# Technology, Preparation and Properties of the Cast Glass-Coated Magnetic Microwires

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## Abstract

**:**

## 1. Introduction

^{4}–10

^{7}K/s [2,6,7].

- The production of the continuous long pieces of a microwire up to 10
^{4}m (in the case of a drip process). For a continuous process (see Figure 1), the length of a microwire is not limited. - A wide range of variations in the geometric parameters (typically the metallic core diameter D
_{m}is in a range of 0.5–70 µm, and a glass-coating thickness is in a range of 1–50 µm (Figure 2)). - The control and adjustment of the geometric parameters (the inner metallic core diameter D
_{m}and glass thickness) during production. - The reproducibility of the physical properties and geometric parameters of the microwires in mass production.

## 2. Experimental Installation for Measuring Magnetic Properties

## 3. The Effect of Heat Treatment on the Coercive Force

_{c}) allowing optimization of magnetic properties of microwires. At temperatures above 400 °C, the formations of a nano- or microcrystalline phases in CGCFMWs were observed (this temperature is designated as T

_{cr}) [11,12,13,14,15,16,17].

_{cr}can improve the CGCFMWs magnetic properties, i.e., decrease the values of σ

_{H}and H

_{c}[14,15,16].

_{cr}-value. In the present study, we used the specimens with the glass coating thickness of about (7–10) μm. The coercive force, H

_{cm}is varied in the range of (0.5–1.5) Oe at room temperature. In spite of a quite broad range of initial H

_{cm}-values, the correlation of H

_{c}/H

_{cm}remains unchanged within the error bars (-up to 20%) and fits within the dependence shown in Figure 5 [5,6,7,8].

_{s}/H

_{sm}~ exp{E

_{1,2}/kT}/[1 + exp{−G/kТ}]

_{1,2}/kT}, describes the low-temperature processes with the energy of activation E

_{1,2}.

_{s}/H

_{sm}~ exp{E

_{1,2}/kT}

_{s}/H

_{sm}~ exp{E

_{1,2}/kT}exp{G/kT}

_{1}+ S

_{2})

_{1}is the entropy change decreasing with the relaxation of the initial amorphous state into more stable one but also amorphous state, and S

_{2}is the entropy change increasing during the transition from the metastable state to the stable polycrystalline state.

_{c}for the irreversible processes that are described by the exp (G/kT) product during the high-temperature treatment.

- Reduced weight
- Reduced losses on the coil due to reduction in number of turns
- Extended temperature range
- Increased durability and stability of properties.
- High accuracy for measuring devices

## 4. Evaluation of Residual Stresses

_{g}(the outer radius of the glass shell) is estimated as follows:

_{d}is the casting rate; σ

_{s}is the surface tension; and η is the dynamical viscosity of the glass:

^{2}kJ/mol, R is the universal gas constant, η

_{0}and c are the material constants (c ~ 0.4–0.9).

- If the casting rate is sufficiently high, we obtain for R
_{c}:$${R}_{C}~\frac{{\mathsf{\eta}}^{4/3}}{{V}_{d}^{2/3}{\mathsf{\sigma}}_{s}^{1/3}\mathsf{\rho}}$$ - At the theoretical limit of extremely high casting rate, i.e., when k→1, we obtain:$${R}_{C}~\frac{\mathsf{\eta}}{\mathsf{\rho}{V}_{d}}$$

_{m}is the metallic core radius of microwire;

_{m}is the Young’s modulus of metal and Y

_{g}is the Young’s modulus of glass:

_{m}and α

_{g}are the thermal expansion coefficients of the metal and glass, respectively; T* is the effective solidification temperature of the composite microwire (when both the metallic core and the glass-coating (cladding) are solidified) and T is the experimental temperature.

_{m}, which has only the elastic residual stresses, freezes earlier.

_{m}, this model gives (see [5,8]):

## 5. Application of Microwires

## 6. Conclusions

## Acknowledgments

## Author Contributions

## Conflicts of Interest

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**Figure 3.**Machine for preparation of cast glass-coated amorphous micro- and nanowires (CGCFMWs) by the Taylor–Ulitovsky method.

**Figure 5.**Dependence of coercivity, H

_{c}/H

_{cm}. (H

_{cm}initial coercivity at room temperature), on annealing temperature, T/T

_{cr}.

**Figure 7.**Viscosity η (see (2)) as a function of temperature [6].

**Figure 8.**Qualitative pattern of the change of residual stresses σ

_{r}

_{,z}along the microwire radius. Curve α presents the initial state of these stresses in the metal strand and in the glass (in glass it is shown by black color and dashed line). In the metal strand these stresses are always tensile, i.e., positive [5,8]. The residual stresses in the silicate glass always compress the glass envelope (i.e., they are negative). Inside the region of plastic relaxation there is shown a section (near the strand center) where the applicability of the continuum model is violated. Curve β is a hypothetic form of residual stresses in the metal strand after removal of the glass envelope or disruption of its bound with the metal strand. As an example, there are presented the qualitative results of the calculation of residual stresses according to [10] (curve γ) which are different from our data.

**Figure 9.**Permeability profile of a single layer, with a resonance frequency value of 0.67 GHz. (see [9]) The composition of microwires investigated here is Co

_{68.15}Fe

_{4.35}Si

_{12.5}B

_{15}which has a very small saturation magnetostriction constant (λ

_{s}) of 1.1 × 10

^{−7}.

**Figure 10.**Absorption characteristics of shielding by a microwire composite (Fe

_{69}C

_{5}B

_{16}Si

_{10}) in a high-frequency field in the range of frequencies 10–12 GHz (see [9]): initial state of the screen is then turned by (2) 90°, (3) 180°, and (4) 270° about the perpendicular axis shown in the figure.

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**MDPI and ACS Style**

Baranov, S.A.; Larin, V.S.; Torcunov, A.V. Technology, Preparation and Properties of the Cast Glass-Coated Magnetic Microwires. *Crystals* **2017**, *7*, 136.
https://doi.org/10.3390/cryst7060136

**AMA Style**

Baranov SA, Larin VS, Torcunov AV. Technology, Preparation and Properties of the Cast Glass-Coated Magnetic Microwires. *Crystals*. 2017; 7(6):136.
https://doi.org/10.3390/cryst7060136

**Chicago/Turabian Style**

Baranov, Serghei A., Vladimir S. Larin, and Alexander V. Torcunov. 2017. "Technology, Preparation and Properties of the Cast Glass-Coated Magnetic Microwires" *Crystals* 7, no. 6: 136.
https://doi.org/10.3390/cryst7060136