WO2015144201A1

WO2015144201A1 - Fiber varistor and method of manufacturing a fiber varistor

Info

Publication number: WO2015144201A1
Application number: PCT/EP2014/055852
Authority: WO
Inventors: Marco Cologna; Stefan Denneler; Carsten Schuh
Original assignee: Siemens Aktiengesellschaft
Priority date: 2014-03-24
Filing date: 2014-03-24
Publication date: 2015-10-01

Abstract

The invention refers to a varistor electronic component comprising a compound made of a plurality of grains, each of which comprises a grain boundary phase enveloping the grain and the grains are electrically connectable along an applicable electric field via their grain boundary phases mechanically contacting each others. The invention is characterized by that the grains with their grain boundary phases are provided by elongate fibers (1) each of which having a length, a thickness and a core-shell structure, whereby for each fiber (1) each core (3) provides the grain and is a fiber bulk; and each shell (5) provides the grain boundary phase. Also a corresponding method of manufacturing is suggested.

Description

Fiber varistor and method of manufacturing a fiber varistor The invention refers to a fiber varistor according to the preamble of claim 1 and methods of manufacturing a fiber varistor according to preambles of ancillary claims 12 and 13. Varistors, which means variable resistors, are well estab^¬ lished technologically important devices because of their highly non-linear electrical characteristics. Their principle use is as reversible solid-state switch with large-energy- handling capabilities.

Varistors are typically used in parallel with circuits to protect them from voltage surges. In normal use, when they are subject to a voltage below their characteristic switch voltage, they behave as highly resistive elements and pass only a very low current, the so called leakage current. When the voltage exceeds the switch voltage, the varistor becomes highly conducting above such switch voltage. Two regimes can be identified above such switch voltage. First the varistor is highly non-linear, where the current varies by orders of magnitude with only small changes in voltage. Second in the so called "up-turn region", where the varistor at the highest voltage is, with its material is again ohmic.

A prior art leading varistor for high voltage applications is ZnO based. These varistors are polycristalline ceramics made from ZnO doped with a Bi203-rich phase. Usually at least two types of dopants are used: First a type which is soluble in the grains, to affect grain resistivity, which are e. g. transition metals, and the second type is of a larger insoluble ion, which segregates to the grain bounda^¬ ries. These additives are the origin of nonlinearity in ZnO matrix by forming electron trapping states at grain boundary regions that are responsible for potential barrier for- mation, causing a critical voltage for breakdown per boundary. At a macroscopic scale, a total breakdown voltage of the device is observed, which is proportional to the number of grain boundaries between two electrodes. Dopants affect also the non-linearity and degradation characteristics. The grain size of a typical commercial device ranges from few micrometers to few tenths of micrometers.

Prior art varistors are typically produced by traditional ceramic powder technologies, such as powder mixing, pressing and sintering. The usual sintering mechanism is liquid phase sintering through the B12O3 rich phase. The properties of the final device depend strongly upon additives type, dis^¬ tribution and amount, sintering conditions, or grain sizes.

A good high voltage varistor should have high voltage gradi^¬ ent and sharp change from linear to nonlinear, low leakage current and low degradation. The following prior art methods have been employed to in^¬ crease the voltage gradients in ZnO varistors. First is low^¬ ering of sintering temperatures and time. As a result, the growth of ZnO grains is limited and the macroscopic break^¬ down voltage per unit of length increases. However, the other electrical performance parameters become worse.

Second method is the use of additives as grain growth in^¬ hibitors, e. g. Sb2C>3, S1O2, ^2^3 and various rare-earth ox^¬ ides. Although the breakdown voltage increases, the leakage currents also increase and the nonlinear coefficients worsen .

Third is the strategy that is being intensively researched with some positive results, which is in particular the use of nanosized powders. However, ZnO has high tendency to grain growth and the grain size resulting from conventional sintering is limited to few micrometers. Fourth non conventional sintering methods are also being pursued, but their industrial applicability is still de^¬ bated . It is an object of the present invention to provide a fiber varistor with in comparison to the prior art increased switch voltage field, which is also called breakdown volt^¬ age, a decreased degradation and decreased failure risk, also a decreased leakage current. Accordingly it is an ob- ject of the present invention to disclose a new concept of a varistor and a method for producing the same. The performances of such device are improved compared to the prior art, in terms of the higher breakdown voltage, which is also called switch voltage, per unit of length, which leads to lower material cost and volumetric hindrance, in terms of lower degradation and failure risk and in terms of a better electrical performance. The object is solved by a varistor electronic component according to the main claim, and the methods of the ancillary claims 12 and 13.

According to a first aspect a varistor electronic component is provided comprising a compound made of a plurality of grains, each of which comprises a grain boundary phase enveloping the grain and the grains are electrically connect- able along an applicable electric field via their grain boundary phases mechanically contacting each other, characterized by that the grains with their grain boundary phases are provided by elongate fibers each of which having a length, a thickness and a core-shell structure, thereby for each fiber each core provides the grain and is a fiber bulk and each shell provides the grain boundary phase.

A phase is a region of space throughout which all physical properties of a material are essentially uniform.

According to a second aspect a method of production of an in^¬ ventive varistor electronic component characterized by per^¬ forming the following steps is suggested. Providing, particu^¬ larly aligned, fibers comprising a core-shell structure; heat treatment of the aligned core-shell fibers to create an array of the core-shell fibers.

The fibers form a dense array, such that the fibers contact each other and there is no or merely a small space between neighboring fibers.

The invention consists in conceiving a new concept for a varistor, composed of fibers, especially of ZnO, each com- prising a shell being used as a grain boundary phase, so that a potential barrier will arise at the grain boundary regions between two adjacent fibers. The benefits of the varistor of the invention arise from the size in the direc^¬ tion along the applied electric field and from the fibrous microstructure perpendicular to the field. The varistor of the invention has the potential to outperform the state of the art varistors in many aspects. In specific, the advan^¬ tages are: First a higher breakdown field in a great order of magnitude arising from the small grain size along the direction of the field. The threshold voltage of a varistor is in fact at a first approximation proportional to the number of grain boundaries in series between the electrodes. For example a decrease of a factor of 10 in grain size, which is from 2 ym to 0,2 ym, would bring a tenfold increase in the threshold voltage per centimetre of varistor thickness.

Second a lower varistor thickness results in a weight and volumetric hindrance reduction. For example the thickness of a varistor needed for the application under a certain voltage difference would be only on tenth of the better state of the art varistors, with obvious material and volume reduc^¬ tions .

Third a lower degradation arises from well defined and homo^¬ geneous fiber diameters and grain boundary thickness distri^¬ butions. State of the art varistors show non-uniform grain size distribution and grain boundary thicknesses. Such inho- mogenities give rise to concentration of current that in^¬ duces rapid local degradation of the device and eventually failure. The well defined microstructure of the invention will give the advantage of a homogeneous current distribu^¬ tion within the varistor electronic component.

Fourth a sharper change from linear to non-linear behaviour is provided in a better overall electrical performance, de- riving from a better distribution of the various grain and grain boundary dopants and moreover from a uniform thickness of all active grain boundaries. It has been in fact reported that a homogeneous distribution of dopants plays a major role in the electrical performance of a varistor. The origin of varistor behaviour lies in the potential barriers at the grain boundary regions which sum up to give the microscopic electrical characteristics of the device. When a transient energy surge is absorbed, it is distributed between the various fibers intergranular junctions. Each boundary shows the same electrical response, given by same thickness and same distribution of dopants, the microscopic electrical re^¬ sponse of the varistor is well defined.

Fifth a lower leakage current can be achieved. In the leak- age region current flows preferentially through the skeleton of the grain boundary phase respectively Bi-rich phase. With a fibrous microstructure the paths for leakage current flow are substantially reduced compared to a prior art multigrain microstructure .

Sixth a higher thermal uniformity and a higher resistance to thermal shocks due to the fibrous and homogeneous micro- structure can be achieved. Seventh the bending strength can also be increased because of the fibrous nature of the varistor electronic component. Further advantageous embodiments are claimed by the sub^¬ claims .

According to an embodiment of the invention the lengths of the fibers can be in the range of micrometers to millimeters and the thicknesses of the fibers are in the range of na^¬ nometers, especially 1 to 5000nm. According to a specific embodiment the fibers can be nanofibers. According to a further embodiment of the invention the fibers can be equally aligned to each other and/or vectors of the electric field can be applicable perpendicular to the elon^¬ gate extensions of the fibers. According to a further embodiment of the invention the fibers can form an array, whereby layers of fibers are stacked.

According to a further embodiment of the invention cross sec^¬ tions and/or lengths of the fibers can be equal

According to a further embodiment of the invention cross sec^¬ tions of the fibers can be circular, oval, rectangular or square . According to a further embodiment of the invention the fiber bulk can consist of a material out of ZnO, SiC, Sn02, Ti0₂,

According to a further embodiment of the invention for im- proving electrical performance of the fiber bulk it's materi^¬ al can be doped by at least one dopant.

According to a further embodiment of the invention for creating the shell at least one dopant can be added to a basic fi- ber material before creating the core-shell structure. According to a further embodiment of the invention the dopant can be at least one out of B1₂O₃ , Sb₂<0₃, Pr₂0₃, Mn0₂, Si0₂, Cr₂0₃, C03O4, AI 2O3 . According to a further embodiment of the invention the shell can be at least one out of B12O3 , Sb2<03, Pr₂03, Mn02, Si0₂, Cr₂0₃, C03O4, AI 2O3 .

According to a further embodiment of the invention a produc- ing a basic fiber material consisting of basic fibers each being modified by means of at least one modifier to obtain a core-shell structure, and an aligning the core-shell fibers mechanically contacting each others with their shells can be performed .

According to a further embodiment of the invention a produc^¬ ing an aligned green monolithic fiber material consisting of basic fibers comprising a homogeneous structure, and a calcinating the fiber material without removing inter-fibers porosity areas and infiltrating the porosity areas with at least one modifier to make the basic fiber material obtain a core-shell structure can be performed.

Green monolithic fiber material is organic polymer material or a compound of anorganic particles embedded in an organic matrix. The fibers can be made by conventional fiber spin^¬ ning. It can be a short or long fiber mat with already aligned fibers. According to a further embodiment of the invention producing and aligning of the core-shell fibers to each others, partic^¬ ularly with their elongate extensions parallel to each other, by electro spinning with a coaxial needle or by rotary jet spinning can be performed.

According to a further embodiment of the invention each fiber bulk can be made out of a basic fiber material precursor so^¬ lution or suspension. According to a further embodiment of the invention the basic fiber material can be one out of ZnO, SiC, Sn0₂, Ti0₂, SrTi0₃,

According to a further embodiment of the invention for improving electrical performance of the fiber bulk at least one dopant can be directly added to the fiber material precursor. According to a further embodiment of the invention the dopant can be out of acetates, nitrates or citrates or at least one out of Bi₂0₃, Sb₂0₃, Pr₂0₃, Mn0₂, Si0₂, Cr₂0₃, Co₃0₄, A1₂0₃.

According to a further embodiment of the invention each shell can be made by adding to the basic fiber material the at least one modifier, which is a dopant or a grain boundary phase material, particularly as a precursor solution or suspension . According to a further embodiment of the invention the shell and/or the shell creating dopant can be rich of at least one out of Bi₂0₃, Sb₂0₃, Pr₂0₃, Mn0₂, Si0₂, Cr₂0₃, Co₃0₄, A1₂0₃ or their respective acetate, nitrate or citrate precursors. According to a further embodiment of the invention a provid^¬ ing the fibers in shape of a mat, and a cutting, a stacking and a laminating of the fiber mat can be performed.

According to a further embodiment of the invention an infil- trating with an additional fiber bulk material matrix or dopant rich phase can be performed.

According to a further embodiment of the invention after a or instead of the heat treatment, a sintering, especially rapid sintering, to achieve final microstructure of the varistor electronic component can be performed. According to a further embodiment of the invention a

calcinating the core-shell fibers by a or the heat treatment before a sintering can be performed.

According to a further embodiment of the invention an application of electrodes and a packaging to the varistor can be performed .

According to a further embodiment of the invention, in order to increase a thermal uniformity, an incorporating of an amount of metallic fibers or layers or fibrous mats perpen^¬ dicular to the electric field direction can be performed.

The invention is described by means of embodiments in rela^¬ tion to the drawings. The drawings show

Fig. 1 a first embodiment of an inventive varistor elec^¬ tronic component;

Fig. 2 a second embodiment of an inventive varistor elec^¬ tronic component;

Fig. 3 a first sight view of the second embodiment of the inventive varistor electronic component;

Fig. 4 a second sight view of the second embodiment of the inventive varistor electronic component;

Fig. 5 a representation of two embodiments of the inventive method of production of a varistor electronic component .

Figure 1 shows a first embodiment of an inventive varistor electronic component. Figure 1 shows a cross sectional view of a compound made of a plurality of grains, each of which comprises a grain boundary phase, enveloping the grain and the grains are electrically connectable along an applicable electric field via the grain boundary phases mechanically contacting each other. According to the invention the grains with their grain boundary phases are provided by elongate fibers 1 each of which having a length, a thickness and a core-shell structure, whereby for each fiber each core 3 provides the grain and is a fibre bulk and each shell 5 pro^¬ vides the grain boundary phase. According to figure 1 the profile of a fiber 1 is round.

The invention solves the problems by introducing a concept of a varistor that is composed of micro- or nanofibers, which can be randomly oriented or aligned. Greater benefits are expected from an aligned configuration. The microstruc- ture of such a varistor electronic component consists of a dense array of aligned for example ZnO fibers 1 with a core 3-shell 5 cross-section. A fiber bulk consists of for exam^¬ ple ZnO, eventually doped, and the shell 5 provides a con^¬ ventional grain boundary phase, which can be rich of for example B12O3. Such varistor electronic component would show varistor behaviour when an electric field is applied perpen- dicular to a length of the fibers 1.

Figure 2 shows a second embodiment of an inventive varistor electronic component, whereby the compound of figure 1 is provided by core-shell fibers 1 comprise an angled cross section. A higher density of an array can be reached in comparison to the round cross-sections.

Figure 3 shows a sight view of a compound according to fig^¬ ure 2. The elongate fibers 1 have an equal thickness but different length. These fibers 1 are arranged such that vec^¬ tors of an electrical field are applicable perpendicular to the elongate extensions of the fibers 1 to provide the varistor functionality. According to figure 3 can be also aligned to each other, such that they extend out of the plain of the cross section view.

Figure 4 shows another sight view on the compound according to figure 2, whereby all fibers 1 have the same length and they are all equally align to each other. They also extend along a common plain.

Figure 5 shows a representation for two embodiments of in- ventive methods of production of an inventive varistor elec^¬ tronic component. By a first step SI a basic fiber material consisting of basic fibers each being modified by means of at least one modifier to obtain a core-shell structure is produced. By a second step S2 the core-shell fibers can be arranged mechanically contacting each other with their shells. These steps SI and S2 can be performed simultane^¬ ously by producing and performing aligning of the core-shell fibers to each other, especially with the elongate exten^¬ sions parallel to each other, by applying electrical spin- ning with a coaxial needle rotary jet spinning method. This possible production method consists correspondingly of the following steps SI and S2 by production of aligned nanofi- bers with a core-shell microstructure by electro spinning with a coaxial needle or rotary jet spinning. Electro spin- ning is a technique suitable for lab scale or mass produc^¬ tion of randomly oriented or aligned fibers or nanofibers, through the use of an electric field. See for example "Poly^¬ mer Advanced Technology, 2011, 22, pages 326, -338". Rotary jet spinning is a newly developed technique that allows the creation of highly aligned fibers or nanofibers by utilizing high-speed, rotating solution jets to extrude fibers. See for example "Nanoletters , 10 [6], 2257-2261 (2010)". The bulk of the fibres consists for example of the ZnO fiber precursor solution or suspension. The dopants that are needed for this ZnO grain bulk can be added directly to the

ZnO precursor, for example in form of acetates, nitrates or citrates. The shell of the fibers consist of the precursor solution/suspension of the for example Bi203-rich grain boundary phase. By a third step S3 a heat treatment of the aligned core-shell fibers is performed to create an array of the core-shell fibers. According to a fourth step S4 a sin^¬ tering, particularly a rapid sintering is performed to achieve a highest possible density and for limiting the growth of the fiber diameters. Conventional or field as^¬ sisted sintering techniques may be used.

The fiber diameter is expected to show limited growth and remain in the sub-micron range during sintering. The grain boundary dopants, such as the Bi-rich phase that gives rise to liquid phase sintering in traditional ZnO varistors pro^¬ duction, are already distributed around the parameter of the fiber 1. A rapid heating may be sufficient to achieve rapid inter-fiber sintering without causing excessive fiber growth .

The heat treatment according to the third step S3 can be re^¬ placed by the sintering step S4. Further processing steps are for example an application of electrodes performed by a fifth step S5 and a packaging step S6, which can be performed unvaried from the conventional technology.

Other method steps can be cutting, stacking and laminating. A fiber or nanofiber mat can be created, cut, stacked and laminated in order to increase the thickness of the varistor electronic component and the pre-sintering density. Addi^¬ tionally to remove organics calcination can be performed during a heat treatment before the sintering.

If needed, the density or electrical properties of an inven^¬ tive varistor may be increased by infiltration with a further ZnO matrix or Bi203-rich phase. According to Figure 5 an alternative production method is stated. This alternative method consists of the following steps. According to a first step SI' a green monolithic fi^¬ ber, eventually doped with grain bulk dopants, is produced. Such a monolithic fiber can consist of ZnO or ZnO precur- sors . By a second step S2 ' a calcination to create dense fi^¬ bers without removing the inter-fibers porosity and an in^¬ filtration of the still porous mictrostructure with the grain boundary dopants is executed. Afterwards a heat treat- ment step S3 is performed followed by the further steps of the first production method.

Claims

1. Varistor electronic component comprising

- a compound made of a plurality of grains, each of

which being enveloped by a grain boundary phase and the grains are electrically connectable along an ap^¬ plicable electric field via their grain boundary phases mechanically contacting each others;

characterized by that

- the grains and their grain boundary phases are pro^¬ vided by elongate fibers (1) each of which having a length, a thickness and a core-shell structure, whereby for each fiber (1)

- each core (3) provides the grain and is a fiber bulk; and

- each shell (5) provides the grain boundary phase.

2. Varistor electronic component according to claim 1, characterized in that

the lengths of the fibers are in the range of micrometers to millimeters and the thicknesses of the fibers are in the range of nanometers, especially 1 nm to 5000 nm.

3. Varistor electronic component according to claim 1 or 2, characterized in that

the fibers are equally aligned to each other and/or vectors of the electric field are applicable perpendicular to the elongate extensions of the fibers.

4. Varistor electronic component according to claim 1, 3,

characterized in that

the fibers form an array, whereby layers of fibers are stacked .

5. Varistor electronic component according to claim 1, 2, 3 or 4 ,

characterized in that

cross sections and/or lengths of the fibers are equal

6. Varistor electronic component according to claim 1, 2, 3, 4 or 5,

characterized in that

cross sections of the fibers are circular, oval, rectangu- lar or square.

7. Varistor electronic component according to one of the precedent claims,

characterized in that

the fiber bulk consists of a material out of ZnO, SiC,

Sn0₂, Ti0₂, SrTi0₃, Zn₇Sb₂0i₂.

8. Varistor electronic component according to one of the precedent claims,

characterized in that

for improving electrical performance of the fiber bulk it's material was doped by at least one dopant.

9. Varistor electronic component according to one of the precedent claims,

characterized in that

for creating the shell at least one dopant was added to a basic fiber material before creating the core-shell struc^¬ ture .

10. Varistor electronic component according to claim 8 or 9,

characterized in that

the dopant is at least one out of Bi₂0₃, Sb₂0₃, Pr₂0₃, Mn0₂, Si0₂, Cr₂0₃, C03O4, A1₂0₃. 1 b

11. Varistor electronic component according to one of the precedent claims,

characterized in that

the shell contains at least one out of B1₂O₃, Sb₂C>₃, Pr₂0₃, Mn0₂, Si0₂, Cr₂0₃, Co₃0₄, A1₂0₃.

12. Method of production of a varistor electronic component, especially according to one of the precedent claims, characterized by performing the steps:

providing (SI, S2), particularly aligned, fibers (1) comprising a core-shell structure;

heat treatment (S3) of the aligned core-shell fibers (1) to create an array of the core-shell fibers (1) .

13. Method according to claim 12,

characterized by

producing (SI) a basic fiber material consisting of basic fibers each being modified by means of at least one modifi^¬ er to obtain a core-shell structure;

aligning (S2) the core-shell fibers (1) mechanically con^¬ tacting each others with their shells (5) .

14. Method according to claim 13,

characterized by

producing (SI) and aligning (S2) of the core-shell fibers

(1) to each others, particularly with their elongate extensions parallel to each other, by electro spinning with a coaxial needle or by rotary jet spinning.

15. Method according to claim 12,

characterized by the steps:

producing (SI') an aligned green monolithic fiber material consisting of basic fibers comprising a homogeneous struc^¬ ture ;

calcinating (S2')the fiber material without removing inter- fibers porosity areas and infiltrating the porosity areas with at least one modifier to make the basic fiber material obtain a core-shell structure.

16. Method according to claim 12, 13, 14 or 15,

characterized by

each fiber bulk is made out of a basic fiber material pre- cursor solution or suspension.

17. Method according to claim 16,

characterized by

the basic fiber material is one out of ZnO, SiC, Sn0₂, Ti0₂, SrTi0₃, Zn₇Sb₂0i₂.

18. Method according to claim 16 or 17,

characterized by

for improving electrical performance of the fiber bulk at least one dopant is directly added to the fiber material precursor .

19. Method according to claim 18,

characterized by

the dopant is out of acetates, nitrates or citrates or at least one out of Bi₂0₃, Sb₂0₃, Pr₂0₃, Mn0₂, Si0₂, Cr₂0₃, Co₃0₄, A1₂0₃.

20. Method according to one of the claims 12 to 19,

characterized by

each shell is made by adding to the basic fiber material the at least one modifier, which is a dopant or a grain boundary phase material, especially as a precursor solution or suspension.

21. Method according to claim 20,

characterized by

the shell and/or the shell creating dopant is rich of at least one out of Bi₂0₃, Sb₂0₃, Pr₂0₃, Mn0₂, Si0₂, Cr₂0₃, Co₃0₄, A1₂0₃ or their respective acetate, nitrate or citrate precursors .

22. Method according to one of the claims 12 to 21, characterized by

providing the fibers in shape of a mat;

cutting, stacking and laminating of the fiber mat.

23. Method according to one of the claims 12 to 22,

characterized by

infiltrating with an additional fiber bulk material matrix or dopant rich phase.

24. Method according to one of the claims 12 to 23,

characterized by

after or instead of heat treatment (S3) sintering (S4), es^¬ pecially rapid sintering, to achieve final microstructure of the varistor electronic component.

25. Method according to claim 24,

characterized by

calcinating the core-shell fibers (1) by a or the heat treatment (S3) before sintering.

26. Method according to one of the claims 12 to 25,

characterized by

application (S5) of electrodes and packaging (S6) to the varistor electronic component.

27. Method according to one of the claims 12 to 26,

characterized by

incorporating an amount of metallic fibers or layers or fi^¬ brous mats perpendicular to the electric field direction.