How to Specify a Custom Nanocrystalline Core: RFQ Template, AL, Permeability & Acceptance Tests

1. What "custom" actually changes (and what it does not)

Nanocrystalline cores start as melt-spun Fe-based ribbon (typical FINEMET-class chemistry: Fe–Si–B–Nb–Cu)¹, rapid-solidified in the roughly 18–35 μm ribbon-thickness class used across commercial grades², wound to shape, then magnetic-field annealed to form an ultrafine grain structure on the order of ~10 nm. That anneal, not the winding alone, sets permeability, loss, and hysteresis loop shape.

2. Start from the magnetic job, not the catalog part number

Write one sentence that locks the core's job. Everything else follows from it.

If function is ambiguous ("high μ core for filter"), manufacturers cannot choose loop shape, anneal, or test method correctly.

3. The quantities manufacturers actually quote against

Do not specify "μ = 80,000" alone. Specify what is measured. Nominal μ without test conditions is not enforceable.

3.1 Geometry → magnetic path → A_L

Notation used below:

• K_stack: ribbon stacking / fill factor (geometry)
• k_w: waveform form factor (volts-to-flux), e.g. 4.44 sine, 4.0 square-ish

For a toroid (approximate):

Spec practice: quote either a minimum A_L at stated conditions, or a minimum μ_i / μ_e at stated f, H (or B), and T, plus dimensional tolerances.

3.2 Application μ bands (CenturaCores design starting points)

Custom RFQs should pick a window (e.g. μ_i ≥ 80,000 @ 10 kHz, 0.1 A/m, 25 °C), not a single marketing number.

3.3 Saturation and design flux

For common Fe-based nanocrystalline grades used in CT/EMI/power magnetics, published saturation induction is typically about B_s ≈ 1.2–1.25 T @ 25 °C². Do not write "1.7 T" unless you are on a different alloy family with datasheet proof. The design B_pk table below is CenturaCores RFQ-review guidance, not a standards limit.

3.4 Core loss: give the operating point

Nanocrystalline loss is low, but RFQs fail when they omit (B, f, T). Classical Steinmetz-type models of the form P_v ≈ k · f^α · B^β are useful for relative sizing only. Fitted k, α, and β depend strongly on grade, excitation range, temperature, waveform, and unit system, so we do not publish fixed public coefficients here. Require measured W/kg or W/set at the real operating point.

4. Application-specific specification kits

4.1 Common-mode choke (CMC)

Nanocrystalline CMC value is high μ (often ~80k–90k class on our CMC series, as a CenturaCores starting band) and B_s ~ 1.2 T versus typical Mn-Zn ferrite ~0.3–0.5 T. Spec the imbalance, or the choke can pass a μ test and still fail EMC under load. For unbuffered CMCs or magnetics with DC, prefer a flat / transverse-field-annealed characteristic (our RFQ shorthand: F-loop; see Section 6).

See CMC cores and EMI filter cores.

4.2 Current transformer (metering / protection)

Full worked method: CT selection guide.

4.3 Mid-frequency power transformer (20–100 kHz)

Cross-check with the EV charger core guide for OBC / DCFC.

4.4 RCD / residual current sensors

Specify excitation at mA-level, expected frequency content of fault/leakage, and whether the sensor is voltage or current output. High μ and low magnetizing current dominate; mechanical stress after anneal can destroy the μ you paid for.

5. Shape, cut cores, and drawing rules that prevent scrap

Wound ribbon cores are not machined steel. On the drawing, show OD/ID/HT (or A/B/C) before and after coating/case if both matter, max OD / min ID after finish, cut gap g, mating finish, and clamp force limits.

DIY / adjustable C-pairs with 0–2 mm gaps can swing A_L across roughly ~25–200 nH/turn² class ranges. Production parts should use a fixed, process-controlled gap.

6. Hysteresis loop / anneal: R, Z, and F (specify, don't imply)

Field anneal sets anisotropy and loop shape. Labels such as R / Z / F are common supplier and manufacturing shorthand (including CenturaCores RFQs). They are not a universal IEC/IEEE naming standard. Always pair the letter with physical requirements: remanence ratio, anneal field orientation, and DC-bias or pulse behavior.

Annealing furnace capacity is often the lead-time bottleneck. Unrealistically tight μ windows without test H/f force re-anneals and delays.

7. Mechanical, coating, and post-anneal fragility

After crystallization anneal, cores are high-performance and brittle. Micro-cracks and clamp stress shift μ and loss even when dimensions look fine. Specify finish (bare / epoxy / varnish / PBT case), impregnation, clamp limits, packing, and handling. For 50–100 kHz+ designs where measured loss approaches the budget, call out a thinner ribbon class (often ≤ 18 μm) as a validation option, not an automatic requirement. See also our packaging guide.

8. Quality, tests, and acceptance

Ask for tests that match the design, not a generic certificate of conformity. CenturaCores quality procedures cover material, dimensional, and electrical checks used for custom cores.

Compare CoC numbers to the same conditions written on the PO. μ @ 50 Hz, 0.4 A/m, 25 °C is not μ @ 100 kHz, elevated T.

9. Copy-paste custom core specification (technical contract)

Use this as the body of the RFQ / drawing notes, or download the full template as PDF, DOCX, RTF, or TXT from the RFQ template resource page. Attach a dimensioned PDF; for oval/rect include datum corners. Prefer a filled template back from us? Request a quote with your drawing.

CUSTOM NANOCRYSTALLINE CORE SPECIFICATION
Buyer: ____________________  Date: __________  Rev: ____

1. FUNCTION
   [ ] CMC/EMI  [ ] Metering CT  [ ] Protection CT  [ ] RCD  [ ] HF transformer  [ ] Other: ____
   One-line duty: _______________________________________________

2. ELECTRICAL / MAGNETIC OPERATING POINT
   Frequency: ____ Hz / kHz (range: ____ to ____)
   Waveform: [ ] sine [ ] square [ ] PWM  Duty: ____
   Form factor kw (info): ____ (e.g. 4.44 sine / 4.0 square)
   Bpk or V·s: ____ T or ____ V·µs   Margin for transient: ____%
   Temp: ambient ____°C  hotspot ____°C
   Max core loss: ____ W (or ____ W/kg) at B=____ T, f=____, T=____°C
   Ribbon thickness class: [ ] 20–25 µm  [ ] ≤18 µm  [ ] other: ____

3. PERMEABILITY / AL (enforceable)
   Target: µi or µe ≥ ____  OR  AL ≥ ____ nH/N²
   Test: f=____, H=____ A/m (or B=____ mT), T=____°C, fixture: ____
   Loop type: [ ] R-round  [ ] Z-square  [ ] F-flat (low Br, DC bias)
   Br/Bs limits if any: ____

4. APPLICATION EXTRAS
   CMC: Iline=____ A; Zcm/IL table attached; max ΔI/surge=____ A
   CT: class=____; burden=____ VA; Is=____ A; ALF/Vk=____; Rf=____ kA
   RCD: IΔn=____ mA; bandwidth=____

5. GEOMETRY
   Shape: [ ] toroid [ ] oval [ ] rect [ ] C-cut [ ] E [ ] other
   Drawing No: ________  OD/ID/HT or A/B/C: ________
   Gap (if cut): g=____ mm; faces: ____
   Finish: [ ] bare [ ] epoxy [ ] case ______  Color/marking: ____
   Stacking factor assumption for design: Kstack=____ (info only)

6. ENVIRONMENT / MECHANICAL
   Mounting: ____  Clamp limits: ____  Potting/PCB notes: ____
   Packaging: export anti-vib / moisture barrier required: [ ] Y [ ] N

7. QUALITY
   Reports required: dimensions, µ/AL, loss, (CT excitation), FAI
   Sampling: ____

8. COMMERCIAL
   Qty: proto ____ / prod ____   Target lead time: ____
   Ship to: ____

10. Failure modes we see weekly (spec mistakes)

11. Worked mini-examples (RFQ-grade)

12. Frequently asked questions

References and standards

Numerical material claims above should be read with the cited literature and the edition of each standard applicable to your project. Always confirm against the supplier datasheet for the exact ribbon grade and anneal.

1. Yoshizawa, Y., Oguma, S., Yamauchi, K. "New Fe-based soft magnetic alloys composed of ultrafine grain structure." Journal of Applied Physics, 1988. Foundational FINEMET-class Fe–Si–B–Nb–Cu nanocrystalline alloy description.
2. Commercial Fe-based nanocrystalline ribbon datasheets (e.g. FINEMET / equivalent Fe–Si–B–Nb–Cu families): typical B_s near 1.2–1.25 T @ room temperature and ribbon thicknesses in the tens of micrometers; confirm the exact grade used on the PO.
3. IEC 60404 series, Magnetic materials – Methods of measurement of magnetic properties. Use the parts applicable to your specimen geometry and frequency when writing loss / permeability acceptance methods.
4. IEC 61869-2, Instrument transformers – Additional requirements for current transformers; IEEE C57.13, Standard Requirements for Instrument Transformers. Accuracy class, burden, ALF, and related CT requirements belong in the RFQ when specifying CT cores.

For CenturaCores manufacturing test and acceptance context, see the quality page. Match lot test reports to your PO conditions.

Notation quick reference

Next step inside CenturaCores

Run geometry and turns in the designer, attach the completed spec block and drawing to a quote request, and ask for FAI plus μ/A_L and loss (or CT excitation) on the stated operating point.