Abstract:
Casting compound suitable for casting an electronic module, in particular a large-volume coil such as a gradient coil, which is composed of a support material forming a matrix, one or more fillers made of inorganic microparticles, and at least one filler made of polymer nanoparticles.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention concerns a casting compound suitable for casting an electronic module, in particular a large-volume coil such as a gradient coil. 
     2. Description of the Prior Art 
     Gradient coils of a magnetic resonance apparatus are formed of three sub-coils with which magnetic field gradients are generated in the three spatial directions. The X-coils and Y-coils are typically fashioned as saddle coils; the Z-coil is realized by means of a peripheral winding. The individual coils can be constructed both from bundled individual conductors and by separating structures in an electrically conductive plate (advantageously made of copper or aluminum) generated with suitable processing methods with, the remaining material representing the coil winding. The coil windings produced according to the different methods are then glued to an electrically insulating support plate and formed into the shape of a half-cylindrical shell, for example. The individual coil layers are mounted successively on a cylindrical mandrel. Additional components of the coil structure are insulation and reinforcement layers; one or more cooling layers composed of plastic tubes to conduct a cooling fluid (typically water); and possibly items known as shim coils. Additional layers in the coil structure are, for example, secondary windings that serve to externally shield the magnetic field generated by the primary coils. 
     The complete coil structure is cast with a casting compound (a casting resin based on epoxy resin), with which it is intended that all conductor interstices are impregnated without cavities and bubbles. A multitude of requirements are imposed on the casting compound (thus the casting resin). It must have a sufficiently low processing viscosity in order to be able to fill or penetrate all conductor interstices so as to be free of voids and bubbles. It should exhibit a high modulus of elasticity in order to be able to ensure a high overall rigidity and a positionally accurate fixing of the individual windings of the coil. It should also have good thermal conductivity in order to enable effective heat transport from the conductor structures (which generate heat during operation) to the cooling layer. The glass transition temperature (and therefore also the heat forming resistance) should be as high as possible in order to have an optimally constant property profile in the usage temperature range. The coefficient of thermal expansion should be as low as possible, preferably close to that of the other materials (copper conductors, insulation layers, etc.) that are used in order to minimize mechanical stresses (and thus the formation of cracks and delaminations in the coil composite that are due to these stresses) given heating due to operation or cooling from the curing temperature at which the casting resin is cured. In particular, the cracking resistance should be very high. The cracking resistance is characterized by a high critical stress intensity factor K 1C  associated with a high critical fracture energy G 1C . The partial discharge resistance should also be high in order to avoid a damage to the mold material during the service life. The dielectric loss factor should be low. 
     Heat-curing casting resins based on epoxy resin are typically used as casting compounds, specifically for large-volume coils. Such a casting compound typically contains approximately 65% filler, for example quartz powder, aluminum oxide or wollastonite with particle sizes in the micrometer range, meaning that the filler consists of microparticles. 
     A casting compound and a magnetic coil cast with such a casting compound are known from EP 1 850 145 A2. This casting compound is mixed (compounded) with microparticle fillers and inorganic nanoparticles. A good crack resistance based on a combination of a high critical stress intensity factor K 1C  with a high critical fracture energy G 1C  can be achieved with these inorganic fillers or filler combinations. The filler addition produces positive alterations of the aforementioned properties of the casting compound mold to be cured and the heat curing resistance, the fracture toughness, etc. An optimally high volume fraction of filler would be appropriate for these reasons. Particularly the flow behavior of the prepared casting compound, however, is drastically impaired by the filler content. The higher the filler content, the poorer the flow behavior, and therefore the property of being able to impregnate the given conductor interstices, etc. without voids and bubbles. A compromise with regard to the casting resin composition is thus always sought. Overall there is a need for a casting resin that exhibits an optimally high crack resistance—in particular at a high glass transition temperature—with very good flow properties. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a casting resin with good flow properties and high crack resistance. 
     According to the invention, a casting compound suitable for casting an electronic module (in particular a large-volume coil such as a gradient coil) is provided to solve this problem, the casting compound being composed of a support material forming a matrix, one or more fillers made of inorganic microparticles and at least one filler made of polymer nanoparticles. 
     As is typical, the casting compound according to the invention is characterized by a support material forming a matrix as well as one or more fillers made of microparticles. According to the invention, in addition to the microparticle fillers, at least one filler made from a polymer nanoparticle is provided. Surprisingly, it has been shown that the addition of polymer nanoparticles produces a striking improvement of the parameters characterizing crack resistance, namely the critical stress intensity factor K 1C  and the critical fracture energy G 1C . By adding polymer nanoparticles, these parameters defining the crack resistance can even be markedly improved relative to casting compounds with inorganic nanoparticles. 
     Compared with casting compounds that only contain microparticle fillers, the casting compound according to the invention shows a very good flow behavior since this is not negatively affected by the added polymer nanoparticles (due to the particle size). 
     The polymer nanoparticles can be particles of polybutadiene, polystyrol or a polysiloxane or other elastomers, thermoplastics or hybrid materials or—if these are (as is preferred) core-shell nanoparticles, thus particles that consist of a core made from a first material and a shell made from a second material—can contain one of these materials. However, this list is not conclusive; naturally, other organic polymer nanoparticles can also be used. Polymer nanoparticles of one type or mixtures of different types can be used. The shape and size of the nanoparticles that are used can also ultimately be arbitrary, for example spherical, fibrous or like platelets given a size of preferably at least &lt;1000 nm, in particular &lt;100 nm. 
     Furthermore, it can be appropriate to additionally admix at least one filler made of inorganic nanoparticles. Additional parameters of the casting compound can be adjusted as needed by such inorganic nanoparticles. This means that, according to the invention, either only organic polymer nanoparticles or particle mixtures containing polymer and inorganic nanoparticles can be used. 
     The inorganic microparticles and the inorganic nanoparticles can be selected from SiO 2 , Al 2 O 3 , AlN, CaMg[CO 3 ] 2  (dolomite), TiO 2 , BN, Fe 2 O 3 , Fe 3 O 4 , ZnO, SiC, synthetic ceramics or zeolites, chalk, Mg 3 Si 4 O 10 (OH) 2  (talcum), CaSiO 3  (wollastonite), pure carbon-based particles or mixtures of these. Here the shape of the microparticles or nanoparticles can also be arbitrary, for example spherical, fibrous or similar to platelets. The surface of the microparticles and nanoparticles can moreover be modified, for example surface-heated. 
     Although the possibility exists to add the polymer nanoparticles (possibly also the inorganic nanoparticles) in addition to the microparticle fillers that are unmodified (in terms of their content) compared with known casting compounds, an advantageous development provides that at least the polymer nanoparticles (possibly also the inorganic nanoparticles) substitute a portion of the microparticles. This means that the nanoparticles replace a certain percentage of the microparticles, which causes a reduction of the viscosity (assuming that the filler quantity remains the same compared with known casting compounds). 
     As already described, the polymer nanoparticles (possibly also the inorganic nanoparticles) are preferably core-shell nanoparticles, thus particles that consist of a core made of a first material and a shell made of a second material. These core-shell nanoparticles can be based on polybutadiene, polystyrol or a polysiloxane or other elastomers, thermoplastics or hybrid materials insofar as these are thereby organic polymer nanoparticles. 
     The average diameter or the average length (depending on which shape the nanoparticles have, be they polymers or inorganic) should be ≦1000 nm, in particular ≦100 nm. 
     The nanoparticles—be they polymers or inorganic nanoparticles—can each be of only one type or particle mixtures made up of multiple types. Different combinations are thereby conceivable. For example, two or more different polymer nanoparticle types can be mixed with one type of inorganic nanoparticles and vice versa; arbitrary combinations are conceivable. 
     The content of polymer nanoparticles should at maximum be 20% by weight, advantageously ≦10% by weight. Insofar as inorganic nanoparticles are also used, the total content of nanoparticles should likewise no longer be more than 20% by weight; the total content should likewise preferably be ≦10% by weight. 
     The casting compound with the polymer nanoparticle fillers according to the invention overall shows a markedly better behavior or, respectively, markedly better properties than known casting compounds. Polymer nanoparticles—in particular in spherical form—in principle benefit the compound. Filtration or sedimentation effects do not occur due to the particle size and particle shape. The particles additionally penetrate into narrow gaps or into regions already filled with fiber, meaning that upon casting a gradient coil an increase of the filler concentration occurs in these boundary regions within the coil winding due to polymer nanoparticles. The substitution of polymer nanoparticles (for example core-shell nanoparticles) for microscale (thus microparticle) filler significantly improves the mechanical fracture properties, wherein this is not limited with a deterioration of the glass transition temperature. The impact resistance is also simultaneously increased. This expresses itself in an increased shock tolerance of the nanomodified casting compound. In the very complex, multilayer design of a cast coil, this in particular has the effect of an improved property alteration, in particular in the area of the numerous boundary layers. A homogeneous distribution in the system as it is necessary for reproducibility of the property advantages and quality can be achieved with the casting compound according to the invention. 
     The invention also concerns an electronic module—in particular a gradient coil—cast with the casting compound according to the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-section of a portion of a coil according to the invention. 
         FIG. 2  is a graph showing the viscosity curve of a casting compound according to the invention in comparison with a casting compound containing only microparticle fillers. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The superconducting coil  1  shown in  FIG. 1  has a number of individual windings  2  of coil conductors  3  (shown larger than in reality for illustration purposes) that, in the shown exemplary embodiment, has an essentially rectangular cross section. The coil conductors  3  consist of a core  4  that contains the conductive material and an insulation  5  (for example made of a lacquer or thermoplastic coating or a plastic fiber weave) surrounding the core  4 . The individual windings  2  are wound and packed with sufficient density; the spaces and interstices  6  shown here between the individual vertical and horizontal conductor layers are shown enlarged for clarity. The columns are actually most often markedly smaller. Such a coil also normally contains other layers, for example a cooling layer embodying coolant conduits and the like (not shown here but similarly cast). 
     After the winding of the individual conductors, it is now necessary to cast or impregnate these with a casting compound  7  so that the conductors  2  are completely embedded into this casting compound  7 . The casting compound  7  includes a support material  8  forming a matrix, for example an epoxy resin based on bisphenol-A. Other matrix-forming reaction resins can also be used that have the desired properties and requirements in operation. In addition to the support material  8  forming the matrix, the casting compound  7  also include microparticles—thus fillers of microparticle size—embedded in the support material  8  and largely homogeneously distributed there. These microparticles  9  are, for example, selected from SiO 2 , Al 2 O 3 , AlN, CaMg[CO 3 ] 2  (dolomite), TiO 2 , BN, Fe 2 O 3 , Fe 3 O 4 , ZnO, SiC, synthetic ceramics or zeolites, chalk, Mg 3 Si 4 O 10 (OH) 2  (talcum), CaSiO 3  (wollastonite), purely carbon-based particles or mixtures of these. 
     Furthermore, polymer nanoparticles  10 , which are preferably core-shell nanoparticles (for example based on polybutadiene, polystyrol or a polysiloxane or other elastomers, thermoplastics or hybrid materials or the like), are provided as additional filler. The polymer nanoparticles  10  are likewise mixed into the fluid casting compound; like the microparticle fillers, they definitively distribute essentially homogeneously in all gaps or interstices  6  of the individual conductor windings or, respectively, other layers. The average particle size of the introduced polymer nanoparticles should be between 0.5 nm-1000 nm, preferably ≦100 nm. The particle shape (like that of the microparticle fillers) can ultimately be arbitrary, for example round, spherical, oblong etc. for example. The nanoparticles  10  can be of one particle type but can also be mixtures of different particle types. The maximum concentration of the admixed nanoparticles should not exceed 20% by weight of the resin portion forming the matrix; a concentration of less than 10% by weight is preferable. 
     It is also possible to admix inorganic nanoparticles (not shown in detail here) in addition to the polymer nanoparticles  10 . These nanoparticles can be of the same materials as the microparticles. 
     It has been shown that the mechanical properties—in particular the crack resistance, the impact resistance and the bonding to wetted boundary surfaces—can already be significantly improved by a slight addition of the polymer nanoparticles to the epoxy resin system of the casting compound  6 . The flow capability of the casting compound is not negatively affected by the low active content of nanoparticles and as a result of the homogeneous distribution and small size, meaning that the distribution of the resin is adjusted as before in the same manner and consequently the impregnation behavior of the casting compound does not negatively change. As a result of their minimal size, the nanoparticles are also particularly advantageously not filtered out in regions of narrow gaps; rather, it is possible that they can even penetrate into extremely narrow gaps together with the support material and can largely homogeneously distribute there. 
     Various characteristic property values of a typical casting compound containing 66% by weight of a microparticle filler an a casting compound according to the invention in which 3.4% by weight of polymer nanoparticles and 62.6% by weight of microparticle filler are contained (3.4% of microparticle filler are thus replaced with polymer nanoparticles) are compared with one another in the following table. The support material respectively contains a modified epoxy resin based on bisphenol-A as a resin component. A modified anhydride curer based on methyl hexahydrophthalic acid anhydride is respectively contained as a cure component; a tertiary amine is used as an accelerator. Quartz powder with an average particle size of 20 μm and a silanized surface was used as microparticle filler in both examined casting compounds. Spherical core-shell nanoparticles based on polybutadiene with a particle size ≦100 nm was used as a polymer nanoparticle. 
     The characteristic molding material properties—thus the measurement values of the cured casting compound that were determined from corresponding samples—are respectively shown in the table with regard to the respective two examined casting compounds; the respective measurement methods according to ISO or DIN or ASTM are also specified. The sample bodies were respectively cured in a two-stage curing method. In the first stage the curing took place at 80° C. for 8 hours; in the second, subsequent stage the curing took place at 140° C. for 10 hours. The respective measured characteristic solid values can be learned from the left column; as stated, the second column indicates the sample geometry. The determined characteristic solid values for the respective sample consisting of support material forming the matrix and 66% microparticle filler are indicated in the third column while the respective characteristic solid values for the casting compound according to the invention, with 62.6% microparticle filler and 3.4% polymer nanoparticle filler, are shown in the right column. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 3.4% 
               
               
                 Characteristic Molding 
                 Sample 
                 66% 
                 Polymer Nanop. 
               
               
                 Material Property 
                 Geometry 
                 (Microp.) 
                 (62.6% Microp.) 
               
               
                   
               
             
             
               
                 Coefficient of linear thermal 
                   3 × 3 × 4 mm 
                  34 
                  35 
               
               
                 expansion α [ppm] 
               
               
                 (ISO 11359-2) 
               
               
                 Glass transition T G  [° C.] 
                   3 × 3 × 4 mm 
                 103 
                 109 
               
               
                 (ISO 11359-2) 
               
               
                 Flexural modulus of elasticity E 
                 10 × 15 × 125 mm 
                 8747 ± 511 
                 7050 ± 276 
               
               
                 [MPa] (DIN EN ISO 178) 
               
               
                 5.0 mm/min 
               
               
                 Flexural strength [MPa] 
                 10 × 15 × 125 mm 
                  121 ± 9.2 
                  121 ± 1.3 
               
               
                 (DIN EN ISO 178) 
               
               
                 5.0 mm/min 
               
               
                 Impact toughness [kJ/m 2 ] 
                 10 × 15 × 125 mm 
                     12 ± 1.2 
                     15 ± 3.2 
               
               
                 (DIN EN ISO 179) 
               
               
                 Mechanical dissipation factor 
                 10 × 15 × 125 mm 
                 2.51 · 10− 2   
                 2.41 · 10− 2   
               
               
                 tan δ mech  [—] (DIN 65583) 
               
               
                 Storage modulus E′ [MPa] 
                 10 × 15 × 125 mm 
                 8639  
                 6930  
               
               
                 (DIN 65583) 
               
               
                 Loss modulus E″ [MPa] 
                 10 × 15 × 125 mm 
                 217 
                 167 
               
               
                 (DIN 65583) 
               
               
                 Modulus of elasticity in tension [MPa] 
                 10 × 15 × 125 mm 
                 12543 ± 564  
                 9149 ± 586 
               
               
                 (DIN EN ISO 527-2) 
               
               
                 Tensile strength [MPa] 
                 10 × 15 × 125 mm 
                 76.5 ± 4.9 
                 72.1 ± 0.7 
               
               
                 (DIN EN ISO 527-2) 
               
               
                 Critical stress intensity factor 
                 80 × 40 × 4 mm 
                  1.90 ± 0.04 
                  2.75 ± 0.05 
               
               
                 K 1C  [MPa  {square root over (m)} ] (according to 
                 central V-notch 
               
               
                 ASTM E 399) Double Torsion 
                 60° 
               
               
                 Critical fracture energy G 1C  [J/m 2 ] 
                 80 × 40 × 4 mm 
                 337 ± 19 
                 987 ± 36 
               
               
                 (According to ASTM E 399) Double 
                 central V-notch 
               
               
                 Torsion 
                 60° 
               
               
                   
               
             
          
         
       
     
     An improvement is clearly shown at nearly all determined characteristic solid values with the casting compound according to the invention. In comparison to the casting compound to which only microparticles are added, the glass transition temperature and the impact toughness increase. The significant improvement of the critical stress intensity factor K 1C  and the critical fracture energy G 1C  (which are significant criteria of the crack resistance) is in particular noticeable. This means that the casting compound to which polymer nanoparticles have been added is markedly more resistant to cracking than a comparable casting compound without these polymer nanoparticles. 
     These two central values—namely the critical stress intensity factor K 1C  and the critical fracture energy G 1C  for the casting compound with 66% by weight added microparticle filler; for the casting compound with 62.6% microparticle filler and 3.4% inorganic nanoparticle filler; and for a third casting compound with 62.6% microparticle filler and 3.4% polymer nanoparticle filler in the form of spherical SiO 2  nanoparticles with a particle size from 20-30 nm (D50)—are indicated in Table 2 below. This means that this table enables a good comparison between two casting compounds that in one case possess the polymer nanoparticles added according to the invention and in the other case possess no polymer nanoparticles but rather only inorganic nanoparticles in the same mass concentration. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                 3.4% 
                 3.4% 
               
               
                 Characteristic Molding 
                 Sample 
                 66% 
                 Inorganic Nanop. 
                 Polymer Nanop. 
               
               
                 Material Property 
                 Geometry 
                 (Microp.) 
                 (62.6% Microp.) 
                 (62.6% Microp.) 
               
               
                   
               
             
             
               
                 Critical stress intensity 
                 80 × 40 × 4 mm 
                 1.90 ± 0.04 
                 2.14 ± 0.06 
                 2.75 ± 0.05 
               
               
                 factor K 1C  [MPa  {square root over (m)} ] 
                 central V-notch 
               
               
                 (according to ASTM E 
                 60° 
               
               
                 399) Double Torsion 
               
               
                 Critical fracture energy 
                 80 × 40 × 4 mm 
                 337 ± 19  
                 433 ± 25  
                 987 ± 36  
               
               
                 G 1C  [J/m 2 ] 
                 central V-notch 
               
               
                 (According to ASTM E 
                 60° 
               
               
                 399) Double Torsion 
               
               
                   
               
             
          
         
       
     
     The respective measured characteristic molding material property—namely the critical stress intensity factor and the critical fracture energy—is again indicated in the left column; the respective sample geometry is indicated in the second column; characteristic solid values for the casting compound with only microparticle filler of 66% by weight remain in the third column; the characteristic molding material properties given the addition of 3.4% inorganic nanoparticles are indicated in the fourth column; and the characteristic molding material properties given the addition of 3.4% polymer nanoparticles are indicated in the fifth column. 
     As can be seen from Table 2, a striking improvement can be achieved by exchanging the inorganic nanoparticles for polymer nanoparticles. The critical stress intensity factor K 1C  of 2.14±0.06 given a casting compound with 62.6% microparticles and 3.4% inorganic nanoparticles increases to 2.75±0.05 given the casting compound according to the invention with 62.6% microparticles and 3.4% polymer nanoparticles, an increase of approximately 30%. 
     There is similar behavior with the markedly improved fracture energy: this increases from 433±25 J/m 2  to 987±36 J/m 2  in the casting compound according to the invention. This means that a striking crack resistance improvement can be achieved by exchanging the inorganic nanoparticles for polymer nanoparticles. 
     An examination of the flow behavior has ultimately yielded that the substituted nanoparticle addition produces a very good flow behavior. The course of the flow curve (circle symbols) of a previously used casting compound containing a matrix-forming support material made of modified epoxy resin based on bisphenol-A, a curer made of modified anhydride based on methyl hexahydrophthalic acid anhydride and an accelerator made from tertiary amine (in addition to 66% microparticles) is compared in  FIG. 2  with a casting compound according to the invention that contains the same matrix-forming support materials, 62.4% microparticles and 3.6% polymer nanoparticles made from polybutadiene in core-shell form (box symbols). As  FIG. 2  clearly shows, a marked decrease of the viscosity can be achieved. The shear rate is shown along the abscissa in s −1 ; the viscosity is shown along the ordinate in mPa·s. The preparation temperature was 50° C.; the shear rate range is 0.07-500 s −1 . The viscosity in the shear rate range of 0.03-20 s −1  is markedly reduced (by more than half in broad ranges) by the addition of the polymer nanoparticles. The course of the curve shows no thixotropic effects over the entire shear rate range. 
     Due to the markedly better flow behavior, the proportion of inorganic microfiller could also be increased again in order to counter the reduction of the E-modulus that is seen in Table 1. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.