Densification mechanisms during reactive spark plasma sintering of Titanium diboride and Zirconium diboride

Abstract

In this study, dense fine-grained ZrB₂ and TiB₂ were fabricated using reactive spark plasma sintering (RSPS) of ball-milled Zr/B and Ti/B mixtures. Systematic investigations were carried out to understand the mechanisms of reactive sintering. Two densification mechanisms were found to be operating during RSPS. The first stage of densification was due to self-propagating high temperature synthesis reaction leading to formation of ZrB₂ and TiB₂ compacts having relative density of ~48 and ~65%, respectively. The second stage of densification occurred at temperatures more than 1100 °C and resulted in final relative density of more than 98%. Electron backscatter diffraction and electron microscopy studies on interrupted RSPS samples as well as dense samples showed deformed grains and presence of slip steps while grain orientation spread map and pole figure analysis confirmed plastic flow. Plastic flow-aided pore closure is shown as major mechanism during reactive sintering.     

Pr1-xCexB6阴极材料的原位反应合成及性能研究

本文以CeH2,PrH2纳米粉和B粉为原料,在无氧环境下采用放电等离子原位反应成功制备了单相多元稀土六硼化物Pr1-xCexB6=（x=0．2—0．8）阴极材料．系统研究了掺杂元素Ce对Pr1-xCexB6的物相组成、力学性能及热电子发射性能的影响．结果表明,当烧结温度为1450℃,烧结压强为50MPa时可制得单相的Pr1-xCexB6多晶块体材料并且该系列样品具有良好的力学性能,维氏硬度和抗弯强度最高值分别达到了24．34GPa和226．02MPa,已达到单晶水平．热电子发射性能结果表明,随着Ce掺杂量的增加Pr1-xCexB6的发射电流密度线性增加．当阴极温度为1973K,外加电压为950V时,Pr0．4Ce0．686最大发射电流密度达到47-3A．cm^-2,该值远高于传统热压烧结法制备的发射电流密度．因此,本文该方法制备的Pr1-xCexB6多晶块体具有良好的力学性能和发射性能,作为热阴极材料将会有很好的应用前景．     

放电等离子固相烧结制备高密度LaB_6阴极性能

采用放电等离子烧结(SPS)技术,以碳化硼还原法制备的LaB6粉末为原料,制备了高致密LaB6多晶块体阴极,并系统研究了烧结温度、压力对LaB6烧结样品的物相、结构和性能的影响。确定了SPS烧结LaB6的最佳工艺为:压力50MPa,烧结温度1650℃,保温时间10min。实验结果表明:与其它LaB6多晶制备方法相比,SPS制备得到的LaB6烧结块体的力学及发射性能均有大幅提高,样品相对密度达到96.2%,维氏硬度达到1720kg/mm2,抗弯强度达到203.2MPa。样品在1520℃温度下发射电流密度达到17.41A/cm2,功函数为2.40eV。SPS制备法显著降低了LaB的烧结温度,缩短了烧结时间。     

放电等离子固相烧结制备高密度LaB6阴极性能

采用放电等离子烧结（SPS）技术,以碳化硼还原法制备的LaB6粉末为原料,制备了高致密LaB6多晶块体阴极,并系统研究了烧结温度、压力对LaB6烧结样品的物相、结构和性能的影响。确定了SPS烧结LaB6的最佳工艺为：压力50 MPa,烧结温度1 650 ℃,保温时间10 min。实验结果表明：与其它LaB6多晶制备方法相比,SPS制备得到的LaB6烧结块体的力学及发射性能均有大幅提高,样品相对密度达到96.2％,维氏硬度达到1 720 kg/mm2,抗弯强度达到203.2 MPa。样品在1 520 ℃温度下发射电流密度达到17.41 A/cm2,功函数为2.40 eV。SPS制备法显著降低了LaB6的烧结温度,缩短了烧结时间。     

Microstructural,phase transformation and magnetic properties of Ni-Mn-Ga alloy fabricated by spark plasma sintering

The microstructural,phase transformation and magnetic properties of Ni-Mn-Ga alloy fabricated using the spark plasma sintering method have been investigated. The results show that both the as-sintered and annealed sintered specimens exhibit typical martensitic transformation behaviours. The martensite of the sintered specimen after annealing exhibits a ferromagnetic nature. Moreover,study of the fracture surface indicates that the transgranular fracture contributes to the higher ductility of sintered Ni-Mn-Ga alloy. In addition,the transformation strain in sintered Ni-Mn-Ga alloy is studied for the first time.     

Synthesis and characterization of precipitation hardened amorphous matrix composite by mechanical alloying and pulse plasma sintering of Al65Cu20Ti15

This study reports synthesis of Al₆₅Cu₂₀Ti₁₅ amorphous alloy by mechanical alloying and consolidation of the powder mass by pulsed plasma sintering. During sintering, several intermetallic phases precipitate from the amorphous matrix and cause a significant increase in nano-hardness and elastic modulus. Microstructure in as-milled and sintered conditions was characterized by X-ray diffraction, scanning/transmission electron microscopy and differential scanning calorimetric. Among various conditions of sintering, the composites pulse plasma, sintered at 500°C, show the high compression strength (1745 MPa) and high indentation fracture toughness (4.96 MPa m^1/2); although, the maximum density (3.73 Mg/in³), nano-hardness (14 GPa) and Young's modulus (208 GPa) in the present alloy have been obtained in the composites pulse plasma sintered at 600°C.     

Synthesis, thermionic emission and magnetic properties of (NdxGd1-x)B6

Polycrystalline rare-earth hexaborides(NdxGd1-x)B6(x = 0,0.2,0.6,0.8,1) were prepared by the reactive spark plasma sintering(SPS) method using mixed powder of GdH2,NdH2 and B.The effects of Nd doping on the crystal structure,the grain orientation,the thermionic emission and the magnetic properties of the hexaboride were investigated by X-ray diffraction,electron backscattered diffraction and magnetic measurements.It is found that all the samples sintered by the SPS method exhibit high densities(> 95%) and high values of Vickers hardness(2319 kg/mm2).The values are much higher than those obtained in the traditional method.With the increase of Nd content,the thermionic emission current density increases from 11 to 16.30 A/cm2 and the magnetic phase transition temperature increases from 5.85 to 7.95 K.Thus,the SPS technique is a suitable method to synthesize the dense rare-earth hexaborides with excellent properties.     

Effect of carbon fiber volume fraction on 6H to 4H-SiC polytype transformation

In this study, polycrystalline α-SiC composed of 55.7?wt.% 6H-SiC, 35.1?wt.% 4H-SiC with different volume fractions of carbon fibers (0–5–10–15–20%) was successfully sintered by spark plasma sintering technique at 2000°C and 35?MPa of applied pressure. The micrographs obtained from scanning electron microscopy revealed that the sintered samples were composed of equiaxed SiC grains. Results indicated that the presence of carbon fibers retarded the SiC densification process, decreased their relative densities and increased their porosity. Additionally, according to quantitative phase analysis by the Rietveld method during the sintering step, it was found that the 6H to 4H transformation has taken place. Increasing the carbon fibers content accelerated this trend as the sample containing 20?vol.% carbon fiber was consisted of 85.5?wt.% 4H-SiC.     

Influence of vacuum sintering on microstructure and magnetic properties of magnetostrictive cobalt ferrite

Differences in the microstructure and magnetic properties of highly magnetostrictive cobalt ferrite resulting from the effects of different vacuum sintering temperatures and times have been investigated. A vacuum environment was chosen to allow direct comparison of results with air-sintered samples which are more often reported in the literature. It was found that vacuum sintering resulted in the development of a solid solution second phase with composition Co_1−xFe_xO₄ (x∼0.33). There was a decrease in magnetostriction as a result of the formation of the second phase. Furthermore, differences in sintering temperatures were found to have a greater effect on the magnetostriction than differences in sintering times. It was found that the first order cubic anisotropy coefficient initially increased with both sintering temperature and time, before peaking and decreasing to its lowest measured value. The lowest anisotropy was therefore achieved with samples sintered at higher temperatures and longer times.     

Effects of In-substitution on the microstructure and magnetic properties of Bi-CVG ferrite with low temperature sintering

[Y_1.05Bi_0.75Ca_1.2](Fe_4.4−xIn_xV_0.6)O₁₂(In_x:Bi-CVG) ferrite material has been prepared successfully by a solid-state reaction method. The effects of In³⁺ substitution and sintering temperatures on the bulk density, microstructure and magnetic properties are performed by X-ray diffraction (XRD), scanning electron microscopy (SEM), materials automatic test system (MATS) and microwave ferrite parameters meter. The results show that In³⁺ can lower the sintering temperatures and enhance the magnetic properties of Bi-CVG ferrite. Besides, all sintered specimens with different In³⁺ contents show a single garnet crystal structure. The specimen of [Y_1.05Bi_0.75Ca_1.2](Fe₄In_0.4V_0.6)O₁₂ sintered at 1075 °C shows homogenous distribution of grain size and densified microstructures. The ferromagnetic resonance linewidth (ΔH) has an increase with In³⁺ contents. Additionally, the sample has the optimum magnetic properties: ρ=5.23 g/cm³, B_r=31.3 mT, H_c=378.8 A/m, 4πM_s=506.2×10⁻⁴ T.     

Low temperature sintering of sub-stoichiometric Ni-Cu-Zn ferrites: Shrinkage, microstructure and permeability

We have studied sub-stoichiometric Ni-Cu-Zn ferrites with iron deficiency (i.e., <50mol% Fe₂O₃) of composition Ni_0.20Cu_0.20Zn_0.60+zFe_2−zO_4−(z/2) with 0≤z≤0.06. The temperature of maximum shrinkage rate is shifted from T=1000 °C for z=0 towards lower temperatures down to T=900 °C for a sub-stoichiometric ferrite with z=0.02. Dense samples are obtained after firing at 900 °C for z>0 only. Sub-stoichiometric compositions (z>0) do not form single-phase spinel ferrites after sintering at 900 °C, but rather represent mixtures of CuO and a stoichiometric ferrite with slightly modified composition. The formation of small amounts of CuO at grain boundaries is demonstrated by XRD and SEM. The permeability is increased from μ=80 for stoichiometric ferrites (z=0) to μ=660 for z=0.02. The formation of CuO during sintering of sub-stoichiometric ferrites supports densification and is a prerequisite for low temperature firing of multilayer inductors. Addition of 1 wt% Bi₂O₃ as liquid phase sintering aid is required to provide sufficient densification of the stoichiometric ferrite (z=0) at 900 °C. Addition of 0.37 wt% Bi₂O₃ to a sub-stoichiometric ferrite (z=0.02) results in dense samples after firing at 900 °C; however, the microstructure formation is dominated by heterogeneous grain growth.     

The influence of sintering conditions on the phase purity of bulk EuTiO3 and Eu0.5Ba0.5TiO3 ceramics

EuTiO₃ and Eu_0.5Ba_0.5TiO₃ ceramics were synthesized using mechanochemical activation of oxide precursors and then calcined. The uniaxially as well as isostatically pressed samples were sintered in different kinds of reducing atmospheres, namely Ar?+?(7–10)%H₂, respectively, 99.99%H₂ in the case of pressureless sintering or in vacuum (enriched by CO vapors) in the case of pressure-assisted spark plasma sintering (SPS). The samples prepared by SPS contained the pyrochlore phase as the second phase. In contrast with SPS, pressureless sintered samples were phase pure, although thermodynamics calculations showed that CO atmosphere in SPS is more reducing than pure hydrogen. This is explained by short sintering times in SPS that do not allow establishment of the thermodynamic equilibrium. The proper choice of sintering temperature, time, and atmosphere enabled preparation of dense and phase pure samples of Eu _x Ba_{1? x} TiO₃ ceramics suitable for the evaluation of “true” physical properties (e.g., infrared reflectivity), or for experimental confirmation of specific functionalities proposed from theory.     

Glass additive influence on the sintering behavior, microstructure and microwave magnetic properties of Cu-Bi-Zn co-doped Co2Z ferrites

The Bi₂O₃-B₂O₃-ZnO-SiO₂ (BB35SZ) glass effects on the sintering behavior and microwave magnetic properties of Cu-Bi-Zn co-doped Co₂Z ferrites were investigated to develop low-temperature-fired ferrites. The glass wetting characteristics on the Co₂Z ferrite surface, X-ray diffractometer, scanning electron microscopy and a dilatometer were used to examine the BB35SZ glass effect on Co₂Z ferrite densification and the chemical reaction between the glass and Co₂Z ferrites. The results indicate that BB35SZ glass can be used as a sintering aid to reduce the densification temperature of Co₂Z ferrites from 1300 to 900 °C. 3(Ba_0.9Bi_0.1O)·2(Co_0.8Cu_0.2O)·12(Fe_1.975Zn_0.025O₃) ferrite with 2 wt% BB35SZ glass can be densified below 900 °C, exhibiting an initial permeability of 3.4. This process provides a promising candidate for multilayer chip magnetic devices for microwave applications.     

Effect of sintering aid MoO3 on the microstructure and ionic conductivity of ceria- and lanthanum gallate-based electrolytes

The effect of a small addition of MoO₃ on the microstructure and ionic conductivity of Nd_0.2Ce_0.8O_1.9 (NDC), La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.8 (LSGM) and Nd_0.2Ce_0.8O_1.9-La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.8 (NDC-LSGM) has been investigated. The microstructure and electrical properties of the samples were characterized by X-ray diffraction, field-emission scanning electron microscopy and electrochemical impedance spectroscopy. The results show that MoO₃ doping can obviously increase the densification and grain sizes, and decrease the grain and grain boundary resistances of the NDC, LSGM and NDC-LSGM electrolytes. It expands the oxygen ion channels and reduces the total conductance activation energy of the system. The total conductivities of MoO₃-doped NDC and NDC-LSGM samples are 1.56 and 2.10 times higher than that of the undoped NDC system at 450°C. The total conductivity of LSGM-Mo is 1.46 times higher than that of LSGM at 450°C. These finding suggest that MoO₃ is considered to be an effective sintering aid that optimizes the electrical properties of NDC, LSGM and NDC-LSGM electrolytes.     

钡钨阴极优化与热电子发射性能

分别从基体和铝酸盐两方面优化了钡钨阴极.在基体方面,首先采用窄粒度钨粉结合放电等离子体烧结获得了孔径分布窄的基体;再利用射频等离子体球化技术制备了球形钨粉,采用球形钨粉制备了多孔基体,获得了孔通道光滑、内孔连通性好、孔径分布更加窄的基体.与窄粒度钨粉基体相比,球形钨粉制备的阴极,空间电荷限制区的斜率由1.25增加至1.37,发射均匀性得到提高,拐点电流密度由6.6 A·cm^–2增至6.96 A·cm^–2.在此基础上,采用液相法改善了铝酸盐物相组成,发现空间电荷限制区的斜率增加至1.44,拐点电流密度增加至21.2 A·cm^–2.通过理论计算对钡钨阴极发射的物理本质进行了研究,发现钡钨阴极发射规律遵循偶极子理论.     

Faceting and Roughening of the Asymmetric Twin Grain Boundaries in Zinc

Faceting of grain boundaries (GBs) or surfaces can be considered as a phase transition when the original surface or GB dissociates onto flat segments whose energy is less than that of original surface or GB. Zn [1120] flat single crystals were grown using the modified Bridgman technique from Zn of 99.999 wt% purity. Individual elongated twin plates having very uniform thickness were produced with the aid of slight deformation of single crystals. Parallel elongated sides of the twin plate are formed by the coherent symmetric twin (1102)₁(1102)₂ grain boundary (STGB) facets. Due to its optical anisotropy, zinc allows one to study the shape of the GB with the aid of polarised light. The stationary shape of the slowly migrating tip of the twin plate has been studied in situ. The hot stage of optical microscope was used. The temperature interval from 592 to 692 K was investigated. Below 632 K the twin tip contains only one plane facet 1 which is nearly parallel to the (1102)₂ plane and has the angle of 84° with the coherent STGB. Above 632 K the second facet 2 appears at the tip of the twin plate. This facet is nearly parallel to the (1100)₁ plane and has the angle of 46° with the coherent STGB. Between 632 and 682 K both 84° and 46° facets coexist, and 84° facet gradually disappear with increasing temperature. Above 682 K only 46° facet is present in the twin tip. The indications of the GB roughening phase transition were also observed, namely the edges of the facets become smoother with increasing temperature. The GB phase diagram for the twin GBs in zinc containing the lines of two GB faceting phase transitions has been constructed. Schematic Wulff-Herring diagrams explaining these transitions are presented.     

The influence of time-dependent electric and magnetic fields on the dynamic localization of lattice electrons

In this paper we deal with the derivation of dynamic localization conditions for electrons on the one-dimensional (1D) lattice under the influence of ac electric and magnetic fields of the same frequency. We resort, for convenience, to a tight-binding single-band Hamiltonian. Our emphasis is on a more fundamental theoretical understanding by investigating interplays between such fields and the nearest-neighbor hopping interactions characterizing the Hamiltonian. In general, such conditions get expressed in terms of infinite sums of binary products of Bessel functions of the first kind. These sums are hardly tractable, but we found that selecting in a suitable manner the phases of time-dependent modulations leads to controllable frequency-mixing effects providing appreciable simplifications. Such mixings concern competitions between the number of flux quanta and the quotients of field amplitudes and field frequencies. More exactly, tuning one of the mixed frequencies to zero opens the way to establishing the simplified dynamic localization conditions. By resorting again to the zeros of the Bessel function of zeroth order. This results in quickly tractable relationships between the amplitudes of electric and magnetic fields, the field frequency, and the zeros referred to just above. Pure field limits and superpositions between uniform electric and time-dependent magnetic fields are also discussed. Comments concerning the role of disorder and of the Coulomb interaction are also made.     

薄膜中异常晶粒生长理论及能量各向异性分析

针对柱状晶薄膜，建立了异常晶粒生长理论模型.指出薄膜中的晶粒生长，除像传统的整体材料中的晶粒生长一样考虑晶界能外，还应当考虑表面能、界面能和应变能.对能量的各向异性进行了回顾性分析.从表面能的最小化考虑，面心立方和体心立方薄膜的择优取向或织构应分别为（111）和（110）；而从应变能的最小化考虑，面心立方和体心立方薄膜的择优取向或织构应分别为（110）和（100）. 关键词： 薄膜 异常晶粒生长 模型 织构     

Dependence of grain boundary character distribution on the initial grain size of 304 austenitic stainless steel

Abstract

In order to study the dependence of the grain boundary character distributions (GBCD) on the grain size, annealing treatment was carried out on 304 austenitic stainless steel with different initial grain sizes. The evolution of the GBCD was analysed by electron backscatter diffraction. The experimental results showed that abnormal grain growth (AGG) occurred when grain size was small. With a smaller initial grain size, the number density of abnormally large grains and the fraction of low-Σ CSL boundaries increased but the size of abnormally large grains decreased and the random boundaries presented a continuous network. With a larger initial grain size, the fraction of low-Σ CSL boundaries also increased as well as the size of abnormally large grains but the number density of abnormally large grains decreased and the connectivity of random boundary network was disrupted by low-Σ CSL boundaries, especially Σ3ⁿ (n = 1, 2, 3) boundaries. However, with a very large initial grain size, normal grain growth (NGG) occurred, which had no effect on the fraction of low-Σ CSL boundaries and the connectivity of random boundary network.     

钡-钨阴极的组织特性与热电子发射性能

利用正交试验探讨了放电等离子技术工艺参数（温度、压力、保温时间）对钡-钨（Ba-W）阴极中的W的孔隙度的影响规律,获得了孔隙率在23%～30%内变化时所需要的最佳工艺参数。在此基础上,制备出了具有不同孔隙度的球形W基体和普通不规则的W基体。研究表明：球形多孔W颗粒间堆积、排列有序,无闭孔,孔径分布集中而均匀,在26.3%的孔隙度下中值孔径为1.41 μm;机械性能方面,球形钨粉基体维氏硬度低于传统普通不规则钨多孔体。在脉宽10 μs、频率1000 Hz的条件下,阴极脉冲发射电流密度随着孔隙度的增大,先增大后减小。当基体孔隙度为26.3%时,阴极电流发射密度最大,在1050 ℃,偏离点发射电流密度可达24.62 A/cm²,零场发射电流密度为7.62 A/cm²,功函数为1.95 eV。