Hot-pressed Samples Research Articles

Y3Al5O12-α-Al2O3 composites with fine-grained microstructure by hot pressing of Al2O3-Y2O3 glass microspheres

Yttrium aluminate glass microspheres with the eutectic composition 76.8 mol. % Al2O3 and 23.2 mol. % Y2O3 were prepared by combining the sol-gel Pechini method with flame synthesis. The sol-gel method was applied to achieve the desired composition homogeneity of the prepared glass and hence, improve the microstructure homogeneity and mechanical properties of bulk polycrystalline materials. The latter were prepared by hot pressing, more specifically pressure assisted sintering, at 1050 °C, 1300 °C and 1600 °C using pressures of 30 MPa and 80 MPa and holding times between 0 and 30 min. This also led to the crystallization of the glass. A composite with the Vickers hardness 18.0 ± 0.7 GPa and an indentation fracture toughness 4.9 ± 0.3 MPa.m1/2 was obtained by sintering at 1600 °C, at the pressure of 80 MPa and with 30 min isothermal heating at the maximum temperature. Improved mechanical properties were observed when increasing the temperature of sintering and the holding time. This can be attributed to the formation of a unique microstructure consisting of α-Al2O3 grains in the μm-scale embedded in a YAG (yttrium-aluminium garnet) matrix in the hot-pressed samples.

Texture Development and Grain Alignment of Hot‐Pressed Tetradymite Bi0.48Sb1.52Te3 via Powder Molding

Bi2Te3‐based materials possess excellent thermoelectric (TE) properties near room temperature. The microstructure's orientations of these materials have a great influence on their properties owing to the lamellar structure. Herein, a process for the preparation of bismuth telluride, named hot‐pressing texture (HPT) where the hot‐pressed (HP) samples are preformed by powder molding, leads to the fine alignment of grains. The orientation factor of the HPT sample is improved by 20%, compared with that of the HP sample. After further carrier concentration optimization, the improved ZT of 1.1 at 350 K is achieved, with an average ZT of 0.9 between 300 and 500 K. Herein, it is shown that the proposed preparation process is an effective strategy for the fabrication of layered materials.

Effect of hot-pressing sintering on thermal and electrical properties of AlN ceramics with impedance spectroscopy and dielectric relaxations analysis

The effects of hot-pressing sintering on the phase composition, microstructure, thermal and electrical properties of AlN ceramics with CeO2–CeF3 additives were studied. During hot-pressing sintering, high pressure reduced the grain boundary phase CeAlO3 and decreased the concentration of oxygen in AlN ceramics. The hot-pressing sintered AlN samples had a much higher thermal conductivity of 191.9 W/m·K than pressureless sintered ones because of the great reduction of grain boundary phases and oxygen impurities in AlN ceramic. As the carbon content in hot-pressing sintered sample was very high, carbon contamination led to the decrease in electrical resistivity and changes in polarization mechanisms for AlN ceramics. The relaxation peak in the dielectric temperature spectrum with an activation energy of 0.64 eV for hot-pressing sintered samples was caused by electrons from free carbon at low temperature. Overall, hot-pressing sintering can effectively increase the thermal conductivity and change the electrical properties of AlN ceramics.

Comparative study of hot and cold press and their effect on superconducting properties of coronene added bulk MgB2

In this study, the superconducting properties of 4 wt% coronene (C24H12) added polycrystalline bulk MgB2 samples prepared by hot pressing and cold pressing, were reported in detail. The results were compared to determine one of which pressing methods is more useful to substitute of carbon released from C24H12 into MgB2 structure and to increase the superconducting properties of MgB2. The lattice strain, crystallite size, residual resistivity, superconducting transition temperature (Tc), critical current density (Jc), flux pinning force density (Fp) and vertical/lateral levitation force (Fz, Fx) were analysed for the hot and cold pressed samples. The a-lattice parameter and Tc value reduced sharply for 4 wt% C24H12 added cold pressed sample, resulting in a notable decrease of increment ratio of Jc and Fp values, while the depressing of Tc is lower for 4 wt% C24H12 added hot pressed sample. The cold pressing strongly increased the lattice strain, confirming more C substitution, while the hot pressing ensured smaller crystallite size. The Jc and Fp values were respectively found as 4.6 × 103 A/cm2, 1.3 × 104 A/cm2, 5.8 × 103 A/cm2 and 1.8 × 108 N/m3, 5.1 × 108 N/m3, 2.3 × 108 N/m3 for the pure sample and, 4 wt% C24H12 added hot and cold pressed samples at 20 K, 4 T. The maximum repulsive and attractive levitation force values at 20 K in ZFC and FC regimes were obtained for the 4 wt% C24H12 added hot pressed sample. Finally, the hot pressing provides an effective way to substitute C into MgB2 lattice without a remarkable decrease of Tc value, ensuring more homogenous and compacted structure, more electron–phonon interaction and strong flux pinning force.

Evaluation of mechanical and microstructural of AlA380 based nanocomposite rein-forced with Tungsten Disulfide nanoparticles

In the present study, AlA380-based nanocomposites reinforced with WS2 (0, 2 and 5% volumetric) nanoparticles were fabricated using the hot press technique in vacuum at 510 °C. Microstructural and mechanical (e.g. hardness, compression and wear behavior) properties of the AlA380-WS2 nanocomposites were investigated as a function of the WS2 reinforcement percentage. It was found that adding 5% WS2 increased the compressive strength of the hot-pressed samples from 66 MPa to 195 MPa. Furthermore, microhardness of the hot-pressed specimens significantly increased from 50 HV for AlA380 matrix to almost 100 HV for AlA380%-5% WS2 nanocomposite. Moreover, incorporation of lubricant WS2 nanoparticles significantly reduced the wear rate and the friction coefficient of the AlA380%-5% WS2 up to 40% and 32% respectively in comparison with the AlA380 matrix composites. OM, SEM and FESEM analysis were used to investigate microstructures of the nanoparticle distribution in the nanocomposites and the worn and fracture surfaces of the hot-pressed samples. Results showed a considerable change in morphology after adding WS2 nanoparticles leading to an improvement in strength of the nanocomposite. In the end, the AlA380%-5% WS2 nanocomposite sample represents the greatest hardness, compressive and wear properties to compare with the other experimented samples.

Effect of milling time on the density, hardness and corrosion behavior of Al 7150 alloy produced by high energy ball milling

The effect of milling time on the density, hardness and corrosion behavior of Al 7150 alloy compacts produced by powder metallurgy route, was investigated. The various ingredients of Al 7150 alloy were milled for a duration of 5 h, 10 h and 20 h in a planetary ball mill at 300 RPM with a ball-to-powder ratio of 10:1. The blended powders were uni-axially hot pressed in a punch die setup at 400 °C with a compaction pressure of 500 MPa & a compaction time of 1 h before ejecting the samples. The hot pressed samples developed from the as-received and milled Al 7150 alloy powders (i.e., milled for 5 h, 10 h and 20 h) were characterized by measuring its hardness, relative density and electrochemical behavior. The as-received Al 7150 alloy samples showed lower hardness than that of the 5 h, 10 h or 20 h milled Al 7150 alloy compacts. XRD analysis revealed the presence of MgZn2 precipitate peaks, formed as a result of simultaneous application of heat and pressure. The HRTEM observations revealed a very high density of homogeneously dispersed η′ (MgZn2) precipitates in the base matrix & also the η (MgZn2) phases precipitated along the grain boundaries. The existence of second phase precipitate particles in the matrix, enhanced its mechanical properties. The potentiodynamic polarization results indicate that an increase in the milling time of up to 20 h had shifted the Ecorr values towards nobler direction, exhibiting better corrosion resistance of the samples. Optical & SEM micrographs revealed that the pitting damage is more prominent in the as-received Al 7150 alloy samples than the 20 h milled compacts. AFM analysis of the corroded sample revealed a negligible surface roughness of only 36.6 nm (2D view) and a height scale of 200 nm (3D view) indicating a superior corrosion resistance due to 20 h of ball milling.

Development of highly wear resistant Cu - Al alloys processed via powder metallurgy

Abstract The present work explores the effect of aluminium (Al: 0, 3, 5, 10 & 15 wt%) addition on wear behaviour of copper (Cu). The Cu-Al alloys were fabricated via mechanical alloying and high-pressure hot press. In fact, high-pressure processes are beneficial as they result in obtaining fine microstructure and property enhancement. Depending on the amount of Al, the microstructure of hot pressed samples (Cu with Al up to 5 wt%) consisted of copper-rich solid solution phase (Cu0.92Al0.08) and the presence of both Cu0.78Al0.22 and Cu9Al4 intermetallic phases for Cu containing high amount of Al (≥10 wt%). The relative density of Cu-Al alloys varied between ∼95 and 98% and the hardness of Cu enhanced significantly from 1.32 to 6.16 GPa with Al content. The coefficient of friction (COF) (0.54 to 0.16) and wear rate (18.26 × 10−5 to 0.92 × 10−5 mm3/N-m) of Cu decreased with the addition of Al. A transition in wear mechanisms of Cu was observed with Al. In case of pure copper, adhesion was the major wear mechanism. Adhesion and tribo-oxidation controlled the wear of Cu alloyed with Al (up to 5 wt%). Oxide layer formation influenced the wear of Cu-10Al, while abrasion was the predominant wear mechanism for Cu-15Al. These results clearly indicate the advantage of large amount of Al addition to achieve better wear resistance of Cu.

Processing and Characterization of Yttria-Dispersed INCONEL 718 ODS Alloy

Oxide dispersion strengthening is an effective method to improve the high-temperature mechanical properties of the alloys in which various reinforcements are added in the alloy matrix by mechanical milling and consolidating the mixture by different techniques like hot pressing, hot isostatic pressing and spark plasma sintering. This work presents the development of IN 718 ODS alloy by dispersing yttria in IN 718, which has improved mechanical properties at high temperatures. Yttria powder was added to IN 718 alloy powder in different weight fractions of 0.25, 0.5 and 1% and mechanically milled. The composite powders were consolidated by hot pressing and hot isostatic pressing (HIP). A small fraction of isolated porosities in the matrix was observed in hot-pressed samples which caused degradation of the mechanical properties. Percentage of porosities increased with the increase in yttria content. The hot-pressed samples were HIPed to heal the porosities. 0.5 wt% yttria-dispersed Inconel 718 hot-pressed compact exhibited better properties at 850 °C and showed 50 MPa improvement in ultimate compressive strength compared to Inconel 718 alloy.

Thermoelectric and Transport Properties of n-type Palladium-Doped Chalcopyrite Cu1−xPdxFeS2 Compounds

Semiconducting CuFeS2 with a diamond-like structure has been recently studied as a potential candidate for thermoelectric applications. In the present study, Pd substitution for Cu was examined in terms of thermoelectric properties. A series of Pd-doped Cu1−xPdxFeS2 (x = 0–0.1) samples were synthesized. The thermoelectric and transport properties of hot-pressed sample pellets were characterized. We observed three effects: (1) Pd substituted for Cu behaves as an effective donor, increasing the free electron concentration. (2) Formation of the foreign phase—PdS—was observed above the solubility limit of Pd in CuFeS2 (x ≥ 0.02). (3) Segregation of the foreign phase is accompanied by the formation of $$ {\hbox{Fe}}_{\rm{Cu}}^{2 + } $$ antisite defects. All these effects synergically enhanced both the power factor S2·σ and the thermoelectric parameter ZT. The highest values of the power factor (∼ 1 mW m−1 K−1) and the ZT parameter (∼ 0.19 at 573 K) were achieved in the Cu0.9Pd0.1FeS2 sample.

Microstructure-property relations of biphasic calcium phosphate obtained by hot pressing process

Biphasic calcium phosphate (BCP) was fabricated through hot pressing process from hydroxyapatite (HA) starting powder synthesized by a peroxide-based precipitation method and dried at 250?C. The synthesized HA powder contained some calcium hydrogen phosphate (CHP) as a minor phase. In this work, the dense BCP ceramics with a mixture of HA and ?-TCP phases were successfully obtained by hot pressing at temperature ranging from 700-1100?C. The amount of HA phase in the hot pressed samples decreased with increasing sintering temperature from 700 to 900?C, while the amount of ?-TCP exhibited an opposite trend. The relations between the microstructure and mechanical properties of the dense BCP ceramics sintered in the temperature range 700-1100?C were investigated. The sample hot pressed at 800?C has a dense structure with the highest hardness (4.86 ? 0.19GPa) and the lowest fracture toughness (0.80 ? 0.01MPa?m1/2). In addition, biocompatibility of apatite layer formed after immersion in simulated body fluid (SBF) for 14 days can be observed by SEM images. The results revealed that the samples hot-pressed at 800 and 900?C were covered by calcium phosphate (CaP) crystal layer indicating the strong interaction of BCP ceramics and SBF.

ENHANCEMENT OF MICROSTRUCTURE AND THERMAL EXPANSION COEFFICIENT OF AL/NI-SIC COMPOSITE PREPARED BY POWDER METALLURGY TECHNIQUE

Al/Ni-SiC composite was prepared via powder metallurgy technique, in which SiC particles were mixed with 10 wt % nickel powder for 24 hrs, then mixed by three percents 5, 10 and 15 wt.% with Al powder in a ball mill for 5 hrs. Three types of sintering techniques were used to prepare the composite. Uniaxial cold compacted samples were sintered in a vacuum furnace at 600oC for 1 hr. The second group was the vacuum sintered samples which were post-processed by hot isostatic press (HIP) at 600oC for 1hr under 190 MPa pressure. The third one was the hot pressed samples that were consolidated at 550oC under the uniaxial pressure of 840 MPa. The results showed that the hot compacted samples have the highest densification values (97-100%), followed by the HIP samples (94-98%), then the vacuum sintered ones (91-97%). The XRD analysis indicated the presence of peaks correspond to Al and Ni, also Al3Ni intermetallic and Al4Si2C5 phases which indicated that all SiC particles were consumed with Al particles to form Al4Si2C5. Hardness results revealed that hot compacted samples have the highest hardness values. Also the CTE results referred the decreasing of its value by increasing Ni/SiC percent.

Effects of hot and cold-pressed processes on volatile compounds of peanut oil and corresponding analysis of characteristic flavor components

The volatile compounds were concentrated from hot-pressed and cold-pressed peanut oils by HS-SPME and were analyzed by GC-MS-O. In the present study, 101 volatiles were identified in the hot-pressed peanut oil samples, mainly including pyrazines, aldehydes, furans, alcohols and pyrroles, of which characteristic flavor compounds present fresh, fatty, nutty and baking flavor. Sixty-four volatiles were obtained from the cold-pressed peanut oil samples, mainly consisting of aldehydes, alcohols, hydrocarbons, furans and ketones, of which characteristic flavor compounds show fresh and nutty flavor. Our results indicated that hot-pressed peanut oil has an advantage over cold-pressed peanut oil in the retention of flavor components. Hence, the process method has a significant effect on the flavor components of peanut oil, and the present study may be helpful in evaluating peanut oil quality, as well as discrimination of hot-pressed and cold-pressed peanut oil.

Plasma press for improved adhesion between flexible polymer substrate and inorganic material

A novel adhesion process called “plasma press” has been investigated for achieving stronger bonding between two different materials. It was first developed to be applied in multilayer flexible printed circuit boards for next generation flexible mobile electronics. The plasma process is a hot pressing method employing plasma between the two materials in order to activate the material surfaces. Compared to the conventional hot-pressed sample, the plasma-pressed sample exhibited up to 130% higher adhesion strength. The stronger bond strength achieved in the plasma-pressed sample is attributed to the formation of active carboxyl functional groups and dangling bonds on the material surfaces by the presence of the plasma during the hot pressing process for bonding.

Evaluation of Laundering Durability of Electro-conductive Textile Dip-coated on Para Aramid Knit with Graphene/Waterborne Polyurethane Composite

This study aims to fabricate an electro-conductive textile dip-coated with graphene/waterborne polyurethane (WPU) composite, and an evaluation of the laundering durability of this composite was conducted in order to confirm the application of the protective clothing. Samples were coated five times on the para-aramid knit with the graphene/WPU composite by the dip-coating method, and then hot-pressed processes were applied with various temperatures. The samples were washed 5, 10, 15, and 20 times, and then, morphology, surface resistance, and surface temperature were measured. After five laundering cycles, control and hot-pressed samples maintained surface resistivity from 10 kΩ/sq to 100 kΩ/sq. The surface resistivity of the control sample, however, was gradually increased up to 20 laundering cycles. The sample that was hot-pressed at 120 °C showed the lowest value of about 15 kΩ/sq with almost no variation from zero laundering cycles to 20, thus, it is the most stable sample up to 20 laundering cycles. When applied voltage at 50V, the surface temperature of the nonlaundering samples was presented as over 40.0 °C. In the case of the control sample, there was almost no electrical heating performance remaining after the fifth laundering test, but the hot-pressed samples maintained 40.0 °C or more at 50 V after the 10th laundering test. The sample that was hot-pressed at 120 °C in particular could maintain electrical heating performance at about an 80 % level up to the 10th laundering cycle. Therefore, the sample that was hot-pressed at 120 °C was the most stable in terms of electrical properties after 20 laundering cycles, and its electrical heating performance could be maintained even after 10 laundering cycles. It is expected that this process can be applied and used to make functional clothes for apparel and other applications.

Microstructure and Mechanical Properties of Hot-Pressed 21-4N Oxide-Dispersion-Strengthened Austenitic Stainless Steels

In this study, austenitic stainless steels of composition containing Fe–21Cr–4Ni–9Mn–0.5C–0.4Si with and without addition of Y2O3 were developed from iron and ferro-alloy powders (Fe–Ni, Fe–Mn and Fe–Cr) using mechanical alloying. Both the powders were hot-pressed under a vacuum of 10−3 mbar at 1200 °C and at a pressure of 50 MPa. Further, the hot-pressed billets were solution-annealed at 1150 °C for 90 min and subsequently quenched in water. Further the steels were aged at 750 °C for 90 min followed by air-cooled. The powder morphology and crystal structure were analyzed with respect to milling time using scanning electron microscopy and X-ray diffraction, respectively. The density of hot-pressed samples was found to be 96–98% of its theoretical value. The microstructure revealed austenite grains containing grain boundary carbides in the hot-pressed condition. However, the carbide precipitates disappeared after the solution treatment, which was confirmed by scanning electron microscopy with energy dispersive spectrum analysis. The hot-pressed and oxide-dispersion-strengthened austenitic stainless steel and austenitic stainless steel samples have exhibited a hardness value of 349 ± 10 and 327 ± 10 HV1.0, respectively. Aged steels have attained the intermediate hardness values in the range of 324–336 HV1.0.

Direct SPD-processing to achieve high-ZT skutterudites

For mass fabrication of thermoelectric generators a fast, cheap and simple method is required to produce high ZT leg material. It is well known that severe plastic deformation (SPD) enhances the density of defects and dislocations, and also refines the grains to nano-size. These are the essential key parameters for low thermal phonon conductivity. Consequently and as a first example, SPD via high-pressure torsion (HPT) at elevated temperatures in protective gas atmosphere was used to directly consolidate and plastically deform commercial p-type skutterudite powder, DDyFe3CoSb12, (DD stands for didymium) into a dense thermoelectric solid. The HPT-sample exhibited a high figure of merit, ZT > 1.3 at 773 K, much higher than that of the hot-pressed reference sample (HP) from the same powder. The ZT achieved is comparable with ZTs of corresponding high energy ball milled and hot pressed samples (HBM-HP) as well as of ball milled, hot pressed plus HPT processed skutterudites (BM-HP-HPT). The thermoelectric efficiency is even 4% higher. Synchrotron measurements were performed at temperatures from 300 to 825 K in order to evaluate the changes in grain size and dislocation density of the CP-HPT samples before, during and after annealing. SEM and TEM images served to give a better insight into the temperature dependent diversification of the new material. Due to grain refinement and enhanced dislocation density, the CP-HPT material exhibits higher hardness values than the reference sample. The elastic moduli are, within the error bar, the same.With large high-pressure torsion facilities at hand, a new, fast and therefore cheap mass production of thermoelectric leg material directly from powders can be envisaged. This new production technique furthermore warrants high figures of merit but excludes time-consuming ball milling and hot-pressing steps.

Process-Oriented Structure Tuning of PBT/PTHF Thermoplastic Elastomers

The multiscale structure of thermoplastic elastomers (TPEs) made of poly(butylene terephthalate)/polytetrahydrofuran (PBT/PTHF) segmented block copolymers is investigated in a systematic way with the aim to characterize the impact of the processing route on it. By means of atomic force microscopy and small-angle scattering techniques, we first evidence the presence of thick (ca. 11 nm) and well-aligned ribbons in hot-pressed samples, whereas thinner and branched filaments (ca. 5 nm) are observed when solvent casting is used. Besides, differential scanning calorimetry suggests that all the so-formed structures can “remember” their initial state after being melted, a property that we assign to the persistent phase separation of the constituents in the viscoelastic liquid. At a smaller length scale, combining transmission and reflection wide-angle X-ray scattering experiments further reveals the influence of the processing method on the molecular order within the crystallites. In particular, it makes emerge ...

Control of oxygen content of n-type Bi2Te3 based compounds by sintering process and their thermoelectric properties

Oxygen content in n-type Bi2Te3-based alloys plays a crucial role in determining the thermoelectric properties. We aimed to control the residual oxygen that originated from the ball milling process; for this, sintering processes such as spark plasma sintering (SPS) and plasma activated sintering (PAS) were utilized. The reduction of oxygen content was successfully achieved with PAS, leading to improved electrical properties. With the removal of residual oxygen, the carrier concentration increased, resulting in reduced electrical resistivity. The phonon thermal conductivity was reduced due to the grain size of this sample, which was smaller than that of the hot-pressed (HP) samples. An enhanced value of the dimensionless figure of merit (ZT) of 0.83 at 473 K, an increase of 25.2% compared with that of the HP samples, was achieved. The selection of the appropriate sintering process should be considered to remove oxygen content, as well as to minimize grain growth.

Ultrasonic-assisted hot pressing of Bi2Te3-based thermoelectric materials

In this study, n-type Bi2Te3-based bulk materials were prepared using the ultrasonic-assisted hot pressing. The effects of ultrasonic vibration on the mechanical and thermoelectric properties of the prepared samples were investigated. Field-emission scanning electron microscope (FESEM) tests revealed that the ultrasonic vibration refined the grain size to the submicron level. The grain size became smaller as the ultrasonic vibration duration is extended. The results showed that the sample with an ultrasonic vibration duration of 20 min (namely, Ultra 20) has a hardness of 64 HV0.3 and a flexural strength of 22.4 MPa, which represents an 82.9% and 50.2% improvement, respectively, over the untreated hot-pressed sample (namely, Ultra 0). As the ultrasonic vibration duration increased, the absolute value of the Seebeck coefficient was found to increase, whereas the electrical conductivity and the thermal conductivity decreased. The maximum dimensionless figure of merit (ZTmax) was 0.78 at 480 K for the Ultra 20 sample, which was 10.9% and 16.4% higher than the Ultra 10 and Ultra 0 samples, respectively. The results proved that both the mechanical properties and the thermoelectric properties of the Bi2Te2.7Se0.3 bulk materials are improved by ultrasonic vibration.

Anisotropy of Transport Properties Correlated to Grain Boundary Density and Quantified Texture in Thick Oriented Ca3Co4O9 Ceramics

The misfit-layered Ca3Co4O9 oxide is being seen as a potential thermoelectric (TE) candidate for high-temperature power generation in air. Given the very small size and low strength exhibited by single crystals, grain-oriented Ca3Co4O9 ceramics are worth elaborating to capitalize on their anisotropy. However, the usual textured pellets are too thin to probe the TE properties along their principal crystallographic directions. In this paper, we report on the anisotropy of TE properties in the 350–860 K range within thick textured Ca3Co4O9 ceramics fabricated by moderately pressing at 1173 K stacks of pellets primarily textured using spark plasma sintering (SPS), spark plasma texturing (SPT), and hot pressing (HP). The texture was quantitatively assessed, and the influent microstructural parameters were identified, particularly the grain boundary density parallel (GBDc) and perpendicular (GBDab) to the mean c*-axis. We found that the edge-free processing fostered material texturing and (a,b) plane grain growth, thereby dropping GBDab and increasing GBDc. This resulted in a resistivity ρab reduction, leading to a marked enhancement in power factor PFab, which reached 520 μW·m−1·K−2 at 800 K for the HP sample. The anisotropy ρc/ρab was substantially promoted as the texture was reinforced and the GBDc/GBDab ratio increased, with ρc/ρab (HP) > ρc/ρab (SPT) > ρc/ρab (SPS). The Seebeck coefficient S also revealed an anisotropic behavior, with a ratio Sc/Sab >1 for the SPS-processed materials. This behavior was reversed (Sc/Sab <1) for the more textured SPT and HP specimens. It therefore resulted in a PF anisotropy PFc/PFab (HP) < PFc/PFab (SPT) < PFc/PFab (SPS). The PFab/PFc ratio attained 13.6 at 800 K for the thick HP sample, which is the largest ratio recorded thus far on undoped Ca3Co4O9 ceramics.