07 May Sample positioning in Solenoid Coils
The sample positioning in solenoid coils to heat in the presence of an alternating magnetic field (AMF) is not a trivial matter. Depending on the geometry and other manufacturing parameters of the coil, the field distribution and therefore the effective volume, will be homogeneously different. For this reason it is essential to take into account several space considerations to maximize the field applied due to the fact that the heating induced could differ greatly depending on the position where the sample is placed.
Below some tips are presented to understand the importance of choosing the best configuration for your samples.
The importance of sample positioning to maximize nanoparticle heating in Solenoid Coils
A solenoid is an electromagnet which generates a controlled magnetic field through a coil. Solenoid coils provide numerous possibilities for different applications and they are the perfect solution for unusual setups, for example a custom large diameter enables experiments with rabbits. These kind of coils could be designed horizontally or vertically oriented, as shown in Image 1, and with different diameters and heights.
As previously mentioned, depending on the solenoid coil dimensions, the magnetic field distribution will be different. As an example, two magnetic field maps of two different solenoids are shown below.
Magnetic field distribution in Solenoid Coils
A sectional drawing of two solenoid coils of diameters 32 mm and 72 mm respectively are represented in Image 2. The different values of maximum magnetic field achieved inside the coil are represented with different coloured regions.
When an experiment is planed, it is critical that the sample is placed within the region where the field is most homogeneous. It can be observed in each case of Image 2 how the magnetic field is normalized to the point of the coil where the field is maximum, corresponding with the center of the solenoid coil (point P). Each change of colour corresponds to a reduction of 5% regarding the field at this point P. In both cases, the region indicated by a cylinder is where the field is 90% of the maximum value achieved, therefore this is the recommended space to place the sample. In the case of the 32 mm solenoid (S32), the recommended space measures 22 mm height with base diameter of 28 mm, whereas the height and the base diameter for the 72 mm diameter solenoid (S72), are 34 mm and 50 mm respectively.
It is very common to find different solenoid coils commercially available given they can allocate a variety of different kind of samples so they seem an easy solution.
However, as previously stated, to know in detail the magnetic field distribution and where the maximum field area is for an optimal sample positioning is of vital importance to ensure a good understanding of the results obtained.
Sample positioning in calorimetry experiments
Determination of heating efficiency of magnetic fluids in presence of an AMF is one of the main purposes of a solenoid coil. The heating efficiency is quantified by the Specific Absorption Rate (SAR), also known as Specific Power Absorption (SPA).
This is defined as the thermal power dissipated per unit mass of magnetic material. A SAR/SPA experiment involves the delivery of heat when a sample, that is thermally insulated in an (ideally) adiabatic environment, is exposed to an AMF. SAR/SPA depends on the ferrofluid properties as well as the system where the experiment is carried out.
For calorimetry experiments, the positioning of not only the sample but also the temperature sensor is even further important than in other experiments with solenoid coils due to the fact that a small variation in their position could result in strong SAR/SPA fluctuations.
Our catalogue of products presents a variety of coils providing the user the opportunity to select the configuration which maximizes the heating for different applications (see Image 3).
In this technical note, we propose two options for different applications and how to place the sample properly regarding the maximum magnetic field area in a solenoid coil: Solenoid CoilSets Sn and Calorimetry CoilSet CAL1.
D5 Series: Kit of Solenoid CoilSets (Sn)
As discussed at the beginning of this note, a solenoid coil presents diverse possibilities for multiple applications, and for this purpose there are available in our catalogue four solenoid coils, Sn CoillSet (see Image 4), of different diameters and heights, customizable with other dimensions.
A calibration certificate is provided for each coil with full characterization of the magnetic field distribution for each solenoid indicating the central cylinder with a maximum deviation of 10% for each as follows: S18 (16Ø20H), S32(32Ø24H), S56(48Ø40H) and S76(46Ø34H).
Along with them, stands of different sizes are provided to support the sample and center it within the cylinder of maximum magnetic field homogeneity inside the coil through a mechanism which regulates their heights.
D5 Series: Calorimetry CoilSet (CAL1)
In addition, a Calorimetry CoilSet (see Image 5) is presented for calorimetry experiments in colloids using 1.5 ml vials featuring a sample holder column with a fiber optic probe for high precision temperature measurement.
Image 6 displays a sectional drawing of the coil showing the magnetic field distribution in the area of application inside the sample holder column, found in the calibration certificate included, where the 1.5 ml chromatography vial is also indicated in the simulation. Each change of colour corresponds to a reduction of 5% regarding the maximum field in P.
Therefore, it is observed how the complete sample volume is exposed to more than a 90% of the maximum magnetic field achieved. To allow performing tests with different volume samples, different rings are included to center the sample in respect to the field distribution.
In conclusion, it is demonstrated that a full report on the magnetic field distribution of a solenoid coil and having solved the issue of positioning the sample within the most homogeneous region of the coil is essential to avoid this source of measurement errors.
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