Free Sintering Research Articles

Performance Assessment of Partially Pre-fused Synthetic Flux in Basic Oxygen Steel Making

Lump lime as the most common flux and iron ore as a coolant are used in basic oxygen steel making. However, high melting point, poor dissolution property, fines generation tendency and hygroscopic nature of lump lime often create problems in operation. As the combination of both iron oxide (Fe2O3) and CaO shows eutectic at 1 230 °C, a combined mass of iron oxide and lime melts at lower temperature and dissolves faster in a molten bath. A partially pre-fused synthetic flux (PSF) was prepared through an innovative way in combination of iron oxide fines viz. Linz Donawitz converter sludge and blast furnace flue dust and lime fines by micro-pelletization of the mix followed by coke breeze free sintering. The developed PSF shows good cold handling strength, low melting point (1 180 °C), good thermal shock resistance, etc. As a low melting synthetic flux, its performance was assessed through dissolution/melting study in hot metal bath and refining of hot metal in a simulated bottom blown converter using (i) PSF, (ii) only lump lime and (iii) lump lime with iron ore when keeping other conditions identical. Very fast dissolution (27–80 s for 1–3 g lumps), enhanced removal of C and P (11–12 min), controlled slag foaming, and reduced oxygen consumption was obtained for using PSF.

Lead-free BFN ceramics doped by chromium, lithium or manganese

The material of the study was lead-free BaFe0.5Nb0.5O3 ceramics subject to modification. The base composition BaFe0.5Nb0.5O3 as well as the chromium, lithium and manganese modified ones were obtained using conventional mixed oxides and carbonates method. Synthesis was performed by the powder calcination method at high temperature 1250 °C for 4 h, while the densification was carried out by free sintering method under conditions 1350 °C/4 h. The paper presents a complex study of admixtures influence on the crystal structure, microstructure and dielectric properties of the BFN type samples. The mentioned dopants chromium, lithium or manganese in the BFN-type ceramics among other caused the reduction of the electric permittivity maximum as well as significant decrease in value of dielectric loss.

Investigation of inhomogeneous shrinkage of partially crystallizing Low Temperature Co-fired Ceramics (LTCC)”

The aim of our study is an improved level of geometrical design fidelity of LTCC microsystems by consideration of location-dependent shrinkage under free sintering conditions. To achieve this, the commercial low temperature co-fired ceramic (LTCC) systems: DuPont GT 951, DuPont 9k7 and Heraeus CT 702 have been investigated experimentally with special regard to the local lateral shrinkage over a 4 inch × 4 inch substrate typically used for the fabrication in multiple panels. Partially crystallizing LTCC materials reveal a significant local dependence of the lateral shrinkage. This dependence is introduced as a shrinkage field. Furthermore, we present an approach for shrinkage field approximation and forecasting. Applied to shrinkage allowance, it allows for minimizing the geometrical deviations after sintering from designed dimensions. Alternatively to a design adapted to the forecasted shrinkage field, local shrinkage deviations can be minimized by sintering for homogeneous shrinkage. Detailed analyses of the DuPont GT 951 system indicate a strong influence of the temperature profile during the free sintering on the shrinkage field. By utilizing this influence, a homogenous shrinkage field can be achieved for optimized heating rate and dwell time at the peak temperature 850 °C.

Internal Friction In The PFN Ceramics With Chromium Dopand

Abstract An aim of this work was to determine an influence of an admixture, the chromium (for x from 0.01 to 0.06), on the mechanical properties of the PFN ceramics. The ceramics with chemical composition Pb(Fe0.5−xCrxNb0.5)O3 was synthesized in two steps from simple oxides PbO, Fe2O3, Nb2O5, Cr2O3. The first stage was based on obtaining the FeNbO4 from the Fe2O3 and Nb2O5 simple oxides. At this stage an admixture in a form the Cr2O3 chromium oxide was added to the solution. In the second stage the PbO lead oxide and the doped FeNbO4 (obtained earlier) were synthetized. The sintering of ceramic samples PFCN type was carried out by free sintering method. Temperature measurements of the internal friction were conducted on a computer-controlled automatic resonant mechanical spectrometer (heating cycle with 3 deg/min).

Mechanical and elastic properties of new silicon nitride ceramics produced by cold isostatic pressing and free sintering

The mechanical and elastic properties of new silicon nitride fabricated by cold isostatic pressing and free sintering are considered. Young׳s modulus, Poisson׳s ratio and shear modulus of investigated ceramic were determined. An indentation technique for measuring the indentation modulus was developed. The flexural strength, the compressive strength and the fracture toughness were investigated. Young׳s modulus of the obtained material was 240GPa by resonance and 214GPa by mechanical tests. The microhardness was 1380 HV0.5 and 970 HK2.5. Obtained silicon nitride ceramic is one of a candidate material for a wide range of structural applications due to its high density (2.94g/cm3) and low open porosity (0.1%).

Microstructure evaluation of titanate based layered perovskites: constrained vs. free sintering

Microstructure evaluation of titanate based layered perovskites: constrained vs. free sintering

Effects of Surface Diffusion and Heating Rate on First‐Stage Sintering That Densifies by Grain‐Boundary Diffusion

This paper presents a computational study of the role played by surface diffusion on first‐stage sintering of powders that densify by grain‐boundary diffusion. Coupled grain‐boundary and surface diffusion is considered as the mechanism for matter redistribution. By using several novel approaches of presentation of the numerical data, it is shown that the assumption of fast surface diffusion is invalid for typical sintering conditions and materials in first stage of sintering. The study reveals a simple explanation for the role played by surface diffusion in matter redistribution of combined grain‐boundary and surface diffusion—surface diffusion changes direction from moving atoms away from a contact neck to depositing atoms onto it as the rate of surface diffusion increases. The reverse surface diffusion blunts the neck and retards densification. It is shown that this mechanism is significant not only for free sintering but also for pressure assisted sintering. It is further confirmed that the widely observed beneficial effect of spark plasma sintering on densification can be, at least partially, attributed to its fast heating rate, which quickly passes through sintering at low temperatures where surface diffusion dominates.

Modeling the Microstructural Evolution During Constrained Sintering

A numerical model able to simulate solid‐state constrained sintering is presented. The model couples an existing kinetic Monte Carlo model for free sintering with a finite element model (FEM) for calculating stresses on a microstructural level. The microstructural response to the local stress as well as the FEM calculation of the stress field from the microstructural evolution is discussed. The sintering behavior of a sample constrained by a rigid substrate is simulated. The constrained sintering results in a larger number of pores near the substrate, as well as anisotropic sintering shrinkage, with significantly enhanced strain in the central upper part of the sample surface, and minimal strain at the edges near the substrate. All these features have also previously been observed experimentally.

Ferroelectric-Ferromagnetic Solid Solutions of PZT-Ferrite Type

In presented work the ferroelectric-ferromagnetic solid solutions based on multicomponent PZT type ferroelectric powders and ferromagnetic powder (chromium-manganese-zinc ferrite) were designed and obtained. In solid solutions the ferroelectric powders have 90 percent share, while ferrite powders have 10 percent share. The syntheses of mixed components of solid solutions were performed using the solid phase method and final densification was performed by free sintering (compacting method).The XRD, the microstructure, dielectric and magnetic (and electrical hysteresis loop) investigations were performed. Obtained results shown that solid solution PZT-ferrite type shown at low temperatures coexistence the magnetic and the electrical subsystem. Such properties of these materials give the possibility to use them to construct magnetoelectric transducers.

Preparation and characterisation of porous composite biomaterials based on silicon nitride and bioglass

The combination of bioinert and bioactive material offers new potentialities in bone tissue engineering. The present paper deals with preparation of novel biomaterial composite based on silicon nitride (Si3N4) and bioglass (in amount of 10 and 30wt%) by free sintering at 980°C for 1h in nitrogen atmosphere. The obtained material was characterised by differential thermal analysis (DTA) and X-ray powder diffraction (XRD), porosity and pore size distribution were evaluated by means of mercury intrusion porosimetry (MIP). The bioactivity was examined in vitro with respect to the ability of hydroxyapatite layer formation on the surface of materials as a result of contact with simulated body fluid (SBF). All composites were studied by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) before and after immersion in SBF. The bioglass-free sample was prepared as a reference material to compare the microstructure and bioactivity to the composites.

Preparation and characterization of a self-supporting inorganic membrane based on metakaolin-based geopolymers

This paper is focused on a new self-supporting inorganic membrane prepared by a one-time cast molding process with geopolymer paste. This new preparation method has the advantages of easy fabrication, free sintering and low cost compared with the conventional inorganic membrane preparation methods. This work explores the influence of the H2O/Na2O molar ratio on the strength and porous structure of the inorganic membrane. A linear relationship between the permeated water flux and the membrane thickness or operating pressure was observed after a filtration test. By optimizing processing parameters, this study obtained an inorganic membrane (with the H2O/Na2O molar ratio of 18) between the ultrafiltration membrane and the microfiltration membrane with a high compressive strength of 20MPa, a pore size distribution of 20 to 100nm, a rejection rate of 100% for nano-Al2O3 particles and a permeated water flux of 185kg/m2h when the operating pressure was 0.3MPa and the membrane thickness was 5.0mm. This membrane will be a good choice for the field of water treatment with membranes.

Densification of potassium-doped tungsten during sintering

Tungsten is traditionally sintered at very high temperatures. The master sintering curve (MSC) for densification is a functional model that describes sintering under an arbitrary time–temperature regime of a particular material during heating. The MSC for potassium-doped tungsten (W-K) has been determined by fitting experimental relative density data results versus work of sintering data with a modified sigmoid function. Five independent parameters of the fitting function are identified by minimizing error in terms of mean residual square. For measurement of relative density during sintering, we developed a non-contact high temperature dilatometer experiment using an optical method. Densification was continuously recorded at constant heating rates of 5, 10, 20, 30 and 40°C/min. The work presented here was carried out to predict and control densification evolution of W-K during free sintering. The results demonstrate, that the MSC model of W-K describes densification independently of selected temperature regime.

Constrained Sintering of a Low‐Fire, Polycrystalline Bi2(Zn1/3Nb2/3)2O7 Dielectric

The densification behavior of a low‐fire, polycrystalline Bi2(Zn1/3Nb2/3)2O7 (BZN) dielectric under constrained sintering at 800°C–900°C is investigated. Although the constrained densification is retarded in relative to free sintering, a high sintered density of >95% is obtained at 900°C. No significant anisotropy with similar grain sizes is developed under free and constrained sintering. The densification behavior and stress development during constrained sintering of BZN is thus analyzed by using the well‐known isotropic constitutive laws.

Electromigration Phenomena in Sintered Nanoparticle Ag Systems Under High Current Density

Electromigration (EM) refers to the movement of atoms inside a conductor due to momentum exchange with the conduction electrons. In this work the EM effect in samples of porous Ag fabricated from nanoparticles of Ag in a pressure free sintering process is studied. Current densities of 2.5×104 − 1.7×105 A/cm2 were applied to the samples for periods ranging up to 500 h. In a typical EM setup with a non-porous conductor, void formation occurs at the cathode and hillock formation at the anode. In this study, voids were not directly observed, but cracks were formed after prolonged electromigration, presumably as a result of void accumulation and coalescence. When the samples were placed in 150 °C ambient no hillocks were observed, but at room temperature nanorods were formed with sizes ranging up to 20 μm in length, typically 25 nm in diameter and with aspect ratios ranging from 20 to 1000. It was found that interrupting and restarting the current resulted in growth of new nanorods rather than growth of existing ones, and that growth was limited by welding of individual nanorods when a critical number density was reached. While similar nanorods have been formed from Ag thin films using thermal stress , the location of nanorods was unusual in that while the number density was highest at the anode, significant numbers also appeared at central and cathode locations. Another unusual feature of the observed EM was that the initial porous structure became refined with coarse pores and grains transforming into a fine grained and fine pored structure with elongated and locally orientated pores and grains. Elemental composition studies provide tentative understanding of the nanorod number density, size distribution and growth mechanism. In the geometry utilized for this study, temperature gradients are known to strongly influence the divergence of the EM induced atomic flux and hence resistivity measurements and COMSOL Finite Element modelling was used to determine the temperature in the sample taking into account joule heating, convection and conduction processes.

Development Modes of Prior Free Sintering to Form a High-Density Fine Grain Ceramics after HIP

In this paper the two methods of pre-operation green samples to hot isostatic pressing (HIP) were investigated. The influence the method of formation (uniaxial and cold isostatic pressing) and the temperature-time parameters on property of samples was analyzed. Structural investigation was carried out and physical properties of samples were defined. The recommendations for pre-operation and uses of sintering modes samples for HIP were given.

Effect of TiH2 in the preparation of MMC Ti based with TiC reinforcement

Many studies were carried out on the elaboration Metal Matrix Composites (MMCs) and a wide variety of process is reported in the bibliography. For titanium based MMC, the basis material for these elaboration techniques mainly consists of atomized titanium powder.In this work a titanium hydride powder is used to elaborate Ti/TiC MMC. Although an additional dehydrogenation operation is required a significant decrease of the sintering temperature is expected with this basis powder. In this context, the behavior of titanium hydride powder mixed with 0, 10 and 20vol.% TiC reinforcement is studied during densification by free sintering. The effects of particle size, temperature and rate of sintering reinforcement are discussed. The comparison of the TiH2 process with Ti HDH (Hydride Dehydride) and atomized Ti mixture is made with 10vol.% reinforcement. The results indicate that the sintering temperature is lowered and the final densities achieved are higher if the hydride is used. Interactions between dehydrogenation and sintering mechanisms clearly appear for the higher sintering temperature rate (10°C/min) and need specific attention to prevent porosity nucleation through hydrogen entrapment.

The magnetoelectric coupling effect in multiferroic composites based on PZT–ferrite

In the multiferroic materials, the dielectric and magnetic properties are closely correlated through the coupling interaction between the ferroelectric and magnetic order. We attempted to determine the values of magnetoelectric coupling coefficient , from the temperature dependences of the dielectric permittivity for the ferroelectric–ferromagnetic composite PZT–ferrite type, namely PSZTC–NiZn and PBZTN–NiZn. The main component of the ferroelectric–ferromagnetic composite was PZT type powder (with ferroelectric properties), which was synthesized using sintering of a mixture of simple oxides in solid phase. The second element of the ferroelectric–ferromagnetic composite was the ferrite powder (with ferromagnetic properties). Ferrite powder was synthesized using calcination. Next, the mixed components were synthesized using sintering of the mixture of simple oxides in a solid phase (compaction by a free sintering method). The temperature dependences of the dielectric permittivity ( ε ) for the different frequencies and for both multiferroic composites were investigated. Based on dielectric measurements and theoretical considerations, the values of the magnetoelectric coupling coefficient were specified. • The magnetoelectric effect at two different ferroelectric–ferromagnetic composites based on a PZT and nickel–zinc ferrite. • Multiferroics composite incorporate both ferroelectric and magnetic phases. • The mechanism of the magnetoelectric coupling between ferroelectric and magnetic properties, in multiferroic composites, is caused by the strain. • The determination of the magnetoelectric coupling coefficient based on a theoretical model and the measurements of dielectric permittivity.

Fabrication and Test of High-Temperature Ceramic Transformer

This work describes the fabrication and test of a high temperature (+200°C) capable high frequency transformer. It was manufactured using Low Temperature Co-fired Ceramic (LTCC) technology, which allowed the complex multilayer structure of ceramic and metal windings to be formed. However, the selected LTCC composition is a free sintering ceramic and there is an interaction between the metal conductor and the ceramic substrate during lamination and firing that can lead to significant deformation, presenting a significant engineering challenge. Here the fabrication process for the LTCC is described (screen printing, collation, lamination and firing) for a number of iterations of the transformer design, each of which was analysed for deformation and subjected to electrical tests. In addition a silicone adhesive for assembling the LTCC with the transformer was analysed for high-temperature performance. A test vehicle was assembled and it was subjected to 1000 hours at 210°C. Shear tests were performed at intervals to quantify the loss in bond strength over time. After a good solution for manufacture was found, a batch of transformers was produced, characterized and tested to demonstrate a high reproducibility and manufacturing yield.

Ferroelectric–Ferromagnetic Ceramic Composites Based on PZT with Added Ferrite

The study presents a ferroelectric–ferromagnetic composite based on a doped PZT‐type and ferrite powders. Ferroelectric powder (in amount of 90 wt‐%) was based on multicomponent PZT‐type materials, while nickel–zinc ferrite Ni1–xZnxFe2O4 (in amount of 10 wt‐%) served as the magnetic component of the composite. The syntheses of the ferroelectric–ferromagnetic composite's components were performed using solid‐phase sintering, while final densification of the synthesized powder was achieved using free sintering. X‐ray analysis of the composite confirmed the presence of strong maxima originating from particular PZT‐type material phases, as well as weak peaks from the Ni0.64Zn0.36Fe2O4 ferrite, without foreign phases. The microstructure of the fracture of the ferroelectric–ferromagnetic composites shows that the ferrite grains on the surface of the ferroelectric component are distributed heterogeneously. Magnetic studies have characterized composite as a soft ferromagnetic material. The study indicated the influence of the magnetic subsystem on the electrical properties. In the two‐phase PBZTMC–NZF ceramic composite, the magnetic component causes the decrease in electric permittivity and increased value of the dielectric losses.

Combined effects of interstitial and Laplace pressure in hot isostatic pressing of cylindrical specimens

Sintering of precompacted metallic and ceramic micro and nanopowders is a complex problem influenced by several factors. We quantify the influence of both local capillary stresses acting at the surface of one pore or particle (usually referred to as Laplace pressure) and the gas pressure in pores during sintering of precompacted metallic (micro/nano)powdered cylinders. The latter influences only the third phase of sintering, that is, the phase in which the porosity is closed. The isostatic pressing loading mode, which also covers the case of free sintering, is considered. Whereas the Laplace pressure is demonstrated to have a beneficial effect on sintering, the gas pressure acts against the reduction of the porosity, causing an increase in sintering time. This contribution could reach the sum of the stress due to loading and the interstitial pressure, thereby preventing the desired porosity to be reached. For the sake of illustration, a specific aluminum-zinc-magnesium-copper alloy is examined in this paper. The purpose is to estimate the effects of sintering time and residual porosity and to determine thresholds under which the contributions described above are negligible. In order to determine the effects of Laplace and gas pressure in pores on the stability of the process, a high-order perturbation analysis has been performed.