Papers by Keyword: Two-Step Sintering

Abstract: A comparative analysis of the efficiency of zirconia ceramics high-energy electron beam sintering by one-step mode and two-step mode sintering was performed for compacts prepared from commercial TZ-3Y-E grade and plasmo-chemical powders. The electron energy was 1.4 MeV. The samples were sintered in the temperature range of 1100–1300°C. The extent of influence of one-step and two-step sintering mode on the characteristics of sintered ceramics depends on the initial powders. Сorrectly chosen the temperature mode of two-step sintering (T_s1=1300°C t = 15 min, T_s2=1200°C t=1 h) leads to an increase of the density and microhardness values of ceramics relatively considered of results at one-step and two-step mode of sintering.

Abstract: Transparent polycrystalline alumina with high bending strength has been fabricated using two-step pressureless sintering. The microstructure and mechanical property has been investigated by varying the MgO doping concentration and sintering parameters. The results showed that the grain size markedly decreased with the increasing content of MgO addition. Although the density of alumina could be increased by the higher temperature for the first sintering step, the larger grain would depress the bending strength. Hence, the optimum bending strength was achieved with 0.5 wt% MgO with the two-step sintering of 1450°C for 20 min and 1400°C for 20 h. The Vickers hardness of the transparent PCA increased doping content, and the variation tendency was well accelerated with the relative density, indicated that the porosity plays a more important on hardness than the grain size.

Abstract: Designing and processing of the alloplastic bone grafts represent one of the newest trends in bone tissue engineering, solving a lot of trauma problems of the patients simultaneously with technological and economical achievements. Recent developments in the field provide advantageous aspects concerning the internal architecture, mechanical properties and biocompatibility of the alloplastic bone grafts processed by the powder metallurgy (PM) technology. In this respect, the PM biocomposite materials based on hydroxyapatite powder particles reinforced by metallic or ceramic powders afford great benefits combining classic PM processes with different foaming techniques. The obtained biocomposites present special morphological and structural features matching the genuine bone tissue to be grafted, cortical respectively trabecular. This study focuses on the mechanical testing of the hydroxyapatite-based biocomposites reinforced by different foaming agents, specifically TiH2, CaCO3 and NH4HCO3 up to 25% mass. The overlapping of the obtained experimental results with those reported by the literature leads to the conclusion that the mechanical response of the PM biocomposites studied in this paper may play as a selection criteria to depict their application in hard tissue engineering.

Abstract: Porous hydroxyapatite (HAp) based biocomposites used in biomedical applications were developed in this paper as artificial bone substitute for vertebrae grafts. The trabecular or spongy bone tissue is an open-cell porous network having 40-90% porosity. In order to obtain a porous structure calcium carbonate and ammonium bicarbonate were added as foaming agents to the biocomposite matrix. The biocomposite matrix was prepared using submicronic powder particles of HAp and titanium hydride powder, which was added as reinforcement’s precursor as well as foaming agent. The aim of this study is to emphasize the effect of the foaming agent additions on the pores morphology by performing analyses at both macro and micro scale. These analyses consist of linear measurements, surface auto-detection and statistics of the pores and optical images as well. The powders mixture was homogenized in a planetary ball mill, uniaxial cold compacted and then heated in argon atmosphere using the two step sintering technique.

Abstract: The (Ba_1-xCa_x)(Ti_0.92Sn_0.08)O₃ (x = 0 and 0.02) lead-free ceramics were prepared by using different ball-milling method and time (common ball milling 24 h, high energy ball milling 1 and 3 h). The two-step sintering method was used for sintered the samples. The densification, structure, dielectric and piezoelectric properties of the ceramics were investigated. The results showed that Ca addition could reduce sintering temperature effectively and enhanced densification at lower temperature when using the common ball-milling type. The XRD patterns of (Ba_1-xCa_x) (Ti_0.92Sn_0.08)O₃ ceramics revealed the change in crystal symmetries from tetragonal to cubic phase with increasing Ca content from 0 to 0.02. However, the crystal structure did not change due to the effect of ball-milling method. Ca incorporation caused a decrease of the grain size and Curie temperature. Moreover, the largest grain size was found in the x = 0 ceramics with 24 h-common ball-milling, whereas the small size (< 5 µm) was obtained for the x = 0.02 ceramics. The dielectric and piezoelectric properties were also affected to Ca addition and ball-milling. High piezoelectric coefficient of d₃₃ = 173 pC/N, dielectric constant ε_r ~ 3200 and dissipation factor tand ~ 0.05 were obtained for the x = 0 sample with high energy ball-milling for 1 h.

Abstract: Ceramic materials based on cerate-zirconates have potential application as a solid electrolyte in electrochemical devices such as fuel cells and hydrogen sensors. To become a good electrolyte, the material must exhibits high density sample with homogenize grain size. In this study, we systematically investigated the microstructure of Ba (Ce,Zr)O₃ pellet as a function of sintering temperature. The calcined powder was compressed to become a pellet using hydraulic press with pressure of 5 tons for 5 min. Two-step sintering (TSS) process was applied for the densification of Ba (Ce,Zr)O₃ pellet. The first temperature profile was fixed at T₁ = 1400°C and the second temperature profile was varied from T₂ = 1150°C, 1200°C, 1250°C, 1300°C and 1350°C, respectively. The sintered sample at T₂ = 1350°C showed the highest density compared to others. Its relative density was 82.50%. The lowest density was 64.38% for the sintered sample at T₂ = 1150°C. However, the densification of pellet showed an irregular behavior at T₂ = 1300°C. It might be related to the de-densification phenomenon due to the presence of carbonate species in the calcined powder. Microstructure of the fracture sintered pellet as observed by a scanning electron microscope (SEM) showed the grain size of the samples was in the range of 149.2–336.4 nm. For a comparison, green pellet and pellet prepared with conventional sintering step (CSS) were also discussed in this study.

Abstract: The paper focuses on the mechanical characterization of porous biocomposites based on hydroxyapatite submicronic powders (< 200 nm), respectively micronic powders particle (30-50 μm) as matrix, reinforced by titanium hybrid powders (15 and 20% mass; 100-150 μm) as foaming agent. Another foaming agent used is calcium bicarbonate powder (5 and 10% mass). The mixture homogenization was made in a Frisch-Pulverisette 6 type planetary mill (n=200 rpm), for 30 minutes. The green compacts were processed by unilateral cold compaction at 150 MPa. The two-step sintering technology (TSS) has been applied to the green parts, on the Nabertherm conventional furnace: at 900°C for few minutes (first step) and at 800°C for 450 minutes, respectively 600 minutes (the second step). The mechanical characteristics (compression modulus G [MPa] and ultimate compression strength σ_UTS [MPa]), were studied using the universal mechanical testing machine INSTRON 3382 and compared with the mechanical characterization of the human bone.

Abstract: Ultra-fine powder of Ba (Ce,Zr)O₃ was prepared by a sol-gel method using metal nitrate salts as pre-cursor. The powder was compacted using hydraulic press with pressure of 45 kg/m² for 5 min to produce four different pellets. They were sintered at two temperature profiles using two-step sintering (TSS) process. The first temperature profile was fixed at T₁ = 1450°C and the second temperature profile was varies from T₂ = 1300°C, 1350°C and 1400°C, respectively. For a comparison, the pellet was also sintered using conventional step sintering (CSS) with temperature of T = 1450°C. Scanning electron microscope (SEM) images for the fracture surface showed that the grain growth for the pellet sintered at T₂ = 1350°C was suppressed compared to others. Majority of it grain size was in the range of 100 to 390 nm. For densification study, all the pellets sintered using TSS profile exhibited lower density than CSS method. Even though TSS method can retard the grain growth but in terms of densification, CSS method is better than TSS.

Abstract: In this work, the (Ba_1-xCa_x)(Ti_1-yZr_y)O₃ lead-free ceramics were prepared by the two-step sintering method, and effects of Ca and Zr contents on phase structure, densification, microstructure, and electrical properties were investigated. It was found that all samples showed orthorhombic phase. The highest density of 5.84 ± 0.01 g/cm³ was achieved in x = 0.05, y = 0.05 sample. The average grain size, phase transition temperature, dielectric and piezoelectric properties significantly decreased by introducing of the Ca/Zr content. The ceramics with a small amount of Ca and Zr maintain good piezoelectric properties, and a lower dielectric loss. The composition with x = 0.05, y = 0.025 demonstrated optimum electrical properties of d₃₃ ∼211 pC/N, T_C ∼ 119 °C, ε_r ∼ 1788, and tanδ ∼ 0.04.

Abstract: Alumina materials are widely used in the industry as cutting tool inserts. In order to improve mechanical properties of the Al₂O₃ matrix, different reinforcing phases are introduced. In the presented work, reinforcements in the form of ZrO₂ particles and SiC nanofibres have been used. ZTA-SiC composites were obtained from 0.5%MgO-20%(3Y₂O₃)ZrO₂-79.5%Al₂O₃ powder and 15% of SiC whiskers. Tested materials have been prepared by means of the two-step sintering (TSS) and spark plasma sintering (SPS). TSS method permits to obtain dense materials with fine-grained microstructure. TSS has been carried out for two different sintering times: 1 hour 20 minutes and 8 hours. Due to the reactivity of the components, the SPS method has been used to shorten sintering times. SPS has been performed at two different temperatures: 1500 °C and 1550 °C.