Capsule-free hot isostatic pressing (HIP) represents a cost-effective variant to classical HIP allowing production of ceramics with top properties. For successful application of capsule-free HIP it ...is necessary to close all pores to prevent infiltration of the high-pressure gas into ceramics. This occurs when the sample completely transits from 2nd to 3rd stage of sintering, i.e. all open pores get closed at a density called critical density. A robust experimental study of this transition for several materials (alumina, magnesia-alumina spinel, tetragonal and cubic zirconia) has been carried out and compared with three theoretical models. From theoretical models it follows that critical density is a material parameter depending only on dihedral angle and being independent of particle size, green density and pore size distribution. It varies from 88.1% to 93.7% of theoretical density for studied materials and used models. Measured critical densities agree well with models by Beere and Carter & Glaeser for cubic systems (spinel and cubic zirconia). However, they do not match very well experimental data for alumina (hexagonal system) being in agreement with other open sources data. The sophisticated model by Svoboda et al. significantly underestimates the critical density for all systems. The reason of disagreement is analyzed in detail and several hypotheses explaining differences between the model and reality are proposed. It is recommended to use Beere’s and Carter’s & Glaeser’s models for prediction of critical density, while the model by Svoboda et al. characterizes rather the stage of pore closing initiation.
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Highly dense magnesium aluminate spinel bodies are usually fabricated using pressure-assisted methods, such as spark plasma sintering (SPS), in the presence of lithium fluoride as a sintering aid. ...The present work investigates whether the addition of transition metal fluorides promotes the sintering of MgAl
O
bodies during SPS. At the same time, such fluorides can act as a source of optically active dopants. A commercial MgAl
O
was mixed with 0.5 wt% of LiF, MnF
, and CoF
and, afterwards, consolidated using SPS at 1400 °C. Although MnF
and CoF
promote the densification as effectively as LiF, they cause significant grain growth.
Hydroxyapatite (HA) and bioglass (BG) ceramics have become of prime importance in bone tissue engineering. Besides the appropriate composition, the microstructure of bone replacement plays a crucial ...role. In the present work, particle composites and functionally graded material (FGM) based on HA and BG prepared by electrophoretic deposition were thoroughly characterised in terms of the preparation method, sintering process, phase composition and microstructure. The sintering was monitored by high-temperature dilatometry in two directions, the sintering rates were calculated, and the overall sintering process was discussed. The SEM showed the continuous change in the microstructure of FGM with gradual interconnected porosity favourable for bio-applications. The fundamental fractographic analysis proved the crack development in FGM related to the sintering process, and the recommendations for the reduction of the crack development were given. The phase transformations during thermal treatment were analysed using X-ray diffraction analysis and deeply discussed.
Tetragonal (3mol% Y2O3) and two cubic zirconia (8mol% Y2O3) as well as alumina green bodies were used for the construction of the Master Sintering Curve (MSC) created from sets of ...constant-rate-of-heating (CRH) sintering experiments. The activation energies calculated according to the MSC theory were 770kJ/mol for Al2O3, 1270kJ/mol for t-ZrO2, 620kJ/mol and 750kJ/mol for c-ZrO2. These values were verified by an alternative approach based on an analysis of the densification rate in the intermediate sintering stage. The MSCs established from the Two-Step Sintering (TSS) experiments showed at high densities a significant deflection from those constructed from the CRH experiments. This deflection was explained by lower sintering activation energy in the closed porosity stage. A new two-stage MSC model was developed to reflect the change in sintering activation energy and to describe TSS. The efficiency of TSS of four materials under investigation was correlated with their activation energies during the final sintering stage.
Four nanometer‐sized zirconia powders stabilized by 3 mol% Y2O3 were used for the preparation of dense bulk ceramics. Ceramic green bodies were prepared by cold isostatic pressing at pressures of ...300–1000 MPa. The size of the pores in ceramic green bodies and their evolution during sintering were correlated with the characteristics of individual nanopowders and with the sintering behavior of powder compacts. Only homogeneous green bodies with pores of <10 nm could be sintered into dense bodies (>99% t.d.) at a sufficiently low temperature to keep the grain sizes in the range <100 nm. Powders with uniform particles 10 nm in size yielded green bodies of required microstructure. These nanoparticle compacts were sintered without pressure to give bodies (diameter 20 mm, thickness 4 mm) with a relative density higher than 99% and a grain size of about 85 nm (as determined by the linear intercept method).
Heating of a ceramic green body is a key step in sintering. We have created inside a spark plasma sintering apparatus pressure-less sintering conditions that allow homogeneous and extremely rapid ...heating. Dense and crack-free zirconia ceramic was sintered at heating rates of up to 500°Cmin-1 and dwell times of 2min. This extremely fast sintering process is accompanied by extremely rapid grain growth, indicating a non classical sintering mechanism. No grain size gradients were observed inside the sintered zirconia ceramics.
Previously, we proposed the hypothesis that similarities in the inflammatory response observed in acne vulgaris and degenerative disc disease (DDD), especially the central role of interleukin ...(IL)-1β, may be further evidence of the role of the anaerobic bacterium
(previously
)
in the underlying aetiology of disc degeneration. To investigate this, we examined the upregulation of IL-1β, and other known IL-1β-induced inflammatory markers and neurotrophic factors, from nucleus-pulposus-derived disc cells infected in vitro with
for up to 48 h. Upon infection, significant upregulation of IL-1β, alongside IL-6, IL-8, chemokine (C-C motif) ligand 3 (CCL3), chemokine (C-C motif) ligand 4 (CCL4), nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF), was observed with cells isolated from the degenerative discs of eight patients versus non-infected controls. Expression levels did, however, depend on gene target, multiplicity and period of infection and, notably, donor response. Pre-treatment of cells with clindamycin prior to infection significantly reduced the production of pro-inflammatory mediators. This study confirms that
can stimulate the expression of IL-1β and other host molecules previously associated with pathological changes in disc tissue, including neo-innervation. While still controversial, the role of
in DDD remains biologically credible, and its ability to cause disease likely reflects a combination of factors, particularly individualised response to infection.
This work demonstrates the carbonization of centrifugally spun polyacrylonitrile (PAN) fibers. Initially, the optimal centrifugal spinning conditions for producing homogeneous PAN fibers were ...identified. Second, the process continued by stabilization and carbonization of PAN to ensure a pure carbonaceous fiber material by eliminating all non-carbonaceous matter. The spun PAN fibers were stabilized at 240 °C in air at a heating rate of 1 °C/min, then carbonized between 600 and 1200 °C in argon at 5 °C/min. After carbonization, the fibers were characterized using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and Raman spectroscopy (RS). The SEM results showed that by increasing the carbonization temperature, the prolonged elimination of other functional groups resulted in the formation of thinner carbon fibers. FTIR spectra of PAN fibers revealed that the peaks associated with C≡N bonds were substantially reduced and C–H bonds were eliminated in the fibers during the stabilization. These reductions are attributed to the cyclization of nitrile groups and the stabilizing process, and increasing carbonization temperatures resulted in flatter FTIR curves, supporting the findings. According to XRD, the structure of PAN was disturbed, as desired, and carbonization led to the formation of broad bumps resulting from amorphous carbon. Raman investigations found that increasing the carbonization temperature from 600 to 1200 °C resulted in no significant R values, suggesting that all fibers had no structural ordering. The study results could be used in many other areas, such as the fabrication of electrodes, supporting catalytic reactions, filter media, and energy.
Newly developed software for calculation of activation energy (Q^sub s^ in the following) of sintering using the Wang and Raj model is presented. To demonstrate the practical potential of the ...software and to evaluate the behaviour of the Q^sub s^ during the sintering process, alumina and cubic zirconia ceramic compacts were prepared from nanometric powders. The results obtained with both materials are in agreement with previously published data calculated by different approaches. In the interval of interest (relative densities from 60 % to almost 100 % of theoretical density), both materials show similar behaviour. Three distinct regions can be seen: the initial constant values of Qs 868 kJ/mol and 762 kJ/mol for alumina and cubic zirconia, respectively; a region containing linear drop of Qs and the final region of constant Qs values 625 kJ/mol and 645 kJ/mol for alumina and cubic zirconia, respectively.
Using time-differential perturbed angular correlation of γ rays, we investigated the electric-field gradient of polycrystalline EuTiO3 and Eu2Ti2O7, with 181Hf(181Ta) as a probe, following different ...thermal treatments. The measurements were performed at ISOLDE-CERN following 80 keV implantation at the Bonn Radioisotope Separator. The experimental results indicated successful induction of different phases in the implantation recovery process at 1273 and 1373 K. These observations were combined with ab initio calculations and X-ray diffraction measurements. A comparison of ab initio calculated electric-field gradients with the measured values discriminates between different structures and defects and rules out many possible cases. The Ta probe at the Ti site in the Eu2Ti2O7 phase is found to be the most probable case of site occupation after annealing at 1373 K, while annealing at 1273 K keeps EuTiO3 in the vicinity of the Ta probe. A discussion of the hyperfine interactions that promote variation in the interaction strength at the 181Ta site is presented.
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