Multilayer capacitor

Abstract

A multilayer capacitor according to the present disclosure includes: a capacitor body including a dielectric layer and an inner electrode; and an external electrode disposed on an outer side of the capacitor body, and the dielectric layer includes a plurality of dielectric grains, the dielectric grains including a primary component and a secondary component, and at least one of the plurality of dielectric grains having a core-shell structure, the core-shell structure comprises a core, a shell and a core-shell wall located between the core and the shell, and a grain boundary is arranged between at least two dielectric grains in the plurality of dielectric grains; when the average concentration, expressed as at%, of the minor component in the grain boundary is CGB and the average concentration, expressed as at%, of the minor component in the core-shell wall is CCS, CCS / CGB is in the range of 0.40 to 0.75.

Description

Technical Field

[0001] The present disclosure relates to a multilayer capacitor. Background Art

[0002] Recently, with the rapid development of multifunctionalization and miniaturization of electronic devices, the miniaturization and performance improvement of electronic components are also developing rapidly. In addition, the demand for high reliability of electrical devices used in automobiles, network equipment, etc. and electronic components used in industry has also increased significantly.

[0003] In order to meet such market demands, competition for technological development of passive components such as inductors, capacitors, resistors, etc. is accelerating. In particular, great efforts are required to dominate the market by developing various multilayer ceramic capacitor (MLCC) products, whose applications and uses as passive components are increasing.

[0004] In addition, MLCC is a capacitor made by stacking dielectric layers and internal electrodes, and is used in various electronic devices such as mobile phones, laptop computers, liquid crystal televisions (LCDTVs), etc. In particular, with the development of electronic control technology for automobiles, the demand for electronic devices for vehicles has increased, and as electronic devices for vehicles have become smaller and their functions have improved, the demand for high temperature resistance and moisture resistance of multilayer capacitors has increased.

[0005] Generally, the reliability of multilayer capacitors is ensured by increasing the fraction of dielectric grain boundaries of the dielectric layer to ensure insulation resistance. However, as capacitors become thinner and their capacity becomes higher, there is a limit in increasing the fraction of grain boundaries. Summary of the invention

[0006] The present disclosure aims to provide a capacitor with improved reliability by preventing degradation of insulation resistance by suppressing migration of electron and ion carriers when voltage is applied.

[0007] A multilayer capacitor according to the present disclosure includes: a capacitor body including a dielectric layer and an inner electrode; and an outer electrode disposed on an outer side of the capacitor body, the dielectric layer including a plurality of dielectric grains, the dielectric grains including a main component and a subsidiary component, at least one of the plurality of dielectric grains having a core-shell structure, the core-shell structure including a core, a shell, and a core-shell wall between the core and the shell, a grain boundary between at least two of the plurality of dielectric grains, and when the average concentration of the subsidiary component in the grain boundary expressed in at% is C GB and the average concentration of the secondary component in the core-shell wall expressed in at% is C CS When C CS / C GB In the range of 0.40 to 0.75.

[0008] A ratio of an average diameter of the dielectric grains to an average diameter of the cores may be in a range of 1.2 to 1.8.

[0009] A ratio of an average number of the dielectric grains including the core-shell wall in a unit area to an average number of the dielectric grains in the unit area may be greater than or equal to 70%.

[0010] The average diameter of the dielectric grains may be in the range of 120 nm to 200 nm.

[0011] The average diameter of the core may be in the range of 100 nm to 150 nm.

[0012] The average thickness of the dielectric layer is in the range of 0.05 μm to 0.5 μm.

[0013] The inner electrode may have an average thickness in a range of 0.05 μm to 0.5 μm.

[0014] The main component may include BaTiO3, Ba(Ti,Zr)O3, Ba(Ti,Sn)O3, (Ba,Ca)TiO3, (Ba,Ca)(Ti,Zr)O3, (Ba,Ca)(Ti,Sn)O3, (Ba,Sr)TiO3, (Ba,Sr)(Ti,Zr)O3, (Ba,Sr)(Ti,Sn)O3 or a combination thereof.

[0015] The secondary component may include dysprosium (Dy), vanadium (V), manganese (Mn), chromium (Cr), silicon (Si), aluminum (Al), magnesium (Mg), tin (Sn), antimony (Sb), germanium (Ge), gallium (Ga), indium (In), barium (Ba), or a combination thereof.

[0016] The core-shell wall may include the subcomponent in an amount greater than 2.0 mol and less than or equal to 30.0 mol relative to 100 mol of the main component.

[0017] The shell may include the subsidiary component in an amount greater than 0.1 mol and less than 30.0 mol relative to 100 mol of the main component, and the core may include the subsidiary component in an amount less than 0.1 mol relative to 100 mol of the main component.

[0018] According to the multilayer capacitor of the present disclosure, migration of electrons and ion carriers when a voltage is applied is suppressed to prevent degradation of insulation resistance, thereby providing a multilayer capacitor with improved reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1is a perspective view showing a multilayer capacitor according to an embodiment.

[0020] Figure 2 It is along Figure 1 A cross-sectional view of the multilayer capacitor taken along line II'.

[0021] Figure 3 It is shown Figure 1 An exploded perspective view of the capacitor body.

[0022] Figure 4 FIG. 1 is a diagram schematically showing the structure of dielectric crystal grains in a dielectric layer.

[0023] Figure 5 Shown are a transmission electron microscope (TEM) image of dielectric grains and a graph representing the results obtained by measuring the content of dysprosium (Dy) according to the position in the dielectric grains by energy dispersive X-ray spectroscopy (EDX).

[0024] Figure 6 The dielectric grains of the dielectric layer are observed using a scanning electron microscope (SEM) image.

[0025] Figure 7 It is marked Figure 6 Image of the core-shell walls contained in the dielectric grains.

[0026] Figure 8 is a diagram schematically showing a portion of a cross section of a multilayer capacitor including dielectric grains including core-shell walls.

[0027] Fig. 9 is a TEM photograph showing a line profile for grain boundary measurement.

[0028] Fig.10 It is used based on Fig. 9 The contrast difference observed in the line profile is used to calculate the thickness of the grain boundary. DETAILED DESCRIPTION

[0029] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art can easily implement embodiments of the present invention. The accompanying drawings and descriptions are considered to be illustrative rather than restrictive in nature. Throughout the specification, similar reference numerals represent similar elements. In addition, the accompanying drawings are provided to help easily understand the embodiments disclosed in this specification, and the technical spirit disclosed in this specification is not limited by the accompanying drawings, and it should be understood that the present invention includes all modifications, equivalents and alternatives contained in the spirit and technical scope of the present invention.

[0030] Terms including ordinal numbers such as first and second are used to describe various constituent elements, but the constituent elements are not limited by the terms. The terms are used only to distinguish one constituent element from another constituent element.

[0031] When a component is referred to as being "connected" or "coupled" to another component, it should be understood that the component may be directly connected or coupled to the other component, or face the other component, or other intermediate components may exist. In contrast, when a component is referred to as being "directly connected" or "directly coupled" to another component, it should be understood that no other intermediate components exist.

[0032] In the present application, it should be understood that the terms "include" and "have" are intended to specify the existence of the features, numbers, steps, operations, constituent elements and components or their combinations described in the specification, and do not exclude the possibility of pre-existing or adding one or more other features, numbers, steps, operations, constituent elements and components or their combinations. Therefore, unless explicitly described to the contrary, the word "include" and variations such as "comprises" or "have" will be understood to imply the inclusion of the elements but not the exclusion of any other elements.

[0033] Figure 1 is a perspective view showing a multilayer capacitor 100 according to an embodiment, Figure 2 It is along Figure 1 A cross-sectional view of the multilayer capacitor 100 taken along line II' in FIG. Figure 3 It is shown Figure 1 An exploded perspective view of the capacitor body 110 is shown in FIG.

[0034] In order to clearly describe the present embodiment, directions are defined as follows: the L-axis, W-axis, and T-axis shown in the drawings represent the length direction, width direction, and thickness direction of the capacitor body 110, respectively. Here, the thickness direction (T-axis direction) may be a direction perpendicular to the wide surface (main surface) of the sheet-like constituent element, and, for example, may be used as the same concept as the stacking direction of the stacked dielectric layer 111. The length direction (L-axis direction) may be a direction extending in parallel to the wide surface (main surface) of the sheet-like constituent element, and may be a direction approximately perpendicular to the thickness direction (T-axis direction), and, for example, may be a direction in which the first external electrode 131 and the second external electrode 132 are provided on both sides. The width direction (W-axis direction) may be a direction extending in parallel to the wide surface (main surface) of the sheet-like constituent element, and may be a direction approximately perpendicular to the thickness direction (T-axis direction) and the length direction (L-axis direction), and the length of the sheet-like constituent element in the length direction (L-axis direction) may be greater than the length of the sheet-like constituent element in the width direction (W-axis direction).

[0035] Reference Figures 1 to 3 , the multilayer capacitor 100 according to the embodiment may include a capacitor body 110 and first and second external electrodes 131 and 132 provided on both ends of the capacitor body 110 opposite to each other in a length direction (L-axis direction).

[0036] The capacitor body 110 may have, for example, a substantially hexahedral shape.

[0037] In this embodiment, for the sake of convenience of description, on the capacitor body 110, two surfaces opposite to each other in the thickness direction (T-axis direction) are defined as a first surface and a second surface, two surfaces connected to the first surface and the second surface and opposite to each other in the length direction (L-axis direction) are defined as a third surface and a fourth surface, and two surfaces connected to the first surface and the second surface, connected to the third surface and the fourth surface and opposite to each other in the width direction (W-axis direction) are defined as a fifth surface and a sixth surface.

[0038] As an example, the first surface as the lower surface may be a surface facing the installation direction. In addition, the first to sixth surfaces may be flat; however, the present embodiment is not limited thereto, and for example, the first to sixth surfaces may be curved surfaces having a convex central portion, and the boundaries (i.e., edges) of each surface may be rounded.

[0039] The shape and size of the capacitor body 110 and the number of stacked dielectric layers 111 are not limited to those shown in the drawings of the present embodiment.

[0040] The capacitor body 110 is formed by stacking a plurality of dielectric layers 111 in a thickness direction (T-axis direction) and firing them, and the capacitor body 110 includes a plurality of dielectric layers 111 and a first internal electrode 121 and a second internal electrode 122, the first internal electrode 121 and the second internal electrode 122 are alternately arranged in the thickness direction (T-axis direction), and the dielectric layer 111 is between the first internal electrode 121 and the second internal electrode 122.

[0041] In this case, adjacent dielectric layers 111 in the capacitor body 110 may be integrated with each other so that a boundary between the dielectric layers is difficult to see without using a scanning electron microscope (SEM).

[0042] Furthermore, capacitor body 110 may include an active area and footprints 112 and 113 .

[0043] The active region is a portion contributing to forming the capacitance of the multilayer capacitor 100. As an example, the active region may be a region where the first and second internal electrodes 121 and 122 stacked in the thickness direction (T-axis direction) overlap.

[0044] Covering areas 112 and 113 are edge portions in the thickness direction and may be located on the first surface side and the second surface side of the active area in the thickness direction (T-axis direction). These covering areas 112 and 113 may be stacked on the upper surface and the lower surface of the active area, respectively, and each covering area may be formed by a single dielectric layer or two or more dielectric layers.

[0045] In addition, the capacitor body 110 may further include side covering areas. The side covering areas are edge portions in the width direction and may be located on the fifth surface side and the sixth surface side of the active area in the width direction (W-axis direction), respectively. These side covering areas may be formed by coating the conductive paste layer for the internal electrode on the surface of the dielectric green sheet only on a portion of the surface of the dielectric green sheet, and not coating the conductive paste layer on both side areas of the surface of the dielectric green sheet, and then stacking the dielectric green sheets.

[0046] The cover regions 112 and 113 and the side cover regions serve to prevent the first and second internal electrodes 121 and 122 from being damaged due to physical stress or chemical stress.

[0047] Figure 4 1 is a diagram schematically showing the structure of dielectric grains 1111 of the dielectric layer 111 .

[0048] The dielectric layer 111 includes a plurality of dielectric grains 1111 .

[0049] There is a grain boundary between at least two dielectric grains 1111 .

[0050] The dielectric grains 1111 include a main component and a subcomponent.

[0051] The main component is a base material of the dielectric, has a high dielectric constant, and contributes to forming the dielectric constant of the multilayer capacitor 100 .

[0052] As an example, the principal component may be a m TiO3 (where 0.995≤m≤1.010), (Ba 1-X Ca x ) m (Ti 1- y Zr y )O3(where 0.995≤m≤1.010, 0≤x≤0.10, and 0 <y≤0.20)、Ba m (Ti 1-x Zr x )O3(where 0.995≤m≤1.010, x≤0.10),(Ba 1-X Ca x ) m(Ti 1-y Sn y )O3 (where 0.995 ≤ m ≤ 1.010, 0 ≤ x ≤ 0.10, and 0 < y ≤ 0.20) or a dielectric material of a combination thereof.

[0053] As an example, the main component may include BaTiO3, Ba(Ti,Zr)O3, Ba(Ti,Sn)O3, (Ba,Ca)TiO3, (Ba,Ca)(Ti,Zr)O3, (Ba,Ca)(Ti,Sn)O3, (Ba,Sr)TiO3, (Ba,Sr)(Ti,Zr)O3, (Ba,Sr)(Ti,Sn)O3, or a combination thereof.

[0054] As an example, the sub-component may include dysprosium (Dy), vanadium (V), manganese (Mn), chromium (Cr), silicon (Si), aluminum (Al), magnesium (Mg), tin (Sn), antimony (Sb), germanium (Ge), gallium (Ga), indium (In), barium (Ba), or a combination thereof.

[0055] In addition, the sub-component may further include lanthanum (La), yttrium (Y), actinium (Ac), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), hafnium (Hf), or a combination thereof.

[0056] For example, relative to 100 mol of the main component, the dielectric grains 1111 may include 0.5 mol to 2.0 mol of Dy2O3, 0.1 mol to 0.5 mol of Mn3O4, 0.1 mol to 0.5 mol of V2O5, 0.2 mol to 1.0 mol of BaCO3, 1.0 mol to 2.0 mol of SiO2, 0.2 mol to 1.5 mol of MgCO3, 0.2 mol to 1.0 mol of Al2O3, or a combination thereof as the sub-component. In this case, it is easy to make a relative comparison of dielectric changes.

[0057] There may be a situation where some of the dielectric grains in the dielectric grains 1111 are different in terms of the molar ratio of the sub-component to the main component, and as an example, at least one of the plurality of dielectric grains 1111 has a core-shell structure.

[0058] Each of the dielectric grains 1111 having a core-shell structure includes a core 1111a, a shell 1111b surrounding at least a part of the core 1111a, and a core-shell wall 1111c located between the core and the shell.

[0059] The core 1111a, the shell 1111b and the core-shell wall 1111c differ in the molar ratio of the minor component to the main component. For example, at the boundary between the core 1111a and the shell 1111b, the molar ratio of the minor component to the main component may change rapidly, and as an example, in the core-shell wall 1111c including the boundary between the core 1111a and the shell 1111b, a point where the molar ratio of the minor component to the main component is the largest may be included.

[0060] Therefore, the boundary between the core 1111 a and the shell 1111 b can be easily recognized, and the boundary between the core 1111 a and the shell 1111 b can be confirmed by transmission electron microscopy-energy dispersive X-ray spectrometry (TEM-EDX).

[0061] Figure 5 A transmission electron microscope (TEM) image of dielectric grains and a graph showing the results of measuring the content of dysprosium (Dy) as a subcomponent according to the position in the dielectric grains by energy dispersive X-ray spectroscopy (EDX) are shown.

[0062] Reference Figure 5 , when, in a cross section obtained by cutting the center of the capacitor body 110 in the W-axis direction along the L-axis direction and the T-axis direction, a line analysis is performed on the dielectric grain 1111 of the dielectric layer 111 located in the center of the effective area in any grain direction of the dielectric grain 1111 starting from the center of the core 1111a of the dielectric grain 1111 using an energy dispersive X-ray spectrometer (EDS) installed in a transmission electron microscope (TEM), the core 1111a and the shell 1111b can be distinguished by regarding the portion where the total content of the subcomponents begins to increase rapidly as the boundary between the core 1111a and the shell 1111b.

[0063] Here, the point where the longest major axis of the core 1111a and the shortest minor axis of the minor axes orthogonal to the longest major axis intersect can be determined as the center of the core 1111a. In addition, energy dispersive X-ray spectroscopy (EDS) line analysis can be performed along the longest major axis passing through the center of the core 1111a of the dielectric grain 1111.

[0064] Alternatively, the core-shell wall 1111c including the boundary between the core 1111a and the shell 1111b or the boundary between the shell 1111b and the grain boundary may be defined by distinguishing different tones by a method such as binarizing a transmission electron microscope image.

[0065] Figure 6 is a SEM image obtained by observing the dielectric grains of the dielectric layer using a scanning electron microscope (SEM), Figure 7 It is marked Figure 6 The image of the core-shell wall contained in the dielectric grains in Figure 8is a diagram schematically showing a portion of a cross section of a multilayer capacitor including dielectric grains including core-shell walls.

[0066] The core-shell wall 1111c may include a boundary between the core 1111a and the shell 1111b, and as an example, the core-shell wall 1111c may include a point where a molar ratio of the subcomponent relative to the main component is maximum.

[0067] Reference Figure 6 and Figure 7 , the core-shell wall 1111c may correspond to a local region having a dark tone in a dielectric grain in the dielectric layer in a scanning electron microscope (SEM) image.

[0068] Similar to the crystal grains, the core-shell wall 1111c can be used as a barrier to suppress the migration of electrons and ion carriers when a voltage is applied to the capacitor. Since the core-shell wall 1111c is in an unstable state with high energy, it can attract electrons and ion carriers to reach a stable energy state, thereby suppressing the migration of electrons and ion carriers. Therefore, it is possible to prevent the insulation resistance from decreasing, thereby effectively improving the reliability of the capacitor. Therefore, the dielectric crystal grain 1111 includes the core-shell wall 1111c, which can effectively improve the reliability of the capacitor without additionally increasing the grain boundary fraction in the thin film high capacitance dielectric layer.

[0069] The core-shell wall 1111c may be a wall located at the interface between the core 1111a and the shell 1111b and formed due to excessive use of the secondary component. Since the secondary component diffuses uniformly at a high concentration to the surface of the main component, the core-shell wall 1111c may be formed in the initial stage of mixing and firing the main component and the secondary component. After mixing and firing so that the secondary component diffuses uniformly at a high concentration to the surface of the main component, a liquid phase sintering step of the main component and the secondary component and a solid phase sintering step through grain boundary migration may be performed, thereby producing dielectric grains having the core-shell wall 1111c at the interface between the core 1111a and the shell 1111b.

[0070] As an example, the core 1111a may include a total amount of less than or equal to 0.1 mol of the secondary component relative to 100 mol of the main component, and the shell 1111b may include a total amount of greater than 0.1 mol and less than or equal to 30.0 mol of the secondary component relative to 100 mol of the main component, or greater than 0.1 mol and less than or equal to 20.0 mol of the secondary component. When the core 1111a includes more than 0.1 mol of the secondary component relative to 100 mol of the main component, the material properties of the pure dielectric material (e.g., BaTiO3) may change, when the shell 1111b includes less than or equal to 0.1 mol of the secondary component relative to 100 mol of the main component, the range of dielectric constant variation according to temperature may become wider, and when the shell includes more than 30.0 mol of the secondary component relative to 100 mol of the main component, the initial insulation resistance may decrease.

[0071] As an example, the core-shell wall 1111c may include the secondary component in an amount greater than 2.0 mol and less than or equal to 30.0 mol relative to 100 mol of the main component. When the core-shell wall 1111c includes the secondary component in an amount greater than 2.0 mol and less than or equal to 30.0 mol relative to 100 mol of the main component, the degradation of the insulation resistance can be effectively prevented.

[0072] In other words, in the core 1111a, there may be no side component, or if there is a side component, there may be only a trace amount of the side component. Therefore, the core 1111a may be composed of only a pure main component without impurities, and the pure main component may generally have a higher dielectric constant than the main component doped with an impurity element. Therefore, the core 1111a can be used to maintain the dielectric constant.

[0073] The shell 1111b contains more accessory components than the core 1111a. In the shell 1111b, the accessory components located at the B site of the main component (perovskite (ABO3) structure) are doped to increase the band gap energy of other rare earth elements and doping elements diffusing into the dielectric grains 1111. Therefore, the shell can be used as a barrier for limiting the diffusion of other rare earth elements and doping elements into the dielectric grains 1111. The shell 1111b can be used to suppress the growth of the dielectric grains 1111, thereby contributing to the micronization of the dielectric grains 1111. In addition, in the shell 1111b, the accessory components located at the A site of the main component can be doped to improve reliability and dielectric constant.

[0074] The core-shell wall 1111c contains the largest amount of the accessory component. Therefore, similar to the grain boundary, when a voltage is applied to the capacitor, the core-shell wall can serve to suppress the migration of electron and ion carriers, thereby preventing the degradation of the insulation resistance.

[0075] In the multilayer capacitor according to the embodiment, when the average concentration of the subcomponent in the grain boundary of the dielectric crystal grain 1111 is C GB (at%), and the average concentration of the secondary components in the core-shell wall 1111c is represented by C CS When expressed as (at%), C CS / C GB In the range of 0.40 to 0.75.

[0076] The average concentration C of the secondary component in the grain boundaries between the dielectric grains 1111 GB The average concentration (at%) of the auxiliary component relative to the concentration of the main component contained in the grain boundary, and the average concentration C of the auxiliary component in the core-shell wall 1111c may be referred to as CS (at %) may refer to an average concentration (at %) of the concentration of the sub-component relative to the concentration of the main component included in the core-shell wall 1111 c .

[0077] When the above range is satisfied, the dielectric constant of the capacitor is excellent and degradation of insulation resistance is effectively prevented. Therefore, a multilayer capacitor with improved reliability can be realized.

[0078] Fig. 9 is a TEM photograph showing a line profile for grain boundary measurement, and Fig.10 It is used based on Fig. 9 The contrast difference observed in the line profile is used to calculate the thickness of the grain boundary to accurately determine the grain boundary map.

[0079] The following will refer to Fig. 9 and Fig.10 A method for measuring the average concentration of the subcomponent in the grain boundary and the average concentration of the subcomponent in the core-shell wall 1111 c is described.

[0080] First, a thin film sample for transmission electron microscopy (TEM) observation was prepared by focused ion beam (FIB) microsampling. Then, the thin film sample was subjected to an Ar milling process, thereby preparing a scanning transmission electron microscopy (STEM) sample with a thickness of about 80 nm for observing the core-shell wall 1111c and the grain boundary.

[0081] Next, the probe diameter of the electron beam is set to about 0.5 nm or less, and the core-shell wall 1111c and the grain boundary are analyzed. At this time, only the core-shell wall 1111c and the grain boundary that are not tilted with respect to the incident electron beam are analyzed.

[0082] Reference Fig. 9 and Fig.10, the full width at half maximum (FWHM) of the peak shown in the line profile for grain boundary measurement, such as a high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) image of the grain boundary (magnification × 2.25), was measured, and the full width at half maximum (FWHM) was defined as the thickness of the grain boundary, and a comparative analysis of regions with the same thickness was performed as a composition analysis of the grain boundary.

[0083] In addition, the full width at half maximum (FWHM) of the peak shown in the line profile for the core-shell wall 1111c measurement is measured and defined as the thickness of the core-shell wall, and a comparative analysis of regions with the same thickness is performed as a composition analysis of the core-shell wall 1111c.

[0084] In addition, the composition analysis of the grain boundary can be performed as follows: EDS analysis is performed by irradiating one point of the dielectric layer with an electron beam under the condition that the grain boundary is not tilted and the probe diameter satisfies the above conditions. At this time, measurement is performed at 20 points of each sample, and the average value is calculated.

[0085] In addition, the composition analysis of the core-shell wall 1111c can be performed as follows: under the condition that the core-shell wall 1111c is not tilted and the probe diameter satisfies the above conditions, EDS analysis is performed by irradiating one point of the dielectric layer with an electron beam. At this time, measurement is performed at 20 points of each sample, and the average value is calculated.

[0086] In an embodiment, the average diameter of the dielectric grains 1111 may be in a range of 120 nm to 200 nm, and the average diameter of the cores 1111 a may be in a range of 100 nm to 150 nm.

[0087] The ratio of the average diameter of the dielectric grains 1111 to the average diameter of the cores 1111a may be in the range of 1.2 to 1.8. When the above range is satisfied, the dielectric constant of the capacitor is excellent and the degradation of the insulation resistance is effectively prevented. Therefore, a multilayer capacitor with improved reliability can be realized.

[0088] The average diameter of the dielectric grains 1111 and the average diameter of the cores 1111 a are measured by the following method.

[0089] As an example, a cross-sectional sample is prepared as follows: the multilayer capacitor 100 is placed in an epoxy resin mixture and cured, the L-axis direction-T-axis direction side surface of the capacitor body 110 is ground to the middle of the W-axis direction, or the center of the capacitor body 110 in the W-axis direction is cut along the L-axis direction and the T-axis direction, and it is fixed and maintained in a vacuum atmosphere chamber.

[0090] Next, the cross-sectional sample was etched for about 10 seconds using a 0.2% hydrofluoric acid solution and a 1% nitric acid solution, and a scanning electron microscope (SEM) image was obtained.

[0091] In the SEM image, an arbitrary dielectric layer 111 is selected at the center point in the length direction (L-axis direction) or the width direction (W-axis direction), and the diameter of the dielectric grain 1111 and the diameter of the core 1111a are measured. At this time, the average value of the length of the longest major axis and the length of the longest minor axis among the minor axes orthogonal to the longest major axis can be calculated as the diameter of the dielectric grain 1111 or the diameter of the core 1111a. In addition, more than 500 dielectric grains can be selected from the same dielectric layer, and the diameter of the dielectric grain 1111 and the diameter of the core 1111a can be measured, and the average value of the diameter of the dielectric grain 1111 and the average value of the diameter of the core 1111a can be calculated.

[0092] In an embodiment, a ratio of the average number of dielectric grains including the core-shell walls 1111 c relative to the average number of dielectric grains 1111 may be 70% or more, for example, 75% or more, or 80% or more.

[0093] When the ratio of the average number of dielectric grains including the core-shell wall 1111c to the average number of dielectric grains 1111 is 70% or greater, the core-shell wall 1111c can suppress the migration of electron and ion carriers when voltage is applied, thereby effectively suppressing the degradation of insulation resistance.

[0094] The ratio of the average number of dielectric grains including the core-shell walls 1111 c relative to the average number of dielectric grains 1111 may be measured by the following method.

[0095] First, an arbitrary dielectric layer 111 is selected from the scanning electron microscope (SEM) image of the above-mentioned cross-sectional sample, an effective area with a size of 1 μm×1 μm (unit area) is set as a measurement object, and the number of dielectric grains 1111 in the unit area and the number of dielectric grains including the core-shell wall 1111c are measured.

[0096] As an example, in the case where the dielectric grains in the dielectric layer are observed by using a scanning electron microscope, Figure 6 In the scanning electron microscope (SEM) image shown in Figure 7 The dielectric grains shown in the image include core-shell walls 1111 c . In such an image, the number of dielectric grains 1111 and the number of dielectric grains including core-shell walls 1111 c can be measured.

[0097] In addition, each of the average value of the number of dielectric grains 1111 and the average value of the number of dielectric grains including the core-shell wall 1111c can be the arithmetic mean of three values ​​measured from three different dielectric layers 111 in the cross-sections taken along the L-axis direction and the T-axis direction, or can be the arithmetic mean of a total of nine values ​​measured at the center, one end, and the other end of the effective area in three different dielectric layers 111 in the cross-sections taken along the L-axis direction and the T-axis direction.

[0098] As an example, the average thickness of the dielectric layer 111 may be greater than or equal to 0.05 μm, greater than or equal to 0.1 μm, or greater than or equal to 0.2 μm, and the average thickness of the dielectric layer 111 may be less than or equal to 0.5 μm or less than or equal to 0.4 μm.

[0099] When the average thickness of the dielectric layer 111 is less than 0.05 μm, it may be difficult to confirm improvement in reliability.

[0100] The average thickness of the dielectric layer 111 may be: in a scanning electron microscope (SEM) image of a cross-sectional sample, when the center point of the dielectric layer 111 in the length direction (L-axis direction) or the width direction (W-axis direction) is set as a reference point, the arithmetic mean of the thickness of the dielectric layer 111 at 10 points at a predetermined interval from the reference point. The interval between the 10 points may be adjusted according to the scale of the SEM image, for example, an interval of 1 μm to 100 μm, an interval of 1 μm to 50 μm, or an interval of 1 μm to 10 μm. In this case, all 10 points should be located within the dielectric layer 111, and when all 10 points are not located within the dielectric layer 111, the position of the reference point may be changed, or the interval between the 10 points may be adjusted.

[0101] The first inner electrode 121 and the second inner electrode 122 may be electrodes having different polarities, and the first inner electrode 121 and the second inner electrode 122 may be alternately arranged along the T-axis direction, so that the first inner electrodes 121 and the second inner electrodes 122 adjacent to each other are opposite to each other, and the dielectric layer 111 is interposed between the first inner electrode 121 and the second inner electrode 122, and one end of each inner electrode may be exposed from the third surface or the fourth surface of the capacitor body 110.

[0102] The first and second internal electrodes 121 and 122 may be electrically insulated from each other by a dielectric layer 111 disposed between the first and second internal electrodes 121 and 122 .

[0103] Ends of the first and second internal electrodes 121 and 122 , respectively exposed from the third and fourth surfaces of the capacitor body 110 , may be electrically connected to the first and second external electrodes 131 and 132 , respectively.

[0104] The first and second internal electrodes 121 and 122 may include a conductive metal, and may include a metal such as Ni, Cu, Ag, Pd, Au, etc., or an alloy thereof such as an Ag—Pd alloy.

[0105] In addition, the first and second internal electrodes 121 and 122 may include dielectric particles of the same composition system as that of the ceramic material included in the dielectric layer 111 .

[0106] The first and second internal electrodes 121 and 122 may be formed using a conductive paste including a conductive metal, and a printing method of the conductive paste may use a screen printing method, a gravure printing method, or the like.

[0107] As an example, the average thickness of the first and second internal electrodes 121 and 122 may be 0.05 μm or more, 0.1 μm or more, 0.2 μm or more, or 0.25 μm or more, and 0.5 μm or less, 0.4 μm or less, or 0.37 μm or less.

[0108] The average thickness of the first internal electrode 121 or the second internal electrode 122 may be: in a scanning electron microscope (SEM) image of a cross-sectional sample, when the center point of the first internal electrode 121 or the second internal electrode 122 in the length direction (L-axis direction) or the width direction (W-axis direction) is set as a reference point, the arithmetic mean of the thickness of the first internal electrode 121 or the second internal electrode 122 at 10 points at a predetermined interval from the reference point. The interval between the 10 points may be adjusted according to the scale of the SEM image, for example, an interval of 1 μm to 100 μm, an interval of 1 μm to 50 μm, or an interval of 1 μm to 10 μm. In this case, all 10 points should be located within the first internal electrode 121 or the second internal electrode 122, and when all 10 points are not located within the first internal electrode 121 or the second internal electrode 122, the position of the reference point may be changed, or the interval of the 10 points may be adjusted.

[0109] The first and second external electrodes 131 and 132 may be applied with voltages of different polarities and may be electrically connected to exposed portions of the first and second internal electrodes 121 and 122 , respectively.

[0110] According to the above configuration, when a predetermined voltage is applied between the first external electrode 131 and the second external electrode 132, charges are accumulated between the first internal electrode 121 and the second internal electrode 122 facing each other. At this time, the capacitance of the multilayer capacitor 100 may be proportional to the overlapping area of ​​the first internal electrode 121 and the second internal electrode 122 overlapping each other in the T-axis direction in the active region.

[0111] The first outer electrode 131 and the second outer electrode 132 may be respectively arranged on the third surface and the fourth surface of the capacitor body 110, and the first outer electrode 131 includes a first connecting portion connected to the first inner electrode 121 and includes a first band portion arranged at the edge where the third surface of the capacitor body 110 intersects with the first surface and the second surface or at the edge where the third surface intersects with the fifth surface and the sixth surface, and the second outer electrode 132 includes a second connecting portion connected to the second inner electrode 122 and includes a second band portion arranged at the edge where the fourth surface of the capacitor body 110 intersects with the first surface and the second surface or at the edge where the fourth surface intersects with the fifth surface and the sixth surface.

[0112] The first and second band portions may extend from the first and second connecting portions, respectively, to a portion of the first and second surfaces or a portion of the fifth and sixth surfaces of the capacitor body 110. The first and second band portions may be used to improve the adhesion strength of the first and second external electrodes 131 and 132 to the capacitor body 110.

[0113] As an example, each of the first and second external electrodes 131 and 132 may include a sintered metal layer contacting the capacitor body 110 , a conductive resin layer disposed to cover the sintered metal layer, and a plated layer disposed to cover the conductive resin layer.

[0114] The sintered metal layer may contain conductive metal and glass.

[0115] As an example, the sintered metal layer may include copper (Cu), nickel (Ni), silver (Ag), palladium (Pd), gold (Au), platinum (Pt), tin (Sn), tungsten (W), titanium (Ti), lead (Pb), their alloys or combinations thereof as the conductive metal, for example, the sintered metal layer may include copper (Cu) or a copper (Cu) alloy. When the conductive metal includes copper, a metal other than copper may be included in an amount of 5 mol or less relative to 100 mol of copper.

[0116] As an example, the sintered metal layer may include a composition comprising an oxide as the glass, for example, the oxide may include one or more selected from the group consisting of silicon oxide, boron oxide, aluminum oxide, transition metal oxide, alkali metal oxide and alkaline earth metal oxide. The transition metal may include zinc (Zn), titanium (Ti), copper (Cu), vanadium (V), manganese (Mn), iron (Fe), nickel (Ni) or a combination thereof. The alkali metal may include lithium (Li), sodium (Na), potassium (K) or a combination thereof. The alkaline earth metal may include magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) or a combination thereof.

[0117] Alternatively, the conductive resin layer may be formed on the sintered metal layer, for example, may be formed to completely cover the sintered metal layer. In addition, the first and second external electrodes 131 and 132 may not include the sintered metal layer, and in this case, the conductive resin layer may directly contact the capacitor body 110.

[0118] The conductive resin layer may extend to the first and second surfaces or the fifth and sixth surfaces of the capacitor body 110, and the length of the region (i.e., the band portion) where the conductive resin layer extends to the first and second surfaces or the fifth and sixth surfaces of the capacitor body 110 may be longer than the length of the region (i.e., the band portion) where the sintered metal layer extends to the first and second surfaces or the fifth and sixth surfaces of the capacitor body 110. In other words, the conductive resin layer may be formed on the sintered metal layer, and may be formed to completely cover the sintered metal layer.

[0119] The conductive resin layer contains resin and conductive metal.

[0120] The resin contained in the conductive resin layer is not particularly limited as long as it has adhesiveness and impact absorption and can be mixed with the conductive metal powder to form a paste, and for example, may contain phenolic resin, acrylic resin, silicone resin, epoxy resin, or polyimide resin.

[0121] The conductive metal contained in the conductive resin layer serves to electrically connect the first and second internal electrodes 121 and 122 to the sintered metal layer.

[0122] The conductive metal contained in the conductive resin layer may have a spherical shape, a flake shape, or a combination thereof. In other words, the conductive metal may be formed only in a flake shape, only in a spherical shape, or in a mixed form of a flake shape and a spherical shape.

[0123] Here, the spherical shape may include a shape that is not a completely spherical shape, for example, a shape in which the ratio of the length of the major axis to the length of the minor axis ([major axis] / [minor axis]) is 1.45 or less. The flake shape may refer to a flat and elongated shape, but is not particularly limited, and, for example, may be a shape in which the ratio of the length of the major axis to the length of the minor axis ([major axis] / [minor axis]) is 1.95 or more.

[0124] The first and second external electrodes 131 and 132 may further include a plating layer disposed on an outer side of the conductive resin layer.

[0125] The plating layer may include one of nickel (Ni), copper (Cu), tin (Sn), palladium (Pd), platinum (Pt), gold (Au), silver (Ag), tungsten (W), titanium (Ti), lead (Pb), etc. or an alloy thereof. As an example, the plating layer may be a nickel (Ni) plating layer or a tin (Sn) plating layer, or may be a nickel (Ni) plating layer and a tin (Sn) plating layer stacked in sequence, or may be a tin (Sn) plating layer, a nickel (Ni) plating layer and a tin (Sn) plating layer stacked in sequence. Optionally, the plating layer may include a plurality of nickel (Ni) plating layers and / or a plurality of tin (Sn) plating layers.

[0126] The plating layer may improve the mounting characteristics of the multilayer capacitor 100 on the substrate as well as the structural reliability, external durability, heat resistance, and equivalent series resistance (ESR) of the multilayer capacitor.

[0127] Hereinafter, specific examples of the present invention will be presented. However, the following examples are only intended to specifically illustrate or describe the present invention and should not be construed as limiting the scope of the present invention.

[0128] (Example)

[0129] To a dielectric raw material powder containing barium titanate (BaTiO3) powder having an average particle size of 100 nm as a main component of a base material powder, 1.5 mol of Dy2O3, 0.2 mol of Mn3O4, 0.2 mol of V2O5, 0.6 mol of BaCO3, 1.5 mol of SiO2, 0.7 mol of MgCO3 and 0.5 mol of Al2O3 are added as auxiliary components relative to 100 mol of the main component. A binder, an organic solvent (such as ethanol / toluene) and zirconia balls are added to the mixture and wet compounded to prepare a dielectric slurry.

[0130] The prepared dielectric slurry was used to prepare a dielectric green sheet having a thickness of about 0.6 μm using a head discharge type roll-on forming coater.

[0131] A conductive paste layer containing nickel (Ni) is printed on the surface of a dielectric green sheet, and the dielectric green sheets with the conductive paste layer are stacked and pressed, thereby preparing a dielectric green sheet laminate. Then, the laminate is pressed and cut.

[0132] Thereafter, the cut laminate is heated to remove the binder, and then fired in a high-temperature reducing atmosphere to form a ceramic body. In the firing process, firing is performed at a temperature of 1050°C to 1250°C for 1 hour in a reducing atmosphere (H2O / H2 / N2 atmosphere, 0.1% H2 / 99.9% N2), and then heat treatment is performed by reoxidation at 1000°C for 3 hours in a nitrogen (N2) atmosphere. At this time, firing is performed while adjusting the average diameter of the core of the dielectric grains and the ratio of the average diameter of the dielectric grains to the average diameter of the core as shown in Table 1, so that in the early stage of firing, the secondary components are diffused uniformly at a high concentration to the surface of the main component.

[0133] Furthermore, in the cooling process, rapid cooling is performed so that the average diameter of dielectric grains in the dielectric layer is adjusted to fall within the range of 120 nm to 200 nm.

[0134] Next, external electrodes were formed on the fired ceramic body using copper (Cu) paste through a termination process and electrode firing, thereby manufacturing a multilayer capacitor according to Example 1, in which the thickness of the dielectric layer was about 0.4 μm and the thickness of the internal electrode was about 0.37 μm.

[0135] Multilayer capacitors of Examples 1 to 3 and Comparative Examples 1 to 6 that differed from each other in (1) the average diameter of the cores and (2) the ratio of the average diameter of the dielectric crystal grains to the average diameter of the cores were prepared and shown in Table 1 below.

[0136] (Experimental example)

[0137] Experimental Example 1: C CS / C GB analyze.

[0138] Regarding the multilayer capacitors of Examples 1 to 3 and Comparative Examples 1 to 6, first, thin film samples for transmission electron microscopy (TEM) observation were prepared by FIB microsampling. Then, the thin film samples were subjected to an Ar grinding process, thereby preparing scanning transmission electron microscopy (STEM) samples with a thickness of about 80 nm for observing core-shell walls and grain boundaries.

[0139] Next, the probe diameter of the electron beam is set to about 0.5 nm or less, and the core-shell wall and the grain boundary are analyzed. At this time, only the core-shell wall and the grain boundary that are not tilted with respect to the incident electron beam are analyzed.

[0140] Reference Fig. 9 and Fig.10, the full width at half maximum (FWHM) of the peak shown in the line profile for grain boundary measurement such as the HAADF-STEM image of the grain boundary (magnification × 2.25) was measured and defined as the thickness of the grain boundary, and comparative analysis of regions with the same thickness was performed as a composition analysis of the grain boundary.

[0141] In addition, the full width at half maximum (FWHM) of the peak shown in the line profile for the core-shell wall 1111c measurement is measured and defined as the thickness of the core-shell wall 1111c, and comparative analysis of regions with the same thickness is performed as a composition analysis of the core-shell wall 1111c.

[0142] In addition, the composition analysis of the grain boundary can be performed as follows: EDS analysis is performed by irradiating one point of the dielectric layer with an electron beam under the condition that the grain boundary is not tilted and the probe diameter satisfies the above conditions. At this time, measurement is performed at 20 points of each sample, and the average value is calculated.

[0143] In addition, the composition analysis of the core-shell wall 1111c can be performed as follows: under the condition that the core-shell wall 1111c is not tilted and the probe diameter satisfies the above conditions, EDS analysis is performed by irradiating one point of the dielectric layer with an electron beam. At this time, measurement is performed at 20 points of each sample, and the average value is calculated.

[0144] Thus, the average concentration C of the accessory components in the grain boundaries is measured. GB (at%) and the average concentration C of the secondary components in the core-shell wall 1111c CS (at%), and C CS / C GB The values ​​of are shown in (3) of Table 1 below.

[0145] Experimental Example 2: Analysis of the ratio of the average diameter of the cores to the average diameter of the dielectric grains.

[0146] The cross-sectional samples are prepared as follows: the multilayer capacitors of Examples 1 to 3 and Comparative Examples 1 to 6 are placed in an epoxy resin mixture and cured, and the L-axis direction-T-axis direction side surface of the capacitor body is ground to the middle of the W-axis direction, or the center of the capacitor body in the W-axis direction is cut along the L-axis direction and the T-axis direction, and fixed and maintained in a vacuum atmosphere chamber.

[0147] Next, the cross-sectional sample was etched for about 10 seconds using a 0.2% hydrofluoric acid solution and a 1% nitric acid solution, and a scanning electron microscope (SEM) image was obtained.

[0148] In the SEM image, an arbitrary dielectric layer 111 is selected at the center point in the length direction (L-axis direction) or the width direction (W-axis direction), and the diameter of the dielectric crystal grain and the diameter of the core are measured.

[0149] At this time, the average value of the length of the longest major axis and the length of the longest minor axis among the minor axes orthogonal to the longest major axis can be calculated as the diameter of the dielectric crystal grain or the diameter of the core.

[0150] In addition, more than 500 dielectric grains were selected from the same dielectric layer, and the diameter of the dielectric grains and the diameter of the core were measured, and the average value was calculated. Then, the ratio of the average diameter of the dielectric grains to the average diameter of the core was obtained, and the ratio of the average diameter of the dielectric grains to the average diameter of the core is shown in (2) of Table 1 below.

[0151] Experimental Example 3: Analysis of the ratio (%) of the average number of dielectric grains including the core-shell wall.

[0152] For the multilayer capacitors of Examples 1 to 3 and Comparative Examples 1 to 6, scanning electron microscope (SEM) images of the above-mentioned cross-sectional samples were prepared, and any dielectric layer was selected, and an area with an effective area size of 1 μm×1 μm (unit area) was set as a measurement object, and the number of dielectric grains in the unit area and the number of dielectric grains including the core-shell wall were measured.

[0153] Then, the average number of dielectric grains and the average number of dielectric grains including core-shell walls were obtained, and the ratio of the average number of dielectric grains including core-shell walls to the average number of dielectric grains is shown in (4) of Table 1 below.

[0154] Experimental Example 4: Reliability evaluation of multilayer capacitors.

[0155] For each of the multilayer capacitors in Examples 1 to 3 and Comparative Examples 1 to 6, 400 samples were taken and evaluated whether the samples were maintained at 125°C under 10V conditions for 10 hours or more without failure, and when the samples were maintained without failure, the reliability was determined to be OK and marked as "○" in (5) of Table 1 below, and when the samples failed, the reliability was determined to be poor and marked as "×" in (5) of Table 1 below.

[0156] (Table 1)

[0157]

[0158] Referring to Table 1, in the case of the multilayer capacitors of Examples 1 to 3, C CS / C GBThe value of is in the range of 0.40 to 0.75, and the ratio of the average diameter of the dielectric grains to the average diameter of the core is in the range of 1.2 to 1.8, and the ratio of the average number of dielectric grains including the core-shell wall is greater than or equal to 70%. Therefore, it can be seen that the reliability of the multilayer capacitors of Examples 1 to 3 is better than that of the multilayer capacitors of Comparative Examples 1 to 6.

[0159] While the present invention has been described in conjunction with what are presently considered to be practical examples, it is to be understood that the invention is not limited to the disclosed examples. Rather, the present invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

[0160] <Description of Reference Numerals>

[0161] 100: Multilayer capacitor

[0162] 110: Capacitor body

[0163] 111: Dielectric layer

[0164] 1111: Dielectric grains

[0165] 1111a: Core of dielectric grain

[0166] 1111b: Shell of dielectric grains

[0167] 1111c: Core-shell wall of dielectric grains

[0168] 112, 113: Coverage area

[0169] 121: First inner electrode

[0170] 122: Second inner electrode

[0171] 131: First outer electrode

[0172] 132: Second external electrode.

Claims

1. A multilayer capacitor comprising: a capacitor body including a dielectric layer and inner electrodes; as well as an outer electrode disposed on an outer surface of the capacitor body, Wherein, the dielectric layer comprises a plurality of dielectric grains, and the dielectric grains comprise a main component and a secondary component. At least one of the plurality of dielectric grains has a core-shell structure, wherein the core-shell structure includes a core, a shell, and a core-shell wall between the core and the shell. At least two of the plurality of dielectric grains have a grain boundary between them, and When the average concentration of the accessory component in the grain boundary expressed in at% is C GB and the average concentration of the secondary component in the core-shell wall expressed in at% is C CS When C CS / C GB In the range of 0.40 to 0.

75.

2. The multilayer capacitor according to claim 1, wherein A ratio of an average diameter of the dielectric grains to an average diameter of the cores is in a range of 1.2 to 1.

8.

3. The multilayer capacitor according to claim 1 or 2, wherein: A ratio of an average number of the dielectric grains including the core-shell wall per unit area to an average number of the dielectric grains per unit area is greater than or equal to 70%.

4. The multilayer capacitor according to claim 1 or 2, wherein: The average diameter of the dielectric grains is in the range of 120 nm to 200 nm.

5. The multilayer capacitor according to claim 1 or 2, wherein: The average diameter of the core is in the range of 100 nm to 150 nm.

6. The multilayer capacitor according to claim 1 or 2, wherein: The average thickness of the dielectric layer is in the range of 0.05 μm to 0.5 μm.

7. The multilayer capacitor according to claim 1 or 2, wherein: The inner electrode has an average thickness in a range of 0.05 μm to 0.5 μm.

8. The multilayer capacitor according to claim 1 or 2, wherein: The main component includes BaTiO3, Ba(Ti,Zr)O3, Ba(Ti,Sn)O3, (Ba,Ca)TiO3, (Ba,Ca)(Ti,Zr)O3, (Ba,Ca)(Ti,Sn)O3, (Ba,Sr)TiO3, (Ba,Sr)(Ti,Zr)O3, (Ba,Sr)(Ti,Sn)O3 or a combination thereof.

9. The multilayer capacitor according to claim 1 or 2, wherein: The secondary component includes dysprosium, vanadium, manganese, chromium, silicon, aluminum, magnesium, tin, antimony, germanium, gallium, indium, barium or a combination thereof.

10. The multilayer capacitor according to claim 1 or 2, wherein: The core-shell wall includes the subcomponent in an amount greater than 2.0 mol and less than or equal to 30.0 mol relative to 100 mol of the main component.

11. The multilayer capacitor according to claim 1 or 2, wherein: The shell contains the secondary component in an amount greater than 0.1 mol and less than or equal to 30.0 mol relative to 100 mol of the primary component, and The core includes the subcomponent in an amount of 0.1 mol or less relative to 100 mol of the main component.

12. The multilayer capacitor according to claim 1 or 2, wherein: The core-shell wall contains the secondary component in an amount greater than 2.0 mol and less than or equal to 30.0 mol relative to 100 mol of the primary component, The shell contains the secondary component in an amount greater than 0.1 mol and less than or equal to 30.0 mol relative to 100 mol of the primary component, and The core includes the subcomponent in an amount of 0.1 mol or less relative to 100 mol of the main component.

13. The multilayer capacitor according to claim 1 or 2, wherein: The core-shell wall includes a portion where the molar ratio of the subcomponent to the main component is highest at the interface between the core and the shell.

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