Densification Of Samples Research Articles

Polyaluminum enhanced dehydrated cement paste activated slag: Mechanical performance and microstructural analysis

Dehydrated cement paste (DCP) activated slag offer a promising sustainable alternative to traditional Portland cement, but their inadequate mechanical strength limits practical applications. This study investigates the potential of polyaluminum chloride (PAC) to enhance the performance of DCP activated slags, utilizing multi-scale characterization methods. Results demonstrate that PAC incorporation significantly promotes strength development, enabling the binder to meet requirements for most engineering applications. PAC accelerates Ca(OH)2 consumption, and facilitates additional hydration product formation that refines the pore structure, leading to the densification of samples. These improvements in strength and microstructure are attributed to the modified slag dissolution and the enhanced polymerization of the C-A-S-H gel. Furthermore, the underlying role of PAC on the hydration, mechanical performance and microstructural development of DCP activated slags are discussed.

Neural Networks to Find the Optimal Forcing for Offsetting the Anthropogenic Climate Change Effects

Abstract Of great relevance to climate engineering is the systematic relationship between the radiative forcing to the climate system and the response of the system, a relationship often represented by the linear response function (LRF) of the system. However, estimating the LRF often becomes an ill-posed inverse problem due to high-dimensionality and nonunique relationships between the forcing and response. Recent advances in machine learning make it possible to address the ill-posed inverse problem through regularization and sparse system fitting. Here, we develop a convolutional neural network (CNN) for regularized inversion. The CNN is trained using the surface temperature responses from a set of Green’s function perturbation experiments as imagery input data together with data sample densification. The resulting CNN model can infer the forcing pattern responsible for the temperature response from out-of-sample forcing scenarios. This promising proof of concept suggests a possible strategy for estimating the optimal forcing to negate certain undesirable effects of climate change. The limited success of this effort underscores the challenges of solving an inverse problem for a climate system with inherent nonlinearity. Significance Statement Predicting the climate response for a given climate forcing is a direct problem, while inferring the forcing for a given desired climate response is often an inverse, ill-posed, problem, posing a new challenge to the climate community. This study makes the first attempt to infer the radiative forcing for a given target pattern of global surface temperature response using a deep learning approach. The resulting deeply trained convolutional neural network inversion model shows promise in capturing the forcing pattern corresponding to a given surface temperature response, with a significant implication on the design of an optimal solar radiation management strategy for curbing global warming. This study also highlights the technical challenges that future research should prioritize in seeking feasible solutions to the inverse climate problem.

Unveiling the role of ZrO2 doping in Sr2SmTi2Nb3O15 ceramics: From structural study to dielectric enhancement

Solid-state processing was used to synthesize tungsten bronze type (TTB) solid solutions, specifically Sr2Sm(Ti1-xZrx)2Nb3O15 with x values of 0, 0.2, 0.3, and 0.4. The study examined the crystalline structure, optical properties, morphology, and electrical and dielectric characteristics of these compounds. X-ray diffraction revealed a tetragonal structure (space group P4bm). Scanning electron microscopy showed significant sample densification with grain sizes decreasing from 6.19 μm to 2.78 μm. Dielectric measurements indicated that the dielectric constant varied from 150 for x = 0 to 567.24 for x = 0.2, then decreased to 380.88 for x = 0.4, with the x = 0.3 compound having the lowest dielectric loss. The Curie temperature (Tc) decreased from 332 °C to 248.5 °C with increasing Zr content, corresponding with a reduction in grain size. The energy gap (Eg) narrowed from 2.914 eV to 2.714 eV as Zr content increased. AC conductivity increased with temperature and frequency. Electrical conductivity followed Jonscher's power law, with activation energies decreasing from 1.54 eV to 0.87 eV, explained by the small polaron tunneling (NSPT) model.

Optimization of process parameters of selective laser melted nickel-based superalloy for densification by random forest regression algorithm and response surface methodology

In this study, Random Forest Regression (RFR) and Response Surface Methodology (RSM) were employed to predict the optimized processing parameters to achieve the highest densification of a nickel-based superalloy Mar-M247LC fabricated by selective laser melting (SLM). The RFR model considered input processing parameters such as laser power, hatch distance, and scanning speed. A dataset of 223 samples, was used to train the RFR model. As a result, the RFR model exhibited accuracy of 99.57 %, R2 value of 0.976, Mean Square Error (MSE) of 0.402, and Mean Absolute Percentage Error (MAPE) of 0.426 % on testing set. In addition to the RFR model, this study also employed Central Composite Design (CCD) and RSM to optimize the processing parameter sets. Subsequently, this study conducted Box-Behnken Design (BBD) to experimentally validate the accuracy of this RFR model. In the end, a set of the optimal processing parameters was tested and resulted a sample densification of 99.959 %, outperformed that in the original database before building the RFR model, which was 99.734 %. In summary, the RFR models was able to predict densification with accuracy, and by coupling with RSM, the optimal processing parameter could be obtained, so better densification of the build could be achieved.

Quantifying the cooperative evolution of microphase segregation and nanostructural order in annealed polyurethanes of MDI‐BDO‐PTHF

AbstractHere, we quantify the relationship between microphase segregation and local structure development in thermoplastic polyurethanes. Samples were prepared with varying ratios of 4,4′‐methylene diphenyl diisocyanate (MDI) and 1,4‐butanediol (BDO) to polytetrahydrofuran (PTHF) and annealed at temperatures between RT–140 °C for 20 h each. Distinct populations of smaller and larger ordered domains are distinguished by pair distribution function analysis. The intermediate‐range order of nanoscale hard domains in the paracrystalline state (form I) is directly extracted and characterized. The results suggest that form I of the MDI‐BDO:PTHF system presents a conformational and packing order similar to the form III structure, typically obtained from stretching/annealing, but with limited spatial coherence of 3–7 nm. Heating above of the hard segments is necessary to achieve a substantial structural response from the annealing treatment. Increasing promotes purification of the soft phase via microphase segregation, coalescence of the hard blocks, and growth of paracrystalline phase, while a minority fraction of longer‐range ordered domains left over from production remains relatively constant. We observe a composition‐dependent decrease in the average nearest‐neighbor distance that can be correlated with the greater number of CC, CN, and CO bonds with increased hard segment content. A non‐linear deviation in the trend is observed to correlate with sample densification. The mechanical and thermal behavior of the samples is intimately tied to the segregation state.

An ultrasonic study of the effect of uniaxial green compaction pressure on the elastic anisotropy of porous copper fabricated using the pore-former route

Porous copper is potentially very attractive for various functional applications such as filters and heat exchanger materials. Within the scope of this study, porous copper samples with dual pore morphologies were fabricated following the space holder technique using a 1:2 mixture ratio of K2CO3 to NaCl as pore formers. The initial compaction pressure required for green body fabrication was systematically varied between 120 and 180 MPa, and the influence of this pressure over porosity, pore morphology, volume shrinkage, and longitudinal elastic constants was systematically studied. Three longitudinal elastic constants of the porous Cu samples were determined following the non-destructive ultrasonic phase spectroscopy method through the measurement of the phase shift vs. frequency spectra along the three orthogonal directions in each sample. When the applied pressure is increased from 120 to 160 MPa and the Cu:PF mixture ratios are 70:30 and 60:40, respectively, the elastic constant values in the green compaction direction (C11) decrease from 34.2 GPa to 7.5 GPa and 23.7 GPa–17.9 GPa. However, for Cu:PF mixture ratios of 70:30 and 60:40, under 180 MPa green compaction pressure C11 increases to 30.9 GPa and 21.2 GPa, respectively. As the applied green compaction pressure increased from 120 to 160 MPa, the samples' average elastic anisotropy ratio initially increased from 0.29 to 0.59 and then decreased to 0.55 at 180 MPa pressure. While the initial increase in anisotropy is due to increased pore flattening with increasing green compaction pressure, the decrease in anisotropy ratio in the samples fabricated under 180 MPa green compaction pressure is due to enhanced sample densification. Contour maps for different properties have been proposed to identify the optimum combination of initial powder composition and green compaction pressure for the intended end application.

High-performance thermoelectric properties of Cu2Se fabricated via cold sintering process

The concept of a ‘phonon-liquid electron-crystal’ has received significant interest in recent years, with copper selenide (Cu2Se) emerging as one of the high-performance thermoelectric materials within this framework. This study focuses on the fabrication of Cu2Se bulk pellets through a low-temperature sintering method known as cold sintering process (CSP). The introduction of a liquid phase (thiol-amine solution) in this process facilitates the dissolution and precipitation of ion/atom clusters, resulting in sample densification. Remarkably, a sample density of nearly 90 % is achieved at a low sintering temperature of only 473 K. Furthermore, this CSP approach preserves the morphology of the Cu2Se precursor powders and effectively inhibits grain growth. Consequently, the lattice thermal conductivity of the CSP samples is significantly reduced, attributed to enhanced grain boundary phonon scattering. This leads to a substantial improvement in the figure-of-merit (ZT), increasing from 0.65 at 800 K in the hot-pressed sample to 2.13 at 800 K in the CSP sample. The advantages of CSP can be extended beyond Cu2Se, as it holds promise for enhancing the performance of various high-performance thermoelectric materials.

Effect of process parameters on the densification of SLM-produced Mg–Y–Sm–Zn–Zr alloy

In this study, Mg–Y–Sm–Zn–Zr samples were prepared using the selective laser melting technique. The influence of process parameters on the densification and mechanical properties of samples was systematically studied from a single-melt track to double melt tracks to block, and the formation mechanism of defects was analysed. Research results show that the appropriate laser power and scanning speed resulted in excellent single-melt path formability. When the scanning spacing was 80 μm, adjacent trajectories were closely connected without spheroidised particles and porosity defects. The densest samples were obtained at an energy density of 125 J/mm3 with maximum microhardness (86.8 ± 1.5 HV), and the compressive yield strength and ultimate strength reached 301.8 ± 5 MPa and 467.9 ± 5 MPa.

A new laser remelting strategy for direct energy deposition of 316L stainless steel

At present, components produced by Direct energy deposition (DED) still have many shortcomings, such as rough surfaces, many internal pores, and the formation of long columnar grains that will lead to anisotropy due to large temperature gradients. Laser remelting (LR) is an effective method to improve the top surface quality and densification of samples. In the present work, in the process of manufacturing 316L stainless steel samples with DED, an additional LR process without powder conveying is added after each deposition layer of the samples is deposited. The smoothness of the sample surface has been greatly improved, and the density of the samples has been increased to nearly fully dense. The number and size of pores inside the samples are significantly decreased. By changing laser power and spot size, the effect of the LR process with different energy densities on microstructure and mechanical properties was studied. After the LR process, the coarse and long columnar grains in samples are transformed into fine equiaxed grains and fine columnar grains. Experimental results show that the LR process with appropriate energy density can reduce the residual stress of the samples made by DED and significantly improve the tensile strength and elongation of the samples. LR with the same processing parameters as the deposition has the most obvious effect on refining the microstructure and improving tensile properties. However, the LR process with low energy density will increase the residual stress of the sample, resulting in poor mechanical properties.

Thickness effects on the sinterability, microstructure, and nanohardness of SiC‐based ceramics consolidated by spark plasma sintering

AbstractThis study aimed to investigate the effects of the original thickness on the densification, microstructure, and nanoindentation hardness of silicon carbide (SiC) ceramics prepared by the spark plasma sintering (SPS) process. The densification of SiC ceramics with different initial thicknesses ranging from 50 to 2000 µm was investigated in combination with varying SPS sintering temperature at 1700–1900°C. The results indicated that the densification of SiC sample with the initial thickness of 50 µm was complete after sintering at 1700°C. On the contrary, when the initial thickness exceeded 50 µm, it resulted in a porous microstructure. When the initial thickness varied from 50 to 100 µm, dense SiC monolithic could be obtained after sintering at 1800°C. All the samples were fully densified after sintering at 1900°C. The predominant factors for the thickness effect were mainly derived from the unique characteristics of SPS. The hardness of SiC ceramics was measured using nanoindentation, and it was found to have a strong correlation with the initial thickness, mainly attributed to the densification status. The dense SiC product demonstrated nanoindentation hardness with high values of ∼28.0 GPa.

Tetragonal phase stabilization and densification in AC flash-sintered 1.5 mol% yttria-stabilized zirconia polycrystals with high toughness

For the first time, isothermal pressureless flash sintering of 1.5 mol% yttria-stabilized zirconia (1.5YSZ) polycrystals designed for high toughness was performed in a 1-kHz alternating current field (120 V·cm−1). The parameters governing tetragonal phase stability and densification needed to maximize the toughness of 1.5YSZ were investigated by varying the applied current density limit and holding time. A sample with relative density exceeding 98% and indentation toughness of about 13 MPa·m0.5 was sintered at 1100°C by applying 40 mA·mm−2 to the green body for 2 min, where the average steady-state temperature during flash reached 1330°C. Unlike flash sintering of conventional oxide ceramics, the applied current density and densification were not positively correlated despite pore closure. While Joule heating intensified when applying higher current densities, sample densification was hindered as the accelerated grain growth resulted in intergranular cracks which were generated by spontaneous tetragonal-to-monoclinic martensitic phase transformation during cooling.

Recent advances on innovative bioactive glass-hydroxyapatite composites for bone tissue applications: Processing, mechanical properties, and biological performance

New Hydroxyapatite-Bioactive Glass composites, xHA-(1-x)BG (x = 25, 50, and 75 wt %), are developed using HA and BGMS10 glass powders co-milled up to 2 h prior to Spark Plasma Sintering (SPS). Ball milling (BM) promoted the consolidation of HA-rich powders, whereas hindered the densification of 25HA-75BG samples. HA crystallite size is reduced from > 200 nm (unmilled) to 60 (x = 25 %) or 88 nm (x = 75 %) when using 2 h milled mixtures. Glass crystallization occurred in 75HA-25BG samples processed by SPS at 950 °C: a negligeable effect in the amount of the residual amorphous phase (12.3–13.3 wt %) is produced by BM, while changes are observed in the relative content of crystalline phases, with SiO2 increases from 8.5 to 13.1 wt %, whereas α- and β-CaSiO3 correspondingly decrease. Superior Young’s modulus and Vickers hardness (130 GPa and 726, respectively) are obtained in HA rich products. Biological tests evidenced that the milling treatment does not determine negative consequences on cells viability.

Response surface analysis, tensile properties, and microstructure of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si fabricated by laser powder bed fusion

In this work, Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy samples under different laser process parameters were successfully fabricated by laser powder bed fusion technology. The influence of three processing parameters (laser power P, scanning speed V, and hatch spacing H) on the forming quality and tensile properties of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si samples was investigated by response surface analysis. The Non-Dominated Sorting Genetic Algorithm-II was employed to optimize and attain laser process parameters with optimal forming quality and tensile properties. Specifically, the response surface was established to reveal the optimization method of two response values (forming densification and ultimate tensile strength). The results demonstrated that hatch spacing (H) and its secondary influencing factor (H2) exerted significant effects on densification. In addition, the secondary influencing factors of laser power and hatch spacing (P2 and H2) exerted significant effects on the ultimate tensile strength of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si samples. The influence mechanism of laser process parameters on the densification and tensile properties of samples was further illuminated from the perspective of melting instability and the grain growth process. The maximum tensile strength of the Ti–6.5Al–3.5Mo–1.5Zr–0.3Si sample obtained after optimization reached above 1300 MPa. The maximum strain of the Ti–6.5Al–3.5Mo–1.5Zr–0.3Si sample with the optimal plastic performance reached 16.6%. The strength and toughness of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si samples were analyzed from the aspects of the microstructure and phase composition.

Is the in-situ ZnAl2O4 formation an alternative for magnesia-alumina spinel zero-carbon shaped refractories?

The ever-increasing demand for cleaner steels is the driving force for developing zero-carbon bricks. These refractories have some advantages such as lower thermal conductivity and no direct CO2 emissions, reducing thermal losses and environmental impacts related to their use. Aiming at enhancing some properties of these refractories, the addition of spinel-like inducers (MgO or ZnO) was compared. These oxides react with alumina forming MgAl2O4 and ZnAl2O4, respectively. The spinel formation of the former phase is well-known to be a counter-diffusional solid-state reaction, in which the Kirkendall effect leads to residual porosity. This work assessed the diffusion mechanisms in the Al2O3–MgO and Al2O3–ZnO systems, addressing the role played by the Kirkendall effect on the physical and thermomechanical properties of shaped refractory. Supported by XRD analyses, the early gahnite formation, which starts close to 800 °C and is fully completed at 1200 °C, was attested. This fast spinelization, the thermal expansion mismatch between ZnAl2O4 and Al2O3, and the residual porosity due to the Kirkendall effect led to a higher thermal shock damage resistance. Further analysis of the microstructural features highlighted that a homogeneous sintering reaction was carried out between the MgO and Al2O3 oxides. However, for the ZnO-containing composition, substantial voids were present between the matrix and aggregates due to the Kirkendall effect, inhibiting the sample densification. Additionally, a coating was formed at the alumina-coarse boundaries, indicating a high ZnO diffusion from the matrix to the aggregates. This resulted in higher porosity, and thus diminishing the mechanical strength, which could not be suitable for the working linings of steel ladles. However, controlled generation of residual porosity enhanced the thermal shock damage resistance. Moreover, the addition of ZnO can help to anticipate the in-situ spinalization for applications in the aluminum industry or to produce pre-reacted spinel-like ZnAl2O4 raw materials.

INFLUENCE OF DOUBLE CALCINATION-MILLING ROUTE ON THE STRUCTURAL AND MICROSTRUCTURAL PROPERTIES OF LEAD-FREE K0.5NA0.5NBO3 (KNN) CERAMICS

Potassium sodium niobate (KNN) has always been one of the most potential candidates to replace lead-based piezoelectric ceramics due to its strong piezoelectric properties and environmentally friendly composition. A strong piezoelectric property is constantly influenced by the sample's densification as well as its microstructural characteristics. One of the current main issues with this KNN lead-free piezoelectric material is the difficulty in creating high-density samples by conventional preparation and sintering. Thus, KNN lead-free ceramics were synthesised using an improved solid-state method by introducing the double calcination-milling route to this process. The outcome demonstrates that, despite the presence of additional KNN secondary phases, the double calcination-milling approach contributed to the early creation of the KNN phase. When sintered pellets are subjected to a double calcination milling process, the XRD pattern revealed that the main peaks of the sample are indexed to orthorhombic K0.5Na0.5NbO3. The double calcination KNN pellet have a relative density of 90% densification which is slightly higher than that of single calcination KNN pellet which shows 88% densification.

Effect of Hot Isostatic Pressing of Water Atomized AISI 316L Manufactured by Laser Powder Bed Fusion

The objective of this work is to study the possibility of obtaining dense parts using water atomized AISI 316L steel powder in the L-PBF process. Despite its irregular, non-spherical, particle morphology, it has a significantly lower cost. 25 samples were produced varying the laser power and the scanning speeds to determine the optimal processing conditions. Additionally, hot isostatic pressing (HIP) was performed after the L-PBF process to further increase densification. Selected samples were subjected to microstructural characterization. The best densification results obtained were for the sample produced with the laser power of 173 W and scanning speed of 600 mm/s, where densifications close to 98% were obtained. HIP post-processing promoted increased densification of samples with closed porosity, allowing samples with densification above 95% to reach values close to 100%. HIP did not promote the closure of open pores. The results indicate that the use of water atomized AISI 316L in the L-PBF process combined with post-processing by HIP can produce dense engineering components and at the same time reduce the production costs of the manufactured components, mainly because it is a lower cost raw material when compared to the commonly used feedstock obtained by gas atomization.

Effect of Ultra-High Pressure Sintering and Spark Plasma Sintering and Subsequent Heat Treatment on the Properties of Si3N4 Ceramics.

In this study, coarse Beta silicon nitride (β-Si3N4) powder was used as the raw material to fabricate dense Si3N4 ceramics using two different methods of ultra-high pressure sintering and spark plasma sintering at 1550 °C, followed by heat treatment at 1750 °C. The densification, microstructure, mechanical properties, and thermal conductivity of samples were investigated comparatively. The results indicate that spark plasma sintering can fabricate dense Si3N4 ceramics with a relative density of 99.2% in a shorter time and promote α-to-β phase transition. Coarse β-Si3N4 grains were partially fragmented during ultra-high pressure sintering under high pressure of 5 GPa, thereby reducing the number of the nucleus, which is conducive to the growth of elongated grains. The UHP sample with no fine α-Si3N4 powder addition achieved the highest fracture strength (822 MPa) and fracture toughness (6.6 MPa·m1/2). The addition of partial fine α-Si3N4 powder facilitated the densification of the SPS samples and promoted the growth of elongated grains. The β-Si3N4 ceramics SPS sintered with fine α-Si3N4 powder addition obtained the best comprehensive performance, including the highest density of 99.8%, hardness of 1890 HV, fracture strength of 817 MPa, fracture toughness of 6.2 MPa·m1/2, and thermal conductivity of 71 W·m−1·K−1.

Pressureless flash sintering of α-SiC: Electrical characteristics and densification

The application of an electric field over a ceramic powder compact at elevated temperatures gives rise to “flash sintering”. The phenomenon occurs at a critical combination of field and temperature, which leads to a rapid increase in the electric power dissipated within the sample. This results in sample densification in far shorter timescales than those required with conventional sintering. This paper reports the first successful pressureless flash sintering of SiC with B and C sintering aids. Mean relative densities of up to 94.4% have been achieved in several minutes using an alumina tube furnace at 1500 °C. The sample temperature during flash sintering was similar to those used for conventional sintering of SiC (2100–2200 °C). The electrical response of SiC during the flash event was consistent with thermal runaway. However, the negative temperature coefficient of resistivity responsible for the runaway originated primarily from the effects of sintering, in contrast to the case with oxide ceramics. The rapid sintering was attributed to the rapid heating and the formation of a liquid phase. The densest specimen had a grain size of 5.9 µm ± 0.5 µm and a Vickers hardness (HV5) of 24.7 GPa ± 0.5 GPa. These values were similar to those of conventionally sintered specimens of the same powder but the production time of the flash sintered samples was reduced by more than 6 h and with a furnace temperature lower by 700 °C.

Effect of Sintering Temperatures, Reinforcement Size on Mechanical Properties and Fortification Mechanisms on the Particle Size Distribution of B4C, SiC and ZrO2 in Titanium Metal Matrix Composites.

Titanium metal matrix composites/TMMCs are reinforced ceramic reinforcements that have been developed and used in the automotive, biological, implants, and aerospace fields. At high temperatures, TMMCs can provide up to 50% weight reduction compared to monolithic super alloys while maintaining comparable quality or state of strength. The objective of this research was the analysis and evaluation of the effect/influence of different sintering temperatures, reinforcement size dependence of mechanical properties, and fortification mechanisms on the particle size distribution of B4C, SiC, and ZrO2 reinforced TMMCs that were produced and fabricated by powder metallurgy/PM. SEM, XRD, a Rockwell hardness tester, and the Archimedes principle were used in this analysis. The composites’ hardness, approximation, tensile, yielding, and ultimate strength were all increased. As the composite was reinforced with low-density ceramics material and particles, its density decreased. The volume and void content in all the synthesized specimens is below 1%; this is the result of good sample densification, mechanical properties and uniform distribution of the reinforced particle samples; 5% B4C, 12.5% SiC, 7.5% ZrO2, 75% Ti develop higher mechanical properties, such as higher hardness, approximation tensile, yielding, and ultimate strength and low porosity.

Improvement of the thermal properties of alkali-activated fly ash after high temperature by the Fe-based solid wastes

Compared with Portland cement, AAM was selected as the high-temperature resistant material. But there is still a trend toward a significant reduction in compressive strength at 600 ℃ and 800 ℃. Thus, in this paper, the Fe-based solid wastes, i.e., copper slag and iron tailing, were used to improve the compressive properties of fly ash-based AAM after high temperatures. The result shows that the residual compressive strength of the Fe-based solid waste samples is higher than that of the blank sample. The residual compressive strength increases by about 20 % at 800 ℃. Though the results of SEM-EDS, MIP, and XPS experiments, the mechanism of the effect of Fe-based solid wastes on the AAM could be summarized as follows: the iron element in the Fe-based solid waste samples is more concentrated after high temperature, which may be caused by the melting of the original iron mineral phase into a new mineral phase, filling the crack gap. And the formation of the Fe-O-Si amorphous phase would improve the densification of samples, resulting in the improvement of compressive strength.