Diamond Unit Research Articles

The Development of a Next-Generation Latticed Resistojet Thruster for CubeSats

CubeSat and small satellite resistojet heat exchanger designs are based on conventional concepts that have been used since the 1960s, being primarily limited to helical or twisted tape heat exchangers. The design flexibility enabled by new additive manufacturing technologies is yet to be properly harnessed. This study introduces a novel resistojet concept that incorporates a highly miniaturized lattice structure as the heat exchanger. A conjugate heat transfer analysis determined that the lattice with a diamond unit cell had superior thermal performance compared to the same lattice with a gyroid unit cell and increased the heat transfer rate by up to 11% compared to a helical heat exchanger of the same volume. Performance testing of the prototype thruster with integral diamond lattice indicated that specific impulses of up to 94 s were possible with a 30-Watt heater using nitrous oxide as the propellant. The prototype thruster weighed only 22 g and demonstrated a 67% reduction in the power required to achieve the same specific impulse as previous nitrous oxide resistojets designed for the small satellite platform. The development of highly miniaturized latticed resistojets is shown to be feasible and highly attractive for CubeSats, where mass and power are of the utmost importance.

Conceptual study on the intensification of gas–liquid mass transfer in trickle bed reactors by the application of strut-based and novel sheet-based periodic open cellular structures (POCS)

Rapid progress in the development of additive manufacturing technology enables the production of structured reactor internals of complex geometry. To address mass transport limitations structured internals can be used in trickle bed reactors (TBRs) as the most frequently used reactor for heterogeneously catalyzed multiphase reactions. However, reactions in TBRs are frequently limited by gas–liquid mass transfer.As periodic open cellular structures (POCS) as reactor internals were not addressed regarding g–l mass transfer yet, this contribution provides first data on gas–liquid mass transfer for different types of POCS. Therefore, desorption of dissolved oxygen in water with nitrogen gas and the two-phase pressure drop were measured. Strut-based Kelvin and diamond unit cell POCS were compared to a sphere random packed bed as a benchmark. A novel sheet-based unit cell was developed realizing meandering channels demonstrating a fivefold increase in volumetric mass transfer coefficient compared to the benchmark. To compare the performance of the setup with literature data, state of the art neural network correlations were used for comparison. This proof of concept highlights the potential of additively manufactured POCS for intensified processes in trickle bed reactors, and demonstrates their versatility in application.

Pentamodes: Effect of unit cell topology on mechanical properties

Pentamodes (first conceived theoretically by Milton and Cherkaev) are a very interesting class of mechanical metamaterials that can be used as building blocks of structures withdecoupled bulk and shear moduli. The pentamodes usually are composed of double cone-shaped struts with the middle diameter being large and the end diameters being tiny (ideally approaching zero). The cubic diamond geometry was proposed by Milton and Cherkaev as a suitable geometry for the unit cell and has since been used in the majority of the works on pentamodes. In this work, we aim to evaluate the degree to which the base unit cell design contributes to high bulk to shear modulus ratio, also known as Figure Of Merit(FOM). In addition to the diamond unit cell, three other well-known unit cell types are considered, and the effect of small diameter size and the ratio of large-to-small diameter, α, on the FOM is evaluated. The results showed that regardless of the base unit cell shape, the FOM value is highly dependent on the d (the smaller diameter size of double-cone) value, while its dependence on the D (the greater diameter of double-cone) value is very weak. For d/h∝0.05 (h representing the linkage length), figures of merit in the range of 103 could be reached for all the studied topologies.

Tailoring mechanical fatigue behaviour of LPBF-produced metal lattice structures while maintaining high manufacturability

Laser Powder Bed Fusion (LPBF) is increasingly being used for producing metal lattice structures (MLS) which offer a large freedom in tailoring strength, stiffness, and fatigue life whilst reducing mass. This can be done not only by altering the parent material but especially by adapting the geometrical characteristics of the unit cells. Unfortunately, not all unit cells can be accurately produced by LPBF. This paper presents a new strategy for connecting the well-known diamond unit cells in different manners, and with a focus on the mechanical performance of resulting samples under cyclic loading. It is shown that by changing the angle between two connected diamond unit cells by 90°, the resulting mechanical behaviour can be significantly altered without affecting manufacturability. Initial results presented in this work indicate that stiffness and fatigue life can be significantly improved compared to conventional diamond based lattices without affecting manufacturability and for equal sample mass.

Detailed numerical investigations of the in operando adjustable flow field in a diamond unit cell-based interpenetrating periodic open cellular structure (interPOCS)

In the field of heterogeneous catalysis, additively manufactured so-called Periodic Open Cellular Structures (POCS) applied as catalyst supports have demonstrated their advantages over conventional randomly packed bed reactors in terms of heat management and pressure drop (Busse et al., 2018; Inayat et al., 2016). Since the internal flow field and the pressure drop are mainly influenced by the geometrical properties of the unit cell (unit cell type, strut diameter and cell size), the resulting transport characteristics of the structure is already determined during the design phase of the unit cell (Horneber, 2015). By inserting one POCS based on a diamond unit cell (offset structure) into the cavities of a second diamond cell structure (fix structure), a completely new type of so-called interpenetrating Periodic Open Cellular Structures (interPOCS) is formed which allows for an in operando adjustment of the global and local morphological properties of the structure (Do et al., 2017; Do et al., 2020). With detailed, spatially resolved computational fluid dynamics (CFD) simulations using the open source toolbox OpenFOAM® (Weller et al., 1998) and an additional in-house random-walk particle tracking implementation called disTrackFoam (Trunk et al., 2021), we systematically investigated the local and global flow characteristics within the interPOCS in dependency of the relative positioning of the fixed and offset structure. The obtained results show that this highly adjustable structure allows for a broad variation of flow characteristics in a chemical reactor without changing the system’s periphery. This study demonstrates that interPOCS represent a completely new application of additively manufactured catalyst support structures enabling highly flexible in operando tuning of the flow field and mass transport in a heterogeneously catalyzed reactor system.

Process intensification in mass-transfer limited catalytic reactors through anisotropic periodic open cellular structures

We propose an advanced design of catalyst supports based on Periodic Open Cellular Structures (POCS) with a modified diamond unit cell. The diamond unit cell is modified by changing the angle between the struts and the fluid direction, thus changing the fluid-solid interaction and consequently the gas-solid mass and momentum transfer properties. Computational Fluid Dynamics is employed to fundamentally investigate the transport phenomena, and to carry out a parametric investigation of the effects of the morphological properties (i.e., struts angle, porosity and unit cell size). A drastic reduction of the friction factor (i.e., up to 50%) at moderate reduction of the mass transfer coefficient (i.e., up to 30%) is observed in the modified configuration upon progressively aligning the struts with the flow direction. This results in an overall improvement of the trade-off between the two properties leading to superior overall performances compared to state-of-the-art structured substrates. Unifying engineering correlations are then developed based on the numerical results for the evaluation of mass transfer coefficients and friction factors of the POCS, describing the behaviour of both the conventional and the modified cellular structure.

Field-driven design and performance evaluation of dual functionally graded triply periodic minimal surface structures for additive manufacturing

Triply periodic minimal surface (TPMS) structures prepared by additive manufacturing (AM) have been widely used in aerospace and thermal management systems due to their functional advantages such as high specific strength and excellent heat transfer coefficient. However, lattices with uniform or simple gradients cannot match the regional diversity needs of functional advantages in service. This study proposed a field-driven AM data processing framework for dual functionally graded TPMS lattices. For unified data processing, mathematical functions, simulated outcomes, and geometric models were transformed into fields. I-WP and Diamond units were selected as the primitive and filling structures of Ti-6Al-4V lattices, respectively. According to the functional requirements, the gradient distribution design guided by the simulated stress field is completed through the proposed framework. The results illustrate that the field-driven dual graded lattices present superior compressive mechanical response. At almost equal volume fractions, their compressive strength is approximately 100% more than that of primitive lattices and 69% greater than that of dual uniform lattices. Furthermore, the dual lattices have larger specific surface areas than the primitive lattices, making them better at heat insulation and heat dissipation. This field-driven digital framework will show enormous potential in the matching design of functionally graded lattices.

Mechanical Properties Directionality and Permeability of Fused Triply Periodic Minimal Surface Porous Scaffolds Fabricated by Selective Laser Melting.

Titanium alloy porous scaffolds possess excellent mechanical properties and biocompatibility, making them promising for applications in bone tissue engineering. The integration of triply periodic minimal surface (TPMS) with porous scaffolds provides a structural resemblance to the trabecular and cortical bone structures of natural bone tissue, effectively reducing stress-shielding effects, enabling the scaffold to withstand complex stress environments, and facilitating nutrient transport. In this study, we designed fused porous scaffolds based on the Gyroid and Diamond units within TPMS and fabricated samples using selective laser melting technology. The effects of the rotation direction and angle of the inner-layer G unit on the elastic modulus of the fused TPMS porous scaffold were investigated through quasi-static compression experiments. Furthermore, the influence of the rotation direction and angle of the inner-layer G unit on the permeability, pressure, and flow velocity of the fused TPMS porous scaffold structure was studied using computational fluid dynamics (CFD) based on the Navier-Stokes model. The quasi-static compression experiment results demonstrated that the yield strength of the fused TPMS porous scaffold ranged from 367.741 to 419.354 MPa, and the elastic modulus ranged from 10.617 to 11.252 GPa, exhibiting stable mechanical performance in different loading directions. The CFD simulation results indicated that the permeability of the fused TPMS porous scaffold model ranged from 5.70015 × 10-8 to 6.33725 × 10-8 m2. It can be observed that the fused porous scaffold meets the requirements of the complex stress-bearing demands of skeletal structures and complies with the permeability requirements of human bone tissue.

Combined Effects of HA Concentration and Unit Cell Geometry on the Biomechanical Behavior of PCL/HA Scaffold for Tissue Engineering Applications Produced by LPBF.

This experimental study aims at filling the gap in the literature concerning the combined effects of hydroxyapatite (HA) concentration and elementary unit cell geometry on the biomechanical performances of additively manufactured polycaprolactone/hydroxyapatite (PCL/HA) scaffolds for tissue engineering applications. Scaffolds produced by laser powder bed fusion (LPBF) with diamond (DO) and rhombic dodecahedron (RD) elementary unit cells and HA concentrations of 5, 30 and 50 wt.% were subjected to structural, mechanical and biological characterization to investigate the biomechanical and degradative behavior from the perspective of bone tissue regeneration. Haralick's features describing surface pattern, correlation between micro- and macro-structural properties and human mesenchymal stem cell (hMSC) viability and proliferation have been considered. Experimental results showed that HA has negative influence on scaffold compaction under compression, while on the contrary it has a positive effect on hMSC adhesion. The unit cell geometry influences the mechanical response in the plastic regime and also has an effect on the cell proliferation. Finally, both HA concentration and elementary unit cell geometry affect the scaffold elastic deformation behavior as well as the amount of micro-porosity which, in turn, influences the scaffold degradation rate.

Fatigue behaviour of diamond based Ti-6Al-4V lattice structures produced by laser powder bed fusion: On the effect of load direction

In this article, the fatigue properties of cellular lattice structures have been investigated focusing on the anisotropic fatigue behaviour of the diamond unit cell. An extensive fatigue test campaign has been conducted on diamond-based lattice samples produced using laser powder bed fusion in Ti-6Al-4V, designed with different orientation of the unit cell reference system with respect to the load direction. Results indicate that, form a global stress point of view, diamond-based lattice structures exhibit a severe anisotropic fatigue behaviour, that is enhanced by geometrical deviations induced by the manufacturing process. A local stress based method is proposed to account for this unit cell behaviour in predicting its fatigue life.

Superior toughness and hardness in graphite–diamond hybrid induced by coherent interfaces

Finding a material with both a high hardness and toughness is a key area of research in materials science and condensed matter physics. Herein, we propose a new kind of superior mechanical material, a graphite–diamond hybrid (termed Gradia, Nature, 2022, 607, 486), consisting of diamond and graphite nanodomains interlocked via coherent interfaces using first-principles calculations. Upon strain, unusual sequential structural transitions appear in the Gradia, driven by the movement of coherent graphite–diamond interfaces towards the diamond units. This feature allows Gradia to withstand large strains and absorb more energy before structural collapse, leading to superior toughness. More interestingly, Gradia has the potential to be a superhard material based on the estimated Vickers hardness above 40 GPa. This work could play an exemplary role in the exploration of excellent mechanical materials through the modulation of the linking styles of carbon building blocks.

Forced convection heat transfer in heat sinks with topologies based on triply periodic minimal surfaces

This study numerically and experimentally investigates the forced convection heat transfer in three mathematically designed heat sinks with lattice topologies based on triply periodic minimal surfaces (TPMS). The TPMS lattices consist of periodically arranged Diamond (D) or Gyroid (G) unit cells of 10 mm size and 80% porosity, considering two TPMS topologies; solid and sheet networks. This investigation examines the thermohydraulic performance of these novel heat sinks and compares them with other heat sinks in the literature. The hydrodynamic characteristics are studied with an airflow channel, and the thermal performance is explored with a validated numerical model. The results show that form drag dominates pressure drop in the range 2000 < Re < 7000. Due to the lowest surface area and largest pore size, the G-Solid heat sink reported the lowest friction factor ( f ). Concerning the thermal performance, G-Sheet showed the highest areal convection heat transfer coefficient ( hA ) and the lowest thermal resistance ( R th ) as a result of having the highest surface area. For a given pumping power, D-Solid exhibits the highest thermal efficiency ( η ). This work opens the door for designing novel 3D printable heat sinks and investigating their performance in thermal management systems.

Effect of Surface Curvature on the Mechanical and Mass-Transport Properties of Additively Manufactured Tissue Scaffolds with Minimal Surfaces.

The design of scaffolds for tissue engineering has to consider two trade-off properties: mechanical and mass-transport properties. This is particularly true for additively manufactured scaffolds with the structures of minimal surfaces, and notably, the influence of the surface curvature of the structure on the mechanical and mass-transport properties remains unclear. This work presents our study on the scaffolds designed with the structure of triply periodic minimal surfaces (TPMS), with a focus on discovering the influence of surface curvature on the mechanical response and the mass-transport property or permeability of the scaffolds. Based on the entropy weight fuzzy comprehensive evaluation method, a model representative of both mechanical and permeable properties of scaffolds was developed; scanning electron microscopy (SEM) and finite element analysis (FEA) were also used to reveal the influence mechanism of curvature on structural fracture and deformation behavior. AlSi10Mg samples of scaffolds designed with different surface curvatures were manufactured using selective laser melting (SLM), and their mechanical and permeable properties were examined and characterized by both experiments and simulations. Our results illustrate that at the same porosity, the more concentrated the curvature distribution of the same type of unit, the better trade-off mechanical and mass-transport properties the scaffolds have. Particularly, at the porosity of 55%, the compressive elastic modulus and permeability of the Dte structure are increased by 2.03 times and 1.95 times compared with the Diamond unit, respectively. The fusion structure can greatly improve permeability performance at the cost of mechanical properties. Our results also show that porosity has the greatest influence on mechanical and permeable properties, followed by the surface curvature. The study illustrates that the surface curvature has a significant influence on the mechanical and permeable properties of scaffolds, and that the developed scaffold performance evaluation scheme is an effective means for the optimization and evaluation of scaffold performance.

The compressive response of additively-manufactured hollow truss lattices: an experimental investigation

The mechanical response of additively-manufactured hollow truss lattices is experimentally investigated under quasi-static compression testing. Exploiting the recent developments in the Fusing Deposition Modelling (FDM) technique, two families of lattices have been fabricated, obtained as tessellation in space of octet-truss and diamond unit cells. Four specimens for each family of lattices have been designed with prescribed relative density, selecting different inner-to-outer radius ratios r/R of their hollow struts. Compression experiments prove that mechanical properties and failure mechanisms of hollow truss lattices are significantly dependent on the r/R ratio. In particular, a shift from quasi-brittle to ductile mechanical response at increasing r/R values has been revealed for the octet-truss lattice, leading to a stable collapse mechanism and increased energy absorption capacity. On the other hand, a more compliant behaviour has been observed in the diamond lattice response, with a monotonic improvement of mechanical properties as a function of the r/R ratio. Such results substantiate the potentialities of additively-manufactured hollow lattice structures as an attractive solution when lightweight, resistant and efficient energy absorption materials are required.Graphic

Optimisation of single contour strategy in selective laser melting of Ti-6Al-4V lattices

PurposeSelective laser melting (SLM) is increasingly used to manufacture bone implants from titanium alloys with particular interest in porous lattice structures. These complex constructs have been shown to be capable of matching native bone mechanical behaviour leading to improved osseointegration while providing numerous clinical advantages, encouraging their broad use in medical devices. However, producing lattices with a strut diameter similar in scale to a typical SLM melt pool or using the same process parameters and scan strategies intended for bulk solid components may lead to geometric inaccuracies. The purpose of this study is to evaluate and optimise the single contour strategy for the production of Ti-6Al-4V lattices.Design/methodology/approachHerein, the potential of an unfilled single contour (SC) scanning strategy to improve the reproducibility of porous lattices when compared with a single contour and fill approach (SC + F) is explored. For this purpose, two parametric analysis were carried out on Ti-6Al-4V diamond unit cell lattices with different strut sizes and scan strategies. Porosity and accuracy measurements were correlated with processing parameters and printing strategy to provide the optimal processing window for lattice manufacturing.FindingsSC is shown to be a viable strategy for production of Ti-6Al-4V lattices with a strut diameter below 350 µm. Parametric analysis highlights the limits of this method in producing fully dense struts with energy density presented as a useful practical tool to guide some aspects of parameter selection (design strut diameter achieved at approximately 0.1 J/mm in this study). Finally, a process map combining data from both parametric studies is provided to guide, predict and control lattice strut geometry and porosity obtained using the SC strategy.Originality/valueThese results explore the use of non-standard SC scanning strategy as a viable method for producing strut-based lattice structures and compare against the traditional contour and fill approach (SC + F).

The osteogenic effects of porous Tantalum and Titanium alloy scaffolds with different unit cell structure

Porous scaffolds have long been regarded as optimal substitute for bone tissue repairing. In order to explore the influence of unit cell structure and inherent material characteristics on the porous scaffolds in terms of mechanical and biological performance, selective laser melting (SLM) technology was used to fabricate porous tantalum (Ta) and titanium alloy (Ti6Al4V) with diamond (Di) or rhombic dodecahedron (Do) unit cell structure. The mechanical strength of all the porous scaffolds could match that of trabecular bone, while the biological performance of each scaffold was diverse from each other. Moreover, the ILK/ERK1/2/Runx2 signaling pathway had been verified to be involved in the osteogenic differentiation of rat bone mesenchymal stem cells (rBMSCs) cultured on those porous scaffolds. Unit cell structure and material characteristics of the porous Ta and Ti6Al4V scaffolds can synergistically modulate this axis and further impact on the osteogenic effects. Our results hence illustrate that porous Ta scaffold with diamond unit cell structure possesses excellent osteogenic effects and moderate mechanical strength and porous Ti6Al4V scaffold with rhombic dodecahedron unit cell structure has the highest mechanical strength and moderate osteogenic effects. Both porous Ta and Ti6Al4V can be applied in different settings requiring either better biological performance or higher mechanical demand.

Revisiting the electronic nature of nanodiamonds

Nanodiamonds, commonly described as fragments of diamond, have been theoretically found to have lower HOMO-LUMO energy splitting compared to the bandgap of bulk diamond. This apparent lack of correlation between theory and experiment is caused by the position of the LUMO, which is placed in the surface of the ND. An eventual enlargement of the ND towards a macroscopic size will turn the LUMO into the unoccupied surface states, which are not accounted if the bandgap of a bulk material is measured. Here, the electron structure of the nanodiamonds is evaluated, demonstrating that due their nature they should be described as discrete systems instead of bulk materials. Hence, the word bandgap should be avoided in the case of the nanodiamonds, using HOMO-LUMO gap instead. Additionally, our obtained ionization potentials show a satisfactory degree of correlation with the experiment, while the electron affinities are found to be positive. Although this feature fits the estimation performed from experimental data, it opposes the generally accepted idea of a negative electron affinity for hydrogenated nanodiamonds. The present article clarifies common misunderstandings regarding the electronic nature of the NDs, and provides some guidelines for the correct computation of this systems. Finally, as a helpful tool, an estimation of the content of carbon atoms and its surface to volume ratio is provided starting from the diamond unit cell.

Development of novel hybrid TPMS cellular lattices and their mechanical characterisation

Uniform lattices composed of one type of lattice structure repeated periodically have been extensively investigated in literature for their mechanical and physical properties. Their promising properties, which include a desirable combination of high strength, stiffness and toughness, suggest that hybrid structures made of two or more lattice types can exhibit even more advantageous and desired properties. In this work, the mechanical properties of hybrid cellular structures designed using implicit functions are investigated both experimentally and numerically. Two proposed samples are investigated comprised of a Gyroid and a Diamond unit cells hybridised linearly and radially. First, a finite element computational model was utilised in LS-DYNA to capture the mechanical properties of the additively manufactured constituent lattices (i.e., Gyroid and Diamond) made of stainless steel 316L and tested under dynamic and quasi-static loading conditions. The model was validated for three different relative densities. Then, the validated computational model was then tested to predict the mechanical behaviour of the proposed hybrid lattices. Finally, the proposed hybrid lattices were fabricated and mechanically tested to obtain their mechanical properties. A good agreement between experimental and computational results was achieved. The validated computational models will be used to evaluate other designs of TPMS lattices and their crashworthiness performance for protective equipment applications.

Detailed investigation of liquid distribution and holdup in periodic open cellular structures using computed tomography

Periodic open cellular structures (POCS) show great potential for process intensification in catalytic reactors due to their favorable properties regarding heat transfer, pressure drop and liquid distribution. In multiphase applications, POCS can be employed as multifunctional catalyst carrier with integrated liquid distribution function for enhancing the performance of heterogeneously catalyzed reactions. This applies particularly to trickle bed reactors where a uniform liquid distribution is of great importance. In this contribution, the liquid distribution of three POCS of different unit cell geometry is investigated using a custom-made computed tomography scanner with high spatial resolution. POCS with the Kelvin, diamond and DiaKel unit cell, respectively, were manufactured via fused deposition modeling out of acrylonitrile butadiene styrene. The liquid distribution was examined in dependence of the unit cell as well as for different liquid and gas flow rates. The three POCS show different behavior for the liquid distribution and holdup within the structures. POCS with Kelvin and DiaKel unit cell geometry show a higher liquid holdup as well as a more uniform liquid distribution compared to diamond unit cell POCS. Also, the liquid filling between the individual POCS that are stacked in the column is dependent on the unit cell.

Biomechanical performances of PCL/HA micro- and macro-porous lattice scaffolds fabricated via laser powder bed fusion for bone tissue engineering

The present experimental study aims to extend know-how on resorbable polycaprolactone/hydroxyapatite (PCL/HA, 70/30 wt%) scaffolds, produced by Laser Powder Bed Fusion (LPBF) technology, to geometrically complex lattice structures and micro porous struts. Using optimized LPBF printing parameters, micro- and macro-porous scaffolds for bone tissue regeneration were produced by regularly repeating in space Diamond (DO) and Rhombic Dodecahedron (RD) elementary unit cells. After production, scaffolds were submitted to structural, mechanical, and biological characterization. The interaction of scaffolds with human Mesenchymal Stem Cells (hMSCs) allowed studying the degradative processes of the PCL matrix. Biomechanical performances and biodegradation of scaffolds were compared to literature results and bone tissue data. Mechanical compression test, biological viability up to 4 days of incubation and degradation rate evidenced strong dependence of scaffold behavior on unit cell geometry as well as on global geometrical features.