By Gerald Koebrugge, Knuth Albertsen, Wiljan Coppens,
and Wilma Derks
Ferro Electronic Materials Systems B.V., The Netherlands
Today’s automobiles contain an ever-increasing number of electronic gadgets. The need for safer, more comfortable, pollution-free, and cost-effective transportation has created a great opportunity for the electronic industry to develop new systems to fulfill these needs. In the last several years, the use of programmed fuel injection, antilock brake systems, cruise control, airbag systems, air conditioning, and advanced entertainment systems have become standard. In the near future, more technology, such as drive-by wire and hybrid electrical cars, will become commonplace.
This trend creates a challenge for the electronics industry to supply components that can operate in the harsh New X8R Material The X8R compositions are produced by mixing and milling dopant components and BaTiO3. Oxides and carbonates were used as dopant components for the solid state process routing. For the coated version, soluble dopant components were selected. Figure 1 presents the procedure for preparing the formulations.
For the solid state process, BaTiO3 is mixed with the dopant components and the material is milled to the required particle size. For the coated version, BaTiO3 is milled to the target particle size, then the dopants are added underhood environment where they are exposed to high temperatures and a variety of chemicals. For devices using ceramic capacitors, this requirement can be met with X8R materials, which are rated for temperatures to 150°C. Noble metal PME X8R materials have been developed recently with special emphasis on reducing firing temperature and, therefore, lead content, making the material RoHScompliant. However, even though the material can be used to the solution and fixed to the BT.
Disc capacitors are then prepared from granulated powder and pressed in a dye.Electrode ink is applied on each side of the disc capacitor. with up to 95% silver electrodes, these parts are expensive for higher layer count and larger amounts of MLCCs due to the noble metals. Another challenge is the increased use of electronics which can add to vehicle weight because each device requires copper wire to connect the electronic system to the control unit. To reduce weight while increasing available power, many automakers are replacing the current 12 to 24 V system with 42 V systems. In addition, the use of buss systems will reduce weight even further. Because these systems employ a network with addresses for each electronic device, more electronics can be found under the hood and near the engine. As a result, the electronics must operate at higher temperatures. To extend the temperature range of electronics, X7R capacitors (rated at -55° to 125°C) must be replaced with X8R capacitors, which can operate at 150°C.
Noble metal X8R capacitors, based on palladium/silver (PdAg) electrode systems combined with Bi2O3-TiO2, are widely available. However, these parts are expensive and cost-prohibitive for widespread use
in automotive applications. To reduce costs, X8R multilayer ceramic capacitors (MLCCs) constructed of base metal electrodes (BME) are being developed for use in automotive applications. In particular, a barium titanate composition meeting X8R requirements for use with nickel electrodes has been developed that incorporates an improved coating process to help distribute dopants homogeneously over the surface of the BaTiO3 grains. The adhesion of this layer is strong enough to allow dispersion with yttrium-stabilized zirconia (YTZ) beads. MLCCs prepared from the coated powders demonstrate higher insulation resistance and voltage breakdown. The high voltage breakdown and stable DC bias performance of the material makes it applicable for high voltage X7R applications as well.
New X8R Material
The X8R compositions are produced by mixing and milling dopant components and BaTiO3. Oxides and carbonates were used as dopant components for the solid state process routing. For the coated version, soluble dopant components were selected. Figure 1 presents the procedure for preparing the formulations. For the solid state process, BaTiO3 is mixed with the dopant components and the material is milled to the required particle size. For the coated version, BaTiO3 is milled to the target particle size, then the dopants are added to the solution and fixed to the BT. Disc capacitors are then prepared from granulated powder and pressed in a dye.
Electrode ink is applied on each side of the disc capacitor.
MLCCs are prepared from PVB-based green tapes, stacked on a Keko Pal (IX), and use a pure nickel inner
electrode paste. The plates are isostatically pressed and cut into individual MLCCs and the layer thickness of the tape was targeted for 8 to 10 μm after sintering. The firing atmosphere used for the disc capacitors and
MLCCs during sintering had an oxygen partial pressure of about 1 x 10-10 bar at 1300°C. The fired parts were re-oxidized at 1000°C for two hours in moistened nitrogen. After firing, the MLCCs were tumbled and copper terminations were applied.
The capacity was measured with an Agilent 4278A LCR meter. This bridge was connected to a Vötsch VT7004 to
measure the temperature dependence of the capacitance. HALT (highly accelerated lifetime testing) was performed by monitoring insulation resistance at 140°C. A voltage of 50 V/μm was applied during this test to discriminate between different materials and to provide an indication of reliability at the severe application conditions. TEM studies were performed using a TECNAI F30ST TEM (FEI) with field emitter gun (FEG) operated at 300 kV. A 4 μl suspension of powder in alcohol was placed on a copper grid supporting a formvar carbon film. Subsequently, the grid was allowed to dry on paper. As a result, part of the suspension was left behind on the carbon film.
Coated Technology vs. Solid State:
The key factor in this study was the distribution of the dopants on the BaTiO3. Therefore, the material was examined by TEM in combination with EDX element mapping. Figure 2 shows the TEM scan for a material produced with solid state mixing and milling. In this experiment, EDX was performed on three different spots.
The elemental spectra clearly demonstrate that for areas 2 and 3, Ba, Ti, and O are the important elements, whereas for area 1 (small grain near the coarser grain), Mn is present. The contribution of Ba and Ti in this spectrum is from the BaTiO3 neighbor. Even though the dopant particle is very fine, it is still a detectable isolated grain. Other dopant elements can also be detected on different spots as isolated grains. For coated materials, single grains of dopants cannot be found. Figure 3 shows a TEM micrograph of the coated material in combination with several elemental maps (over the marked area). The elemental map is represented by a
distribution of intensities, as detected by EDX. The intensity is proportional to the concentration in mass percent.
The micrographs demonstrate the detectable dopant elements Mn, Si, and Y are well distributed over the surface
of the BaTiO3 grain. Dopant elements are mainly observed on the edge of the grain where the concentration of Ba
and Ti is low. At higher Ba and Ti concentrations, signals from other elements become weak or disappear. Milling resistance of the coating is important in applications of coated dielectric powders. During MLCC processing, the powders are dispersed in organic solvents by using ZrO2, which can damage or strip off the coating layer on top of the grains. Adhesion of the coating to the grain is very important to prevent the separation of the coating
from the grain.
Figure 4 presents STEM micrographs of coated material in combination with elemental maps for Ba, Ti, Mn, Si, and
Y after milling four hours in ethanol-toluene with binder and 1 mm YTZ balls. Again, the intensity is related to the concentration in mass percent.
The elemental maps demonstrate the coating remains on top of the grain. For most of the elements, the intensity appears at the edge of each grain. The presence of Zr is due to milling and indicates the intensity of milling during the dispersing process; however, the coating stays on the grains. Full BME-X8R compositions were prepared using both the solid state and the coating process. Figure 5 lists typical powder properties, and Figure 6 shows typical electrical properties for 40 nF MLCCs.
D50 [μm] 0.8
D90 [μm] 1.4
BET [m2/gr] 5.5
The data in Figure 6 indicates both insulation resistance (IR) and breakdown voltage (BDV) are higher for the
coated material at room temperature, thus illustrating processing conditions affect material properties. It seems reasonable to assume the improved homogeneity of the coated material will also improve reliability under elevated temperature and voltage conditions.
Figure 6: Typical Electrical Properties of 8μm MLCCs
Coated Solid state
K Value >2000 >2000
DF [%] 2.3 2.1
IR [GΩ] @ 25°C 93 78
RC [s] @ 25°C 3900 3200
VBD [V/μm] 157 129
HALT [h] 140°C,
50V/μm >100 >100
Figure 7 shows a typical micrograph of an X8R MLCC produced from the coated materials and demonstrates a very dense ceramic is produced after firing at 1320° to 1350°C. Layer thickness is about 8 μm.
The temperature dependence of the formulation is presented in Figure 8. A flat TCC curve has been obtained for 10 μm 1206 MLCCs. Capacity varies by only a few percentage points over a wide temperature range (+7.5% from -55° to 125°C) and drops at temperatures above 130°C to about -11% at 150°C, meeting the X8R temperature specification.
Electrical properties indicate the potential of this material. Typical K values are sufficiently high for use in 10 μm, 0603, 47 nF capacitors. The RC value at room temperature is higher than 3000 s. The stable temperature behavior of the material makes it acceptable not only for automotive applications but also for other areas where use condition temperature limits the suitablility of X7R capacitors. The higher operating temperatures being seen in many electronic applications require components that can withstand these conditions.
In addition, a voltage breakdown above 120 V/μm (150 V/μm for the coated version) implies the possibility of using this material in high voltage applications. This is supported by the DC bias behavior of MLCCs with 10 μm dielectric thickness, as shown in Figure 9. This curve indicates that 80% of the initial capacity is available at 50 V (5 V/μm) and 50% of the capacity is available at 100 V (10 V/μm). Aging tests, in Figure 10, indicate the aging loss is about 1% from 24 to 240 hours.
Higher operating temperatures in automotive electronic applications require the use of X8R MLCCs. To reduce costs, base metal (BME) compositions have been developed to replace precious metal compositions. Furthermore, the addition of specialty coatings improves some of the electrical properties such as IR and BDV. In addition, the BME-X8R material has promising dielectric properties and thermal stability. It may also be suitable for
use in high voltage X7R applications.
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