to be unveiled. Here, the fatigue behavior of SCNCs with different degrees of organic crosslinking is investigated via nanoindentation. The nanocomposites’ fatigue life is assessed, and it emerges that SCNCs without crosslinking are more efficient in dissipating energy under cyclic loading and thus feature a longer fatigue life. Chipping is identified as the main fatigue failure mechanism, whereas different mechanisms, intrinsic or extrinsic, dominate in the indentation depth propagation, again depending on crosslinking

Advancement of tooling for spark plasma sintering

A combined experimental and numerical study is conducted to investigate temperature nonhomogeneities within a Spark Plasma Sintering tooling setup. Radial thermal gradients through a powder compact are encountered, a cause of microstructural nonuniformities in sintered specimens, which tend to become more significant when increasing the setup's characteristic size. In the insulating silicon nitride powder compact employed for the experimental procedures, a double pyrometer arrangement detects a strong temperature disparity between the overheated die and the area adjacent to the tooling's axis. A previous finite-element simulations campaign had individuated a possible solution in a novel punch design, consisting in the drilling of three concentric ring-shaped holes according to a specific geometrical pattern, whose efficacy is here experimentally verified. Further punch optimization strategies are drawn, involving a refinement of the threerings geometry by linearly varying the drilled holes characteristic dimensions along the radial direction, or the selective coating and consequent insulation of the punch cross section with a thin layer of hexagonal boron nitride. Ideal configurations are identified, consisting in a concentration of the graphite punch's mass at its center by means of a tailored holes pattern, or in the coating of a portion of the conventionally shaped punch with boron nitride

Nanoindentation creep of supercrystalline nanocomposites

Supercrystalline nanocomposites (SCNCs) are inorganic-organic hybrid materials with a unique periodic nanostructure, and thus they have been gaining growing attention for their intriguing functional properties and parallelisms with hierarchical biomaterials. Their mechanical behavior remains, however, poorly understood, even though its understanding and control are important to allow SCNCs’ implementation into devices. An important aspect that has not been tackled yet is their time-dependent deformation behavior, which is nevertheless expected to play an important role in materials containing such a distribution of organic phase. Hereby, we report on the creep of ceramic-organic SCNCs with varying degrees of organic crosslinking, as assessed via nanoindentation. Creep strains and their partial recoverability are observed, hinting at the co-presence of viscoelasticity and viscoplasticity, and a clear effect of crosslinking in decreasing the overall material deformability emerges. We rationalize our experimental observations with the analysis of stress exponent and activation volume, resulting in a power-law breakdown behavior and governing deformation mechanisms occurring at the organic sub-nm interfaces scale, as rearrangement of organic ligands. The set of results is reinforced by the evaluation of the strain rate sensitivity via strain rate jump tests, and the assessment of the effect of oscillations during continuous stiffness measurement mode