Microstructure and thermal conductivity of spark plasma sintering AlN ceramics

Abstract

Dense aluminium nitride ceramics were prepared by spark plasma sintering at a lower sintering temperature of 1700°C with Y₂O₃, Sm₂O₃ and Dy₂O₃ as sintering additives respectively. The effects of three kinds of sintering additives on the phase composition, microstructure and thermal conductivity of AlN ceramics were investigated. The results showed that those sintering additives not only facilitated the densification via the liquid phase sintering mechanism, but also improved thermal conductivity by decreasing oxygen impurity. Sm₂O₃ could effectively improve thermal conductivity of AlN ceramics compared with Y₂O₃ and Dy₂O₃. Observation by scanning electron microscopy showed that AlN ceramics prepared by spark plasma sintering method manifested quite homogeneous microstructures, but AlN grain sizes and shapes and location of secondary phases varied with the sintering additives. The thermal conductivity of AlN ceramics was mainly affected by the additives through their effects on the growth of AlN grain and the location of secondary phases.     

添加剂对热压钛酸铝陶瓷性能与结构的影响

ＭｇＯ－Ａｌ２Ｏ３－ＳｉＯ２、Ａｌ２Ｏ３－ＳｉＯ２、Ｓｉ３Ｎ４、ＳｉＣ四种添加剂对热压钛酸铝陶瓷力学与热学性能的影响。结果表明：引入添加剂后钛酸铝陶瓷的抗弯强度得到极大的改善，同时，由于陶瓷结构的变化，其热膨胀系数也明显增加。Ｘ射线衍射结果表明：添加剂在热压条件下与钛酸铝陶瓷发生了反应并形成固溶体。     

Effect of oxide additive in silicon nitride on interfacial structure and strength of silicon nitride joints brazed with aluminium

Silicon nitride ceramics with Y₂O₃ and Al₂O₃ as sintering additives were brazed with aluminium, and the brazed strength and the interfacial structure of the joints were compared with those of the joints made of additive-free silicon nitride ceramics. It is concluded that the additives in silicon nitride ceramics take part in the interfacial reaction, make the reaction layer thicker, and hence increase the brazed strength greatly.     

Influence of AST additives on the stability of PTCR characteristics and microstructure in ferroelectric ceramics

(Ba_0.69Pb_0.31)TiO₃ ceramics were prepared using Al₂O₃, SiO₂, additives and excess of TiO₂ (AST). The characteristics of positive temperature coefficient of resistivity (PTCR) was studied and the corresponding microstructures were investigated using atomic force microscopy and scanning electron microscopy. The results showed that the PTCR effect was related to the AST additives. The maximum value of resistivity in the ceramics with lower content of or without Al₂O₃ and SiO₂ additives was much lower than in those with AST additives. Ceramics with low AST content, which were heated by electric field to a temperature much higher than their Curie temperature, lost the PTCR effect after the electric field stimulation. The microstructure observations revealed that re-crystallization took place in the ceramics with lower content of or without AST additives resulting in the loss of the PTCR effect.     

Bioinerte Implantatkeramiken Teil I. Technologisch relevante Phasenuntersuchungen im Al2O3-SiO2-TiO2-System

Bioinert implant ceramics Part I. Technological relevant phase investigations in the Al₂O₃-SiO₂-TiO₂-system The Al₂O₃-SiO₂-TiO₂-system forms the basis for developing variable, bioinert implant ceramics. This is why its components and their binary systems as well as the ternaryphase diagram for the solid (< 1750 K) and solid-liquid region (1775 K) have been investigated and compared with literature data.     

Corrosion of silicon nitride ceramics in aqueous HF solutions

The leaching behaviours of hot-pressed Si₃N₄ ceramics containing Y₂O₃, Al₂O₃ and AIN as additives and hot isostatically pressed Si₃N₄ without additives were studied in 0.1 to 10 M HF aqueous solutions at 50 to 80° C. Silicon and aluminium ions were dissolved into the HF solutions, but yttrium ion did not dissolve at all and formed insoluble YF₃. The dissolution of silicon and aluminium ions was controlled by the surface chemical reaction and the apparent activation energies were 70.5 to 87.6 kJ mol^–1, respectively. The corrosion rate increased with increasing degree of crystallization of the grain boundary phases. The corrosion resulted in roughness of the surface and degradation of the fracture strength. Si₃N₄ ceramics containing an amorphous phase at the grain boundaries showed the most excellent resistance to corrosion with HF solution, and kept a fracture strength of above 400 M Pa even after leaching 40% of the silicon ions.     

Effect of La2O3 additive on the dielectric properties of barium strontium titanate glass–ceramics

The barium strontium titanate (Ba_xSr_1–xTiO₃) glass–ceramics doped with different content of La were prepared via controlled crystallization. Phase compositions, microstructure and dielectric behaviors were investigated systematically. The results revealed that La₂O₃ additives had little influence on the dielectric constant but significantly changed the microstructure of the glass–ceramics, which led to improved breakdown strength (BDS). The optimized energy-storage density of 3.18 J/cm³ was achieved in the glass–ceramics with 1.0 wt% La₂O₃ content which is 2.56 times higher than pure BST glass–ceramics, suggesting glass–ceramics of this composition could be an attractive candidate for energy-storage applications.     

Effect of magnesia on structure, degradability and in vitro bioactivity of CaO-MgO-P2O5-SiO2 system ceramics

CaO-MgO-P₂O₅-SiO₂ system ceramics with various magnesia contents (0, 5, 10 and 20 mol%) were successfully prepared by sintering the sol-gel-derived powder compacts. The ceramic degradation was evaluated through the weight loss in the tris-(hydroxymethyl)-aminomethane and hydrochloric acid (Tris-HCl) buffer solution, and their ability to form apatite was determined by soaking in simulated body fluid (SBF). Results indicated that the ceramics structure was greatly influenced by magnesia contents. New crystal phases of Ca₂MgSi₂O₇ and SiO₂ were formed when magnesia was added and with an increase of magnesia concentration the phase of Ca₂MgSi₂O₇ increased with a simultaneous decline of β-CaSiO₃. In addition, studies showed that magnesia played an important role in affecting the degradability and apatite forming ability of CaO-MgO-P₂O₅-SiO₂ system ceramics. It is observed that with increasing magnesia concentration, the ceramic degradability gradually decreased and the formation of apatite on samples was delayed.     

Effect of forms of B2O3-SiO2 as an additive on magnetic properties of SrFe12O19 ferrites

Strontium ferrites (SrO · 5.5Fe₂O₃) were prepared using hot-rolled mill scale as a source of iron oxides. Calcination was performed at 1200°C for 2 h. The fused B₂O₃-SiO₂, with various mole ratios of B₂O₃ to SiO₂, was added to the calcined ferrites during the milling stage. The ferrites were formed anisotropically. The fused additives are quite effective to enhance the magnetic properties; (BH)_max can reach 3.0 MGOe for the ferrite sintered at 1220°C for 2 h. In general, (BH)_max is independent of the mole ratio of B₂O₃ to SiO₂ and dominantly influenced by the sintering temperature. The addition of 0.3 or 0.5 wt% fused additives showed no significant difference in the magnet quality. The quality of the magnet was decreased with increasing mole ratio of B₂O₃ to SiO₂ when unfused B₂O₃-SiO₂ was used under the same processing conditions.     

Effect of Zr doping on the microstructure and electric properties of BaHf0.1Ti0.9O3 ceramics

Pure and Zr-doped barium titanate hafnate (BaHf_0.1Ti_0.9O₃, short for BHT) ceramics were prepared by conventional solid state reaction method. The microstructures, dielectric and ferroelectric properties of BaHf_0.1Ti_0.9-x Zr _x O₃ (x = 0, 0.02, 0.04, 0.10) ceramics have been investigated. From the X-ray diffraction patterns it is indicated that Zr⁴⁺ ions have entered the unit cell maintaining the tetragonal perovskite structure of solid solution and the lattice constant of Zr-doped BHT ceramics increases with the increase of Zr content. There is an obvious difference between the grain shape of pure BHT ceramics and that of Zr-doped BHT ceramics. The temperature dependences of dielectric constant indicated that all of the three phase transition temperatures increase after doped zirconium. It is found that well-behaved hysteresis loops can be observed in pure and Zr-doped BHT ceramics. The remanent polarization (2P _r) and the coercive electric field (2E _C) of BaHf_0.1Ti_0.9-x Zr _x O₃ ceramics gradually decrease as the Zr content increases from 2 to 10 mol %.     

The effect of Ni, ZrB2 and MoS2 additives on certain physico-chemical and mechanical properties of special glasses in the ZnO-PbO-B2O3-SiO2 system

The influence of additives of metal compounds (ZrB₂, MoS₂) and nickel to the abrasive masses based on devitrified glasses of ZnO-PbO-B₂O₃-SiO₂ system was studied by investigating both the wettability of submicrocrystalline sintered corundum, and the size, structure and chemistry of the interfaces. XRD analysis, scanning electron microscopy with X-ray microanalysis, wettability tests and microhardness measurements were performed. The experimental results were compared with the probability of reaction between the abrasive grain (cBN, submicrocrystalline sintered corundum) and: (1) the additives of metal compounds, (2) devitrified glass binders identified by using a VCS algorithm designed for thermodynamic calculations. It was confirmed that 18–20 compounds (6–9 in condensed phase) out of 230 exhibited chemical stability.     

Structure,fracture toughness,and fatigue of two aluminium matrix compositesproduced by vertical squeeze casting technique

Abstract

Composite materials produced from ceramic reinforcement of aluminium alloys have some properties that are better (higher modulus and strength, lower thermal expansion coefficient and density, and good creep and wear resistance) than those of the conventional monolithic aluminium alloys. However, they have a poor fracture toughness. The aim of the present work was to characterise the structure and mechanical properties of two different aluminium matrix composites (AS9C1G/20%(Al₂O₃-SiO₂) and 2014/20%(Al₂O₃-SiO₂)) manufactured using the vertical squeeze casting technique. Tensile, plane strain fracture toughness, and fatigue crack growth rate tests were carried out. In particular, the influence of specimen geometry on the toughness tests was examined. It was found that chevron notched short bar specimens gave toughness values ～ 40% higher than other types of specimens. Fatigue crack growth rate data were interpolated using some semiempirical models. An accurate metallographic investigation of both the structures and the fatigue fracture surfaces was carried out using optical microscopy and energy dispersive spectroscopy with SEM.     

The effects of CuO-doping on dielectric and piezoelectric properties of Bi<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>–Ba(Zr,Ti)O<Subscript>3</Subscript> lead-free ceramics

CuO-doped lead-free ceramics based on bismuth sodium titanate (Bi_0.5Na_0.5TiO₃, BNT) and barium zirconate titanate (Ba(Zr_0.07Ti_0.93)O₃, BZT) were prepared via a multi-step solid-state reaction process. The BNT–BZT with CuO dopant ceramics sintered at 1150–1180 °C for 2 h in air showed a pure perovskite structure. SEM images reveal that a small amount of CuO (<2 mol%) play a significant role on the microstructure to improve its sintering attributes, while it will degrade when the dopant is added beyond 2 mol%. The dielectric and piezoelectric properties of CuO-doped BNT–BZT ceramics were evaluated. At room temperature, the sample doped with 2 mol% CuO shows quite good properties such as a high piezoelectric constant (d ₃₃ ∼156.5 pC/N) and a high electromechanical coupling factor (k _t ∼52%). The depolarization temperature increased dramatically and the maximum permittivity temperature decreased slightly.     

Relationship between microstructural evolution and electric properties of B2O3–CaCu3Ti4O12 composite ceramics

Calcium copper titanate (CaCu₃Ti₄O₁₂, CCTO) powder was mixed with boric oxide (B₂O₃) glassy phase up to 8% by weight to prepare the composite ceramics. The effects of B₂O₃ incorporation on the microstructures, electric and dielectric properties of CCTO ceramics have been systematically investigated. Adding B₂O₃ results in both the decrease of the CCTO grain size. These microstructural changes conjointly affect the dielectric constant. In addition, further B₂O₃ increase leads to its aggregation at grain boundaries. Meanwhile, the B₂O₃ addition reduces the nonlinear coefficient and influences the breakdown electric field.     

Phase separation and toughening of SiC-AIN solid-solution ceramics

SiC-AIN solid-solution ceramics were prepared by pressureless sintering using Al₂O₃ and Y₂O₃ as the sintering additives. The resulting ceramics were subjected to annealing treatments over a range of temperatures from 1400 °C to 1800 °C in the spinodal region. The fracture toughness of the annealed ceramics was examined, by the indentation method, in relation to the annealing temperature and annealing time. X-ray diffraction profiles revealed that phase separation occurred during annealing. In ceramics containing 50 mol % SiC annealed at 1800 °C, the morphology of the phase separation is the characteristic modulated stratiform structure. Energy-dispersive X-ray spectroscopy (EDS) showed that the structure consisted of alternations of silicon-rich and aluminium-rich composition. The fracture toughness of the annealed ceramics increased compared to the as-sintered solid-solution ceramics. The phase separation is expected to contribute to the toughening of ceramics with nanometre-scale texture.     

Effect of synthesized BaTiO3 doping on the dielectric properties of ultra temperature-stable ceramics

The high performance X9R ceramics could be sintered at as low as 1,120?°C by doping 3?mol% synthesized BaTiO₃ (SB) additives into the BaTiO₃-based ceramics, with a dielectric constant greater than 2,200 at 25?°C and dielectric loss lower than 1.7?%. The effects of SB additives on the microstructure and dielectric properties of BaTiO₃-based ceramics were investigated. The dielectric constant of BaTiO₃-based ceramics doped with 3?mol% SB was increased due to the promotion of the densification of ceramics. With SB content up to 4.5?mol%, Ti⁴⁺’s polarization was depressed, which resulted in the decrease of augmented dielectric constant at 25?°C. The partial solid solution was formed between Pb(Ti, Sn)O₃ and BaTiO₃, and the substitutions of Pb at A-sites and Sn at B-sites were existed. The strengthen of Ti–O bonds and higher Curie point of Pb(Ti_0.55Sn_0.45)O₃ was helpful to increased the Curie point of the ceramics effectively. Doped with SB additives, the volume of ferroelectric core was increased, and the sharp peak intensity at Curie point was increased accordingly. Capacitance temperature characteristics was improved attributed to the mutual effects of SB and Pb(Ti_0.55Sn_0.45)O₃. The formation of core–shell structure was sensitive to the sintering temperature, so the dielectric properties of ceramics were highly depended on the sintering temperature.     

Dielectric and microstructural behaviour of strontium titanate borosilicate glass ceramic system

65(SrO·TiO₂)−35(2SiO₂·B₂O₃) wt% glass was synthesized. Differential thermal analysis study shows one exothermic peak which shifts towards higher temperature with increasing heating rate. Glass ceramics prepared by controlled crystallization of strontium titanate borosilicate glass produce uniform distribution of crystallites in a glassy matrix. Attempt was made to crystallize strontium titanate phase in this glass ceramic. Different phases precipitated out during ceramization have been identified by X-ray diffraction. It appears that due to high reactivity of SrO with B₂O₃, strontium borate crystallizes as principal phase followed by TiO₂ (rutile) and Sr₃Ti₂O₇ phases. Dielectric constant of these glass ceramics was observed to be more or less temperature independent over wide range of temperatures with low values of dielectric constant and dissipation factor.     

Stability of the perovskite phase in PMN-PZN-PT ceramics

The thermal stability of ceramics based on PMN, PZN and their solid solutions PMN-PT, PZN-PT (PMN = PbMg_1/3Nb_2/3O₃, PZN = PbZn_1/3Nb_2/3O₃, PT = PbTiO₃) during sintering is investigated. The present work confirms that the incorporation of a limited excess of MgO in the starting materials is actually quite beneficial for the synthesis of a pure PMN perovskite phase, and that it prevents the formation of a secondary pyrochlore-type phase, after PbO loss during firing. An excess of ZnO is much less efficient for the stabilization of the PZN compound. Examination of the microstructural evolution as well as analysis of second phases in these materials and in their solid solutions formed with lead titanate by wavelength dispersive spectroscopy and/or energy dispersive X-ray spectroscopy techniques, allows a mechanism to be proposed for the stabilization of the PMN-based perovskite phases by an excess of MgO.     

Microstructure and properties of spark plasma sintered AlN ceramics

Spark plasma sintering (SPS) is a newly developed technique that enables poorly sinterable aluminum nitride (AlN) powder to be fully densified. It is addressed that pure AlN sintered by SPS has relatively low thermal conductivity. In this work, SPS of AlN ceramic was carried out with Y₂O₃, Sm₂O₃ and Li₂O as sintering aids. Effects of additives on AlN densification, microstructure and properties were investigated. Addition of sintering aids accelerated the densification, lowered AlN sintering temperature and was advantageous to improve properties of AlN ceramic. Thermal conductivity and strength were found to be greatly improved with the present of Sm₂O₃ as sintering additive, with a thermal conductivity value about 131 Wm⁻¹K⁻¹ and bending strength about 330 MPa for the 2 wt% Sm₂O₃-doped AlN sample SPS at 1,780 °C for 5 min. XRD measurement revealed that additives had no obvious effect on the AlN lattice parameters. Observation by SEM showed that AlN ceramics prepared by SPS method manifested quite homogeneous microstructure. However, AlN grain sizes and shapes, location of secondary phases varied with the additives. The thermal conductivity of AlN ceramics was mainly affected by the additives through their effects on the growth of AlN grain and the location of liquid phases.     

Effect of Bi2O3 doping on the dielectric properties of medium-temperature sintering BaTiO3-based X8R ceramics

Medium-temperature sintering X8R ceramics were fabricated based on BaTiO₃-based ceramics with Bi₂O₃ additives. The effects of sintering aids Bi₂O₃ on crystalline structure and electrical properties of BaTiO₃-based ceramics were investigated. The sinterability of BaTiO₃ ceramics was significantly improved by adding Bi₂O₃, whose densification sintering temperature reduced from 1,260 to 1,130 °C. However, the dielectric constant (ε) of BaTiO₃-based ceramics doped with Bi₂O₃ was decreased dramatically. Both low ε phase Bi₄Ti₃O₁₂ and the decrease of the tetragonality (c/a ratio), which are demonstrated by XRD pattern, are resulted in the decrease of ε. The ε of samples doped with 5.5 wt% Bi₂O₃ was higher than the other doped samples. The substitution of Bi³⁺ for the Ba²⁺ in BaTiO₃ resulted in the increase of electrovalence (from +2 to +3) of A-site ion, so the attractive force between A and B (Ti⁴⁺) sites becomes stronger. Thus Ti⁴⁺’s polarization enhances, then ε was increased to some extent. The X8R BaTiO₃-based ceramics could be sintered at as low as 1,130 °C by doping 5.5 wt% Bi₂O₃ additives into the BaTiO₃-based ceramics, with a ε greater than 2,430 at 25 °C, dielectric loss lower than 1.3 % and temperature coefficient of capacitance <±15 % (?55–150 °C).